@page
@node Data Types
@chapter Data Types for Generic Use

This chapter describes all the data types that Guile provides for
``generic use''.

One of the great strengths of Scheme is that there is no straightforward
distinction between ``data'' and ``functionality''.  For example,
Guile's support for dynamic linking could be described

@itemize @bullet
@item
either in a ``data-centric'' way, as the behaviour and properties of the
``dynamically linked object'' data type, and the operations that may be
applied to instances of this type

@item
or in a ``functionality-centric'' way, as the set of procedures that
constitute Guile's support for dynamic linking, in the context of the
module system.
@end itemize

The contents of this chapter are, therefore, a matter of judgement.  By
``generic use'', we mean to select those data types whose typical use as
@emph{data} in a wide variety of programming contexts is more important
than their use in the implementation of a particular piece of
@emph{functionality}.

@ifinfo
The following menu
@end ifinfo
@iftex
The table of contents for this chapter
@end iftex
@ifhtml
The following table of contents
@end ifhtml
shows the data types that are documented in this chapter.  The final
section of this chapter lists all the core Guile data types that are not
documented here, and provides links to the ``functionality-centric''
sections of this manual that cover them.

@menu
* Booleans::                    True/false values.
* Numbers::                     Numerical data types.
* Characters::                  New character names.
* Strings::                     Special things about strings.
* Regular Expressions::         Pattern matching and substitution.
* Symbols and Variables::       Manipulating the Scheme symbol table.
* Keywords::                    Self-quoting, customizable display keywords.
* Pairs::                       Scheme's basic building block.
* Lists::                       Special list functions supported by Guile.
* Vectors::                     One-dimensional arrays of Scheme objects.
* Records::                     
* Structures::                  
* Arrays::                      Arrays of values.
* Association Lists and Hash Tables::  Dictionary data types.
* Hooks::                       User-customizable event lists.
* Other Data Types::            Data types that are documented elsewhere.
@end menu


@node Booleans
@section Booleans
@tpindex Booleans

The two boolean values are @code{#t} for true and @code{#f} for false.

Boolean values are returned by predicate procedures, such as the general
equality predicates @code{eq?}, @code{eqv?} and @code{equal?}
(@pxref{Equality}) and numerical and string comparison operators like
@code{string=?} (@pxref{String Comparison}) and @code{<=}
(@pxref{Comparison}).

@lisp
(<= 3 8)
@result{}
#t

(<= 3 -3)
@result{}
#f

(equal? "house" "houses")
@result{}
#f

(eq? #f #f)
@result{}
#t
@end lisp

In test condition contexts like @code{if} and @code{cond} (@pxref{if
cond case}), where a group of subexpressions will be evaluated only if a
@var{condition} expression evaluates to ``true'', ``true'' means any
value at all except @code{#f}.

@lisp
(if #t "yes" "no")
@result{}
"yes"

(if 0 "yes" "no")
@result{}
"yes"

(if #f "yes" "no")
@result{}
"no"
@end lisp

A result of this asymmetry is that typical Scheme source code more often
uses @code{#f} explicitly than @code{#t}: @code{#f} is necessary to
represent an @code{if} or @code{cond} false value, whereas @code{#t} is
not necessary to represent an @code{if} or @code{cond} true value.

It is important to note that @code{#f} is @strong{not} equivalent to any
other Scheme value.  In particular, @code{#f} is not the same as the
number 0 (like in C and C++), and not the same as the ``empty list''
(like in some Lisp dialects).

The @code{not} procedure returns the boolean inverse of its argument:

@rnindex not
@deffn {Scheme Procedure} not x
@deffnx {C Function} scm_not (x)
Return @code{#t} iff @var{x} is @code{#f}, else return @code{#f}.
@end deffn

The @code{boolean?} procedure is a predicate that returns @code{#t} if
its argument is one of the boolean values, otherwise @code{#f}.

@rnindex boolean?
@deffn {Scheme Procedure} boolean? obj
@deffnx {C Function} scm_boolean_p (obj)
Return @code{#t} iff @var{obj} is either @code{#t} or @code{#f}.
@end deffn


@node Numbers
@section Numerical data types
@tpindex Numbers

Guile supports a rich ``tower'' of numerical types --- integer,
rational, real and complex --- and provides an extensive set of
mathematical and scientific functions for operating on numerical
data.  This section of the manual documents those types and functions.

You may also find it illuminating to read R5RS's presentation of numbers
in Scheme, which is particularly clear and accessible: see
@xref{Numbers,,,r5rs}.

@menu
* Numerical Tower::             Scheme's numerical "tower".
* Integers::                    Whole numbers.
* Reals and Rationals::         Real and rational numbers.
* Complex Numbers::             Complex numbers.
* Exactness::                   Exactness and inexactness.
* Number Syntax::               Read syntax for numerical data.
* Integer Operations::          Operations on integer values.
* Comparison::                  Comparison predicates.
* Conversion::                  Converting numbers to and from strings.
* Complex::                     Complex number operations.
* Arithmetic::                  Arithmetic functions.
* Scientific::                  Scientific functions.
* Primitive Numerics::          Primitive numeric functions.
* Bitwise Operations::          Logical AND, OR, NOT, and so on.
* Random::                      Random number generation.
@end menu


@node Numerical Tower
@subsection Scheme's Numerical ``Tower''
@rnindex number?

Scheme's numerical ``tower'' consists of the following categories of
numbers:

@itemize @bullet
@item
integers (whole numbers)

@item
rationals (the set of numbers that can be expressed as P/Q where P and Q
are integers)

@item
real numbers (the set of numbers that describes all possible positions
along a one dimensional line)

@item
complex numbers (the set of numbers that describes all possible
positions in a two dimensional space)
@end itemize

It is called a tower because each category ``sits on'' the one that
follows it, in the sense that every integer is also a rational, every
rational is also real, and every real number is also a complex number
(but with zero imaginary part).

Of these, Guile implements integers, reals and complex numbers as
distinct types.  Rationals are implemented as regards the read syntax
for rational numbers that is specified by R5RS, but are immediately
converted by Guile to the corresponding real number.

The @code{number?} predicate may be applied to any Scheme value to
discover whether the value is any of the supported numerical types.

@deffn {Scheme Procedure} number? obj
@deffnx {C Function} scm_number_p (obj)
Return @code{#t} if @var{obj} is any kind of number, @code{#f} else.
@findex number?
@end deffn

For example:

@lisp
(number? 3)
@result{}
#t

(number? "hello there!")
@result{}
#f

(define pi 3.141592654)
(number? pi)
@result{}
#t
@end lisp

The next few subsections document each of Guile's numerical data types
in detail.

@node Integers
@subsection Integers

@tpindex Integer numbers

@rnindex integer?

Integers are whole numbers, that is numbers with no fractional part,
such as 2, 83 and -3789.

Integers in Guile can be arbitrarily big, as shown by the following
example.

@lisp
(define (factorial n)
  (let loop ((n n) (product 1))
    (if (= n 0)
        product
        (loop (- n 1) (* product n)))))

(factorial 3)
@result{}
6

(factorial 20)
@result{}
2432902008176640000

(- (factorial 45))
@result{}
-119622220865480194561963161495657715064383733760000000000
@end lisp

Readers whose background is in programming languages where integers are
limited by the need to fit into just 4 or 8 bytes of memory may find
this surprising, or suspect that Guile's representation of integers is
inefficient.  In fact, Guile achieves a near optimal balance of
convenience and efficiency by using the host computer's native
representation of integers where possible, and a more general
representation where the required number does not fit in the native
form.  Conversion between these two representations is automatic and
completely invisible to the Scheme level programmer.

@c REFFIXME Maybe point here to discussion of handling immediates/bignums
@c on the C level, where the conversion is not so automatic - NJ

@deffn {Scheme Procedure} integer? x
@deffnx {C Function} scm_integer_p (x)
Return @code{#t} if @var{x} is an integer number, @code{#f} else.

@lisp
(integer? 487)
@result{}
#t

(integer? -3.4)
@result{}
#f
@end lisp
@end deffn


@node Reals and Rationals
@subsection Real and Rational Numbers
@tpindex Real numbers
@tpindex Rational numbers

@rnindex real?
@rnindex rational?

Mathematically, the real numbers are the set of numbers that describe
all possible points along a continuous, infinite, one-dimensional line.
The rational numbers are the set of all numbers that can be written as
fractions P/Q, where P and Q are integers.  All rational numbers are
also real, but there are real numbers that are not rational, for example
the square root of 2, and pi.

Guile represents both real and rational numbers approximately using a
floating point encoding with limited precision.  Even though the actual
encoding is in binary, it may be helpful to think of it as a decimal
number with a limited number of significant figures and a decimal point
somewhere, since this corresponds to the standard notation for non-whole
numbers.  For example:

@lisp
0.34
-0.00000142857931198
-5648394822220000000000.0
4.0
@end lisp

The limited precision of Guile's encoding means that any ``real'' number
in Guile can be written in a rational form, by multiplying and then dividing
by sufficient powers of 10 (or in fact, 2).  For example,
@code{-0.00000142857931198} is the same as @code{142857931198} divided by
@code{100000000000000000}.  In Guile's current incarnation, therefore,
the @code{rational?} and @code{real?} predicates are equivalent.

Another aspect of this equivalence is that Guile currently does not
preserve the exactness that is possible with rational arithmetic.
If such exactness is needed, it is of course possible to implement
exact rational arithmetic at the Scheme level using Guile's arbitrary
size integers.

A planned future revision of Guile's numerical tower will make it
possible to implement exact representations and arithmetic for both
rational numbers and real irrational numbers such as square roots,
and in such a way that the new kinds of number integrate seamlessly
with those that are already implemented.

@deffn {Scheme Procedure} real? obj
@deffnx {C Function} scm_real_p (obj)
Return @code{#t} if @var{obj} is a real number, @code{#f} else.
Note that the sets of integer and rational values form subsets
of the set of real numbers, so the predicate will also be fulfilled
if @var{obj} is an integer number or a rational number.
@findex real?
@end deffn

@deffn {Scheme Procedure} rational? x
@deffnx {C Function} scm_real_p (x)
Return @code{#t} if @var{x} is a rational number, @code{#f}
else.  Note that the set of integer values forms a subset of
the set of rational numbers, i. e. the predicate will also be
fulfilled if @var{x} is an integer number.  Real numbers
will also satisfy this predicate, because of their limited
precision.
@end deffn


@node Complex Numbers
@subsection Complex Numbers
@tpindex Complex numbers

@rnindex complex?

Complex numbers are the set of numbers that describe all possible points
in a two-dimensional space.  The two coordinates of a particular point
in this space are known as the @dfn{real} and @dfn{imaginary} parts of
the complex number that describes that point.

In Guile, complex numbers are written in rectangular form as the sum of
their real and imaginary parts, using the symbol @code{i} to indicate
the imaginary part.

@lisp
3+4i
@result{}
3.0+4.0i

(* 3-8i 2.3+0.3i)
@result{}
9.3-17.5i
@end lisp

Guile represents a complex number as a pair of numbers both of which are
real, so the real and imaginary parts of a complex number have the same
properties of inexactness and limited precision as single real numbers.

@deffn {Scheme Procedure} complex? x
@deffnx {C Function} scm_number_p (x)
Return @code{#t} if @var{x} is a complex number, @code{#f}
else.  Note that the sets of real, rational and integer
values form subsets of the set of complex numbers, i. e. the
predicate will also be fulfilled if @var{x} is a real,
rational or integer number.
@end deffn


@node Exactness
@subsection Exact and Inexact Numbers
@tpindex Exact numbers
@tpindex Inexact numbers

@rnindex exact?
@rnindex inexact?
@rnindex exact->inexact
@rnindex inexact->exact

R5RS requires that a calculation involving inexact numbers always
produces an inexact result.  To meet this requirement, Guile
distinguishes between an exact integer value such as @code{5} and the
corresponding inexact real value which, to the limited precision
available, has no fractional part, and is printed as @code{5.0}.  Guile
will only convert the latter value to the former when forced to do so by
an invocation of the @code{inexact->exact} procedure.

@deffn {Scheme Procedure} exact? x
@deffnx {C Function} scm_exact_p (x)
Return @code{#t} if @var{x} is an exact number, @code{#f}
otherwise.
@end deffn

@deffn {Scheme Procedure} inexact? x
@deffnx {C Function} scm_inexact_p (x)
Return @code{#t} if @var{x} is an inexact number, @code{#f}
else.
@end deffn

@deffn {Scheme Procedure} inexact->exact z
@deffnx {C Function} scm_inexact_to_exact (z)
Return an exact number that is numerically closest to @var{z}.
@end deffn

@c begin (texi-doc-string "guile" "exact->inexact")
@deffn {Scheme Procedure} exact->inexact z
Convert the number @var{z} to its inexact representation.
@end deffn


@node Number Syntax
@subsection Read Syntax for Numerical Data

The read syntax for integers is a string of digits, optionally
preceded by a minus or plus character, a code indicating the
base in which the integer is encoded, and a code indicating whether
the number is exact or inexact.  The supported base codes are:

@itemize @bullet
@item
@code{#b}, @code{#B} --- the integer is written in binary (base 2)

@item
@code{#o}, @code{#O} --- the integer is written in octal (base 8)

@item
@code{#d}, @code{#D} --- the integer is written in decimal (base 10)

@item
@code{#x}, @code{#X} --- the integer is written in hexadecimal (base 16).
@end itemize

If the base code is omitted, the integer is assumed to be decimal.  The
following examples show how these base codes are used.

@lisp
-13
@result{}
-13

#d-13
@result{}
-13

#x-13
@result{}
-19

#b+1101
@result{}
13

#o377
@result{}
255
@end lisp

The codes for indicating exactness (which can, incidentally, be applied
to all numerical values) are:

@itemize @bullet
@item
@code{#e}, @code{#E} --- the number is exact

@item
@code{#i}, @code{#I} --- the number is inexact.
@end itemize

If the exactness indicator is omitted, the integer is assumed to be exact,
since Guile's internal representation for integers is always exact.
Real numbers have limited precision similar to the precision of the
@code{double} type in C.  A consequence of the limited precision is that
all real numbers in Guile are also rational, since any number R with a
limited number of decimal places, say N, can be made into an integer by
multiplying by 10^N.


@node Integer Operations
@subsection Operations on Integer Values
@rnindex odd?
@rnindex even?
@rnindex quotient
@rnindex remainder
@rnindex modulo
@rnindex gcd
@rnindex lcm

@deffn {Scheme Procedure} odd? n
@deffnx {C Function} scm_odd_p (n)
Return @code{#t} if @var{n} is an odd number, @code{#f}
otherwise.
@end deffn

@deffn {Scheme Procedure} even? n
@deffnx {C Function} scm_even_p (n)
Return @code{#t} if @var{n} is an even number, @code{#f}
otherwise.
@end deffn

@c begin (texi-doc-string "guile" "quotient")
@deffn {Scheme Procedure} quotient
Return the quotient of the numbers @var{x} and @var{y}.
@end deffn

@c begin (texi-doc-string "guile" "remainder")
@deffn {Scheme Procedure} remainder
Return the remainder of the numbers @var{x} and @var{y}.
@lisp
(remainder 13 4) @result{} 1
(remainder -13 4) @result{} -1
@end lisp
@end deffn

@c begin (texi-doc-string "guile" "modulo")
@deffn {Scheme Procedure} modulo
Return the modulo of the numbers @var{x} and @var{y}.
@lisp
(modulo 13 4) @result{} 1
(modulo -13 4) @result{} 3
@end lisp
@end deffn

@c begin (texi-doc-string "guile" "gcd")
@deffn {Scheme Procedure} gcd
Return the greatest common divisor of all arguments.
If called without arguments, 0 is returned.
@end deffn

@c begin (texi-doc-string "guile" "lcm")
@deffn {Scheme Procedure} lcm
Return the least common multiple of the arguments.
If called without arguments, 1 is returned.
@end deffn


@node Comparison
@subsection Comparison Predicates
@rnindex zero?
@rnindex positive?
@rnindex negative?

@c begin (texi-doc-string "guile" "=")
@deffn {Scheme Procedure} =
Return @code{#t} if all parameters are numerically equal.
@end deffn

@c begin (texi-doc-string "guile" "<")
@deffn {Scheme Procedure} <
Return @code{#t} if the list of parameters is monotonically
increasing.
@end deffn

@c begin (texi-doc-string "guile" ">")
@deffn {Scheme Procedure} >
Return @code{#t} if the list of parameters is monotonically
decreasing.
@end deffn

@c begin (texi-doc-string "guile" "<=")
@deffn {Scheme Procedure} <=
Return @code{#t} if the list of parameters is monotonically
non-decreasing.
@end deffn

@c begin (texi-doc-string "guile" ">=")
@deffn {Scheme Procedure} >=
Return @code{#t} if the list of parameters is monotonically
non-increasing.
@end deffn

@c begin (texi-doc-string "guile" "zero?")
@deffn {Scheme Procedure} zero?
Return @code{#t} if @var{z} is an exact or inexact number equal to
zero.
@end deffn

@c begin (texi-doc-string "guile" "positive?")
@deffn {Scheme Procedure} positive?
Return @code{#t} if @var{x} is an exact or inexact number greater than
zero.
@end deffn

@c begin (texi-doc-string "guile" "negative?")
@deffn {Scheme Procedure} negative?
Return @code{#t} if @var{x} is an exact or inexact number less than
zero.
@end deffn


@node Conversion
@subsection Converting Numbers To and From Strings
@rnindex number->string
@rnindex string->number

@deffn {Scheme Procedure} number->string n [radix]
@deffnx {C Function} scm_number_to_string (n, radix)
Return a string holding the external representation of the
number @var{n} in the given @var{radix}.  If @var{n} is
inexact, a radix of 10 will be used.
@end deffn

@deffn {Scheme Procedure} string->number string [radix]
@deffnx {C Function} scm_string_to_number (string, radix)
Return a number of the maximally precise representation
expressed by the given @var{string}. @var{radix} must be an
exact integer, either 2, 8, 10, or 16. If supplied, @var{radix}
is a default radix that may be overridden by an explicit radix
prefix in @var{string} (e.g. "#o177"). If @var{radix} is not
supplied, then the default radix is 10. If string is not a
syntactically valid notation for a number, then
@code{string->number} returns @code{#f}.
@end deffn


@node Complex
@subsection Complex Number Operations
@rnindex make-rectangular
@rnindex make-polar
@rnindex real-part
@rnindex imag-part
@rnindex magnitude
@rnindex angle

@deffn {Scheme Procedure} make-rectangular real imaginary
@deffnx {C Function} scm_make_rectangular (real, imaginary)
Return a complex number constructed of the given @var{real} and
@var{imaginary} parts.
@end deffn

@deffn {Scheme Procedure} make-polar x y
@deffnx {C Function} scm_make_polar (x, y)
Return the complex number @var{x} * e^(i * @var{y}).
@end deffn

@c begin (texi-doc-string "guile" "real-part")
@deffn {Scheme Procedure} real-part
Return the real part of the number @var{z}.
@end deffn

@c begin (texi-doc-string "guile" "imag-part")
@deffn {Scheme Procedure} imag-part
Return the imaginary part of the number @var{z}.
@end deffn

@c begin (texi-doc-string "guile" "magnitude")
@deffn {Scheme Procedure} magnitude
Return the magnitude of the number @var{z}. This is the same as
@code{abs} for real arguments, but also allows complex numbers.
@end deffn

@c begin (texi-doc-string "guile" "angle")
@deffn {Scheme Procedure} angle
Return the angle of the complex number @var{z}.
@end deffn


@node Arithmetic
@subsection Arithmetic Functions
@rnindex max
@rnindex min
@rnindex +
@rnindex *
@rnindex -
@rnindex /
@rnindex abs
@rnindex floor
@rnindex ceiling
@rnindex truncate
@rnindex round

@c begin (texi-doc-string "guile" "+")
@deffn {Scheme Procedure} + z1 @dots{}
Return the sum of all parameter values.  Return 0 if called without any
parameters.
@end deffn

@c begin (texi-doc-string "guile" "-")
@deffn {Scheme Procedure} - z1 z2 @dots{}
If called with one argument @var{z1}, -@var{z1} is returned. Otherwise
the sum of all but the first argument are subtracted from the first
argument.
@end deffn

@c begin (texi-doc-string "guile" "*")
@deffn {Scheme Procedure} * z1 @dots{}
Return the product of all arguments.  If called without arguments, 1 is
returned.
@end deffn

@c begin (texi-doc-string "guile" "/")
@deffn {Scheme Procedure} / z1 z2 @dots{}
Divide the first argument by the product of the remaining arguments.  If
called with one argument @var{z1}, 1/@var{z1} is returned.
@end deffn

@c begin (texi-doc-string "guile" "abs")
@deffn {Scheme Procedure} abs x
Return the absolute value of @var{x}.
@end deffn

@c begin (texi-doc-string "guile" "max")
@deffn {Scheme Procedure} max x1 x2 @dots{}
Return the maximum of all parameter values.
@end deffn

@c begin (texi-doc-string "guile" "min")
@deffn {Scheme Procedure} min x1 x2 @dots{}
Return the minium of all parameter values.
@end deffn

@c begin (texi-doc-string "guile" "truncate")
@deffn {Scheme Procedure} truncate
Round the inexact number @var{x} towards zero.
@end deffn

@c begin (texi-doc-string "guile" "round")
@deffn {Scheme Procedure} round x
Round the inexact number @var{x} towards zero.
@end deffn

@c begin (texi-doc-string "guile" "floor")
@deffn {Scheme Procedure} floor x
Round the number @var{x} towards minus infinity.
@end deffn

@c begin (texi-doc-string "guile" "ceiling")
@deffn {Scheme Procedure} ceiling x
Round the number @var{x} towards infinity.
@end deffn


@node Scientific
@subsection Scientific Functions

The following procedures accept any kind of number as arguments,
including complex numbers.

@rnindex sqrt
@c begin (texi-doc-string "guile" "sqrt")
@deffn {Scheme Procedure} sqrt z
Return the square root of @var{z}.
@end deffn

@rnindex expt
@c begin (texi-doc-string "guile" "expt")
@deffn {Scheme Procedure} expt z1 z2
Return @var{z1} raised to the power of @var{z2}.
@end deffn

@rnindex sin
@c begin (texi-doc-string "guile" "sin")
@deffn {Scheme Procedure} sin z
Return the sine of @var{z}.
@end deffn

@rnindex cos
@c begin (texi-doc-string "guile" "cos")
@deffn {Scheme Procedure} cos z
Return the cosine of @var{z}.
@end deffn

@rnindex tan
@c begin (texi-doc-string "guile" "tan")
@deffn {Scheme Procedure} tan z
Return the tangent of @var{z}.
@end deffn

@rnindex asin
@c begin (texi-doc-string "guile" "asin")
@deffn {Scheme Procedure} asin z
Return the arcsine of @var{z}.
@end deffn

@rnindex acos
@c begin (texi-doc-string "guile" "acos")
@deffn {Scheme Procedure} acos z
Return the arccosine of @var{z}.
@end deffn

@rnindex atan
@c begin (texi-doc-string "guile" "atan")
@deffn {Scheme Procedure} atan z
Return the arctangent of @var{z}.
@end deffn

@rnindex exp
@c begin (texi-doc-string "guile" "exp")
@deffn {Scheme Procedure} exp z
Return e to the power of @var{z}, where e is the base of natural
logarithms (2.71828@dots{}).
@end deffn

@rnindex log
@c begin (texi-doc-string "guile" "log")
@deffn {Scheme Procedure} log z
Return the natural logarithm of @var{z}.
@end deffn

@c begin (texi-doc-string "guile" "log10")
@deffn {Scheme Procedure} log10 z
Return the base 10 logarithm of @var{z}.
@end deffn

@c begin (texi-doc-string "guile" "sinh")
@deffn {Scheme Procedure} sinh z
Return the hyperbolic sine of @var{z}.
@end deffn

@c begin (texi-doc-string "guile" "cosh")
@deffn {Scheme Procedure} cosh z
Return the hyperbolic cosine of @var{z}.
@end deffn

@c begin (texi-doc-string "guile" "tanh")
@deffn {Scheme Procedure} tanh z
Return the hyperbolic tangent of @var{z}.
@end deffn

@c begin (texi-doc-string "guile" "asinh")
@deffn {Scheme Procedure} asinh z
Return the hyperbolic arcsine of @var{z}.
@end deffn

@c begin (texi-doc-string "guile" "acosh")
@deffn {Scheme Procedure} acosh z
Return the hyperbolic arccosine of @var{z}.
@end deffn

@c begin (texi-doc-string "guile" "atanh")
@deffn {Scheme Procedure} atanh z
Return the hyperbolic arctangent of @var{z}.
@end deffn


@node Primitive Numerics
@subsection Primitive Numeric Functions

Many of Guile's numeric procedures which accept any kind of numbers as
arguments, including complex numbers, are implemented as Scheme
procedures that use the following real number-based primitives.  These
primitives signal an error if they are called with complex arguments.

@c begin (texi-doc-string "guile" "$abs")
@deffn {Scheme Procedure} $abs x
Return the absolute value of @var{x}.
@end deffn

@c begin (texi-doc-string "guile" "$sqrt")
@deffn {Scheme Procedure} $sqrt x
Return the square root of @var{x}.
@end deffn

@deffn {Scheme Procedure} $expt x y
@deffnx {C Function} scm_sys_expt (x, y)
Return @var{x} raised to the power of @var{y}. This
procedure does not accept complex arguments.
@end deffn

@c begin (texi-doc-string "guile" "$sin")
@deffn {Scheme Procedure} $sin x
Return the sine of @var{x}.
@end deffn

@c begin (texi-doc-string "guile" "$cos")
@deffn {Scheme Procedure} $cos x
Return the cosine of @var{x}.
@end deffn

@c begin (texi-doc-string "guile" "$tan")
@deffn {Scheme Procedure} $tan x
Return the tangent of @var{x}.
@end deffn

@c begin (texi-doc-string "guile" "$asin")
@deffn {Scheme Procedure} $asin x
Return the arcsine of @var{x}.
@end deffn

@c begin (texi-doc-string "guile" "$acos")
@deffn {Scheme Procedure} $acos x
Return the arccosine of @var{x}.
@end deffn

@c begin (texi-doc-string "guile" "$atan")
@deffn {Scheme Procedure} $atan x
Return the arctangent of @var{x} in the range -PI/2 to PI/2.
@end deffn

@deffn {Scheme Procedure} $atan2 x y
@deffnx {C Function} scm_sys_atan2 (x, y)
Return the arc tangent of the two arguments @var{x} and
@var{y}. This is similar to calculating the arc tangent of
@var{x} / @var{y}, except that the signs of both arguments
are used to determine the quadrant of the result. This
procedure does not accept complex arguments.
@end deffn

@c begin (texi-doc-string "guile" "$exp")
@deffn {Scheme Procedure} $exp x
Return e to the power of @var{x}, where e is the base of natural
logarithms (2.71828@dots{}).
@end deffn

@c begin (texi-doc-string "guile" "$log")
@deffn {Scheme Procedure} $log x
Return the natural logarithm of @var{x}.
@end deffn

@c begin (texi-doc-string "guile" "$sinh")
@deffn {Scheme Procedure} $sinh x
Return the hyperbolic sine of @var{x}.
@end deffn

@c begin (texi-doc-string "guile" "$cosh")
@deffn {Scheme Procedure} $cosh x
Return the hyperbolic cosine of @var{x}.
@end deffn

@c begin (texi-doc-string "guile" "$tanh")
@deffn {Scheme Procedure} $tanh x
Return the hyperbolic tangent of @var{x}.
@end deffn

@c begin (texi-doc-string "guile" "$asinh")
@deffn {Scheme Procedure} $asinh x
Return the hyperbolic arcsine of @var{x}.
@end deffn

@c begin (texi-doc-string "guile" "$acosh")
@deffn {Scheme Procedure} $acosh x
Return the hyperbolic arccosine of @var{x}.
@end deffn

@c begin (texi-doc-string "guile" "$atanh")
@deffn {Scheme Procedure} $atanh x
Return the hyperbolic arctangent of @var{x}.
@end deffn


@node Bitwise Operations
@subsection Bitwise Operations

@deffn {Scheme Procedure} logand n1 n2
Return the bitwise AND of the integer arguments.

@lisp
(logand) @result{} -1
(logand 7) @result{} 7
(logand #b111 #b011 #b001) @result{} 1
@end lisp
@end deffn

@deffn {Scheme Procedure} logior n1 n2
Return the bitwise OR of the integer arguments.

@lisp
(logior) @result{} 0
(logior 7) @result{} 7
(logior #b000 #b001 #b011) @result{} 3
@end lisp
@end deffn

@deffn {Scheme Procedure} logxor n1 n2
Return the bitwise XOR of the integer arguments.  A bit is
set in the result if it is set in an odd number of arguments.
@lisp
(logxor) @result{} 0
(logxor 7) @result{} 7
(logxor #b000 #b001 #b011) @result{} 2
(logxor #b000 #b001 #b011 #b011) @result{} 1
@end lisp
@end deffn

@deffn {Scheme Procedure} lognot n
@deffnx {C Function} scm_lognot (n)
Return the integer which is the 2s-complement of the integer
argument.

@lisp
(number->string (lognot #b10000000) 2)
   @result{} "-10000001"
(number->string (lognot #b0) 2)
   @result{} "-1"
@end lisp
@end deffn

@deffn {Scheme Procedure} logtest j k
@deffnx {C Function} scm_logtest (j, k)
@lisp
(logtest j k) @equiv{} (not (zero? (logand j k)))

(logtest #b0100 #b1011) @result{} #f
(logtest #b0100 #b0111) @result{} #t
@end lisp
@end deffn

@deffn {Scheme Procedure} logbit? index j
@deffnx {C Function} scm_logbit_p (index, j)
@lisp
(logbit? index j) @equiv{} (logtest (integer-expt 2 index) j)

(logbit? 0 #b1101) @result{} #t
(logbit? 1 #b1101) @result{} #f
(logbit? 2 #b1101) @result{} #t
(logbit? 3 #b1101) @result{} #t
(logbit? 4 #b1101) @result{} #f
@end lisp
@end deffn

@deffn {Scheme Procedure} ash n cnt
@deffnx {C Function} scm_ash (n, cnt)
The function ash performs an arithmetic shift left by @var{cnt}
bits (or shift right, if @var{cnt} is negative).  'Arithmetic'
means, that the function does not guarantee to keep the bit
structure of @var{n}, but rather guarantees that the result
will always be rounded towards minus infinity.  Therefore, the
results of ash and a corresponding bitwise shift will differ if
@var{n} is negative.

Formally, the function returns an integer equivalent to
@code{(inexact->exact (floor (* @var{n} (expt 2 @var{cnt}))))}.

@lisp
(number->string (ash #b1 3) 2)     @result{} "1000"
(number->string (ash #b1010 -1) 2) @result{} "101"
@end lisp
@end deffn

@deffn {Scheme Procedure} logcount n
@deffnx {C Function} scm_logcount (n)
Return the number of bits in integer @var{n}.  If integer is
positive, the 1-bits in its binary representation are counted.
If negative, the 0-bits in its two's-complement binary
representation are counted.  If 0, 0 is returned.

@lisp
(logcount #b10101010)
   @result{} 4
(logcount 0)
   @result{} 0
(logcount -2)
   @result{} 1
@end lisp
@end deffn

@deffn {Scheme Procedure} integer-length n
@deffnx {C Function} scm_integer_length (n)
Return the number of bits neccessary to represent @var{n}.

@lisp
(integer-length #b10101010)
   @result{} 8
(integer-length 0)
   @result{} 0
(integer-length #b1111)
   @result{} 4
@end lisp
@end deffn

@deffn {Scheme Procedure} integer-expt n k
@deffnx {C Function} scm_integer_expt (n, k)
Return @var{n} raised to the non-negative integer exponent
@var{k}.

@lisp
(integer-expt 2 5)
   @result{} 32
(integer-expt -3 3)
   @result{} -27
@end lisp
@end deffn

@deffn {Scheme Procedure} bit-extract n start end
@deffnx {C Function} scm_bit_extract (n, start, end)
Return the integer composed of the @var{start} (inclusive)
through @var{end} (exclusive) bits of @var{n}.  The
@var{start}th bit becomes the 0-th bit in the result.

@lisp
(number->string (bit-extract #b1101101010 0 4) 2)
   @result{} "1010"
(number->string (bit-extract #b1101101010 4 9) 2)
   @result{} "10110"
@end lisp
@end deffn


@node Random
@subsection Random Number Generation

@deffn {Scheme Procedure} copy-random-state [state]
@deffnx {C Function} scm_copy_random_state (state)
Return a copy of the random state @var{state}.
@end deffn

@deffn {Scheme Procedure} random n [state]
@deffnx {C Function} scm_random (n, state)
Return a number in [0,N).

Accepts a positive integer or real n and returns a
number of the same type between zero (inclusive) and
N (exclusive). The values returned have a uniform
distribution.

The optional argument @var{state} must be of the type produced
by @code{seed->random-state}. It defaults to the value of the
variable @var{*random-state*}. This object is used to maintain
the state of the pseudo-random-number generator and is altered
as a side effect of the random operation.
@end deffn

@deffn {Scheme Procedure} random:exp [state]
@deffnx {C Function} scm_random_exp (state)
Return an inexact real in an exponential distribution with mean
1.  For an exponential distribution with mean u use (* u
(random:exp)).
@end deffn

@deffn {Scheme Procedure} random:hollow-sphere! v [state]
@deffnx {C Function} scm_random_hollow_sphere_x (v, state)
Fills vect with inexact real random numbers
the sum of whose squares is equal to 1.0.
Thinking of vect as coordinates in space of
dimension n = (vector-length vect), the coordinates
are uniformly distributed over the surface of the
unit n-sphere.
@end deffn

@deffn {Scheme Procedure} random:normal [state]
@deffnx {C Function} scm_random_normal (state)
Return an inexact real in a normal distribution.  The
distribution used has mean 0 and standard deviation 1.  For a
normal distribution with mean m and standard deviation d use
@code{(+ m (* d (random:normal)))}.
@end deffn

@deffn {Scheme Procedure} random:normal-vector! v [state]
@deffnx {C Function} scm_random_normal_vector_x (v, state)
Fills vect with inexact real random numbers that are
independent and standard normally distributed
(i.e., with mean 0 and variance 1).
@end deffn

@deffn {Scheme Procedure} random:solid-sphere! v [state]
@deffnx {C Function} scm_random_solid_sphere_x (v, state)
Fills vect with inexact real random numbers
the sum of whose squares is less than 1.0.
Thinking of vect as coordinates in space of
dimension n = (vector-length vect), the coordinates
are uniformly distributed within the unit n-sphere.
The sum of the squares of the numbers is returned.
@end deffn

@deffn {Scheme Procedure} random:uniform [state]
@deffnx {C Function} scm_random_uniform (state)
Return a uniformly distributed inexact real random number in
[0,1).
@end deffn

@deffn {Scheme Procedure} seed->random-state seed
@deffnx {C Function} scm_seed_to_random_state (seed)
Return a new random state using @var{seed}.
@end deffn


@node Characters
@section Characters
@tpindex Characters

Most of the characters in the ASCII character set may be referred to by
name: for example, @code{#\tab}, @code{#\esc}, @code{#\stx}, and so on.
The following table describes the ASCII names for each character.

@multitable @columnfractions .25 .25 .25 .25
@item 0 = @code{#\nul}
 @tab 1 = @code{#\soh}
 @tab 2 = @code{#\stx}
 @tab 3 = @code{#\etx}
@item 4 = @code{#\eot}
 @tab 5 = @code{#\enq}
 @tab 6 = @code{#\ack}
 @tab 7 = @code{#\bel}
@item 8 = @code{#\bs}
 @tab 9 = @code{#\ht}
 @tab 10 = @code{#\nl}
 @tab 11 = @code{#\vt}
@item 12 = @code{#\np}
 @tab 13 = @code{#\cr}
 @tab 14 = @code{#\so}
 @tab 15 = @code{#\si}
@item 16 = @code{#\dle}
 @tab 17 = @code{#\dc1}
 @tab 18 = @code{#\dc2}
 @tab 19 = @code{#\dc3}
@item 20 = @code{#\dc4}
 @tab 21 = @code{#\nak}
 @tab 22 = @code{#\syn}
 @tab 23 = @code{#\etb}
@item 24 = @code{#\can}
 @tab 25 = @code{#\em}
 @tab 26 = @code{#\sub}
 @tab 27 = @code{#\esc}
@item 28 = @code{#\fs}
 @tab 29 = @code{#\gs}
 @tab 30 = @code{#\rs}
 @tab 31 = @code{#\us}
@item 32 = @code{#\sp}
@end multitable

The @code{delete} character (octal 177) may be referred to with the name
@code{#\del}.

Several characters have more than one name:

@itemize @bullet
@item
@code{#\space}, @code{#\sp}
@item
@code{#\newline}, @code{#\nl}
@item
@code{#\tab}, @code{#\ht}
@item
@code{#\backspace}, @code{#\bs}
@item
@code{#\return}, @code{#\cr}
@item
@code{#\page}, @code{#\np}
@item
@code{#\null}, @code{#\nul}
@end itemize

@rnindex char?
@deffn {Scheme Procedure} char? x
@deffnx {C Function} scm_char_p (x)
Return @code{#t} iff @var{x} is a character, else @code{#f}.
@end deffn

@rnindex char=?
@deffn {Scheme Procedure} char=? x y
Return @code{#t} iff @var{x} is the same character as @var{y}, else @code{#f}.
@end deffn

@rnindex char<?
@deffn {Scheme Procedure} char<? x y
Return @code{#t} iff @var{x} is less than @var{y} in the ASCII sequence,
else @code{#f}.
@end deffn

@rnindex char<=?
@deffn {Scheme Procedure} char<=? x y
Return @code{#t} iff @var{x} is less than or equal to @var{y} in the
ASCII sequence, else @code{#f}.
@end deffn

@rnindex char>?
@deffn {Scheme Procedure} char>? x y
Return @code{#t} iff @var{x} is greater than @var{y} in the ASCII
sequence, else @code{#f}.
@end deffn

@rnindex char>=?
@deffn {Scheme Procedure} char>=? x y
Return @code{#t} iff @var{x} is greater than or equal to @var{y} in the
ASCII sequence, else @code{#f}.
@end deffn

@rnindex char-ci=?
@deffn {Scheme Procedure} char-ci=? x y
Return @code{#t} iff @var{x} is the same character as @var{y} ignoring
case, else @code{#f}.
@end deffn

@rnindex char-ci<?
@deffn {Scheme Procedure} char-ci<? x y
Return @code{#t} iff @var{x} is less than @var{y} in the ASCII sequence
ignoring case, else @code{#f}.
@end deffn

@rnindex char-ci<=?
@deffn {Scheme Procedure} char-ci<=? x y
Return @code{#t} iff @var{x} is less than or equal to @var{y} in the
ASCII sequence ignoring case, else @code{#f}.
@end deffn

@rnindex char-ci>?
@deffn {Scheme Procedure} char-ci>? x y
Return @code{#t} iff @var{x} is greater than @var{y} in the ASCII
sequence ignoring case, else @code{#f}.
@end deffn

@rnindex char-ci>=?
@deffn {Scheme Procedure} char-ci>=? x y
Return @code{#t} iff @var{x} is greater than or equal to @var{y} in the
ASCII sequence ignoring case, else @code{#f}.
@end deffn

@rnindex char-alphabetic?
@deffn {Scheme Procedure} char-alphabetic? chr
@deffnx {C Function} scm_char_alphabetic_p (chr)
Return @code{#t} iff @var{chr} is alphabetic, else @code{#f}.
Alphabetic means the same thing as the isalpha C library function.
@end deffn

@rnindex char-numeric?
@deffn {Scheme Procedure} char-numeric? chr
@deffnx {C Function} scm_char_numeric_p (chr)
Return @code{#t} iff @var{chr} is numeric, else @code{#f}.
Numeric means the same thing as the isdigit C library function.
@end deffn

@rnindex char-whitespace?
@deffn {Scheme Procedure} char-whitespace? chr
@deffnx {C Function} scm_char_whitespace_p (chr)
Return @code{#t} iff @var{chr} is whitespace, else @code{#f}.
Whitespace means the same thing as the isspace C library function.
@end deffn

@rnindex char-upper-case?
@deffn {Scheme Procedure} char-upper-case? chr
@deffnx {C Function} scm_char_upper_case_p (chr)
Return @code{#t} iff @var{chr} is uppercase, else @code{#f}.
Uppercase means the same thing as the isupper C library function.
@end deffn

@rnindex char-lower-case?
@deffn {Scheme Procedure} char-lower-case? chr
@deffnx {C Function} scm_char_lower_case_p (chr)
Return @code{#t} iff @var{chr} is lowercase, else @code{#f}.
Lowercase means the same thing as the islower C library function.
@end deffn

@deffn {Scheme Procedure} char-is-both? chr
@deffnx {C Function} scm_char_is_both_p (chr)
Return @code{#t} iff @var{chr} is either uppercase or lowercase, else @code{#f}.
Uppercase and lowercase are as defined by the isupper and islower
C library functions.
@end deffn

@rnindex char->integer
@deffn {Scheme Procedure} char->integer chr
@deffnx {C Function} scm_char_to_integer (chr)
Return the number corresponding to ordinal position of @var{chr} in the
ASCII sequence.
@end deffn

@rnindex integer->char
@deffn {Scheme Procedure} integer->char n
@deffnx {C Function} scm_integer_to_char (n)
Return the character at position @var{n} in the ASCII sequence.
@end deffn

@rnindex char-upcase
@deffn {Scheme Procedure} char-upcase chr
@deffnx {C Function} scm_char_upcase (chr)
Return the uppercase character version of @var{chr}.
@end deffn

@rnindex char-downcase
@deffn {Scheme Procedure} char-downcase chr
@deffnx {C Function} scm_char_downcase (chr)
Return the lowercase character version of @var{chr}.
@end deffn


@node Strings
@section Strings
@tpindex Strings

Strings are fixed-length sequences of characters.  They can be created
by calling constructor procedures, but they can also literally get
entered at the REPL or in Scheme source files.

Guile provides a rich set of string processing procedures, because text
handling is very important when Guile is used as a scripting language.

Strings always carry the information about how many characters they are
composed of with them, so there is no special end-of-string character,
like in C.  That means that Scheme strings can contain any character,
even the NUL character @code{'\0'}.  But note: Since most operating
system calls dealing with strings (such as for file operations) expect
strings to be zero-terminated, they might do unexpected things when
called with string containing unusal characters.

@menu
* String Syntax::               Read syntax for strings.
* String Predicates::           Testing strings for certain properties.
* String Constructors::         Creating new string objects.
* List/String Conversion::      Converting from/to lists of characters.
* String Selection::            Select portions from strings.
* String Modification::         Modify parts or whole strings.
* String Comparison::           Lexicographic ordering predicates.
* String Searching::            Searching in strings.
* Alphabetic Case Mapping::     Convert the alphabetic case of strings.
* Appending Strings::           Appending strings to form a new string.
* String Miscellanea::          Miscellaneous string procedures.
@end menu

@node String Syntax
@subsection String Read Syntax

The read syntax for strings is an arbitrarily long sequence of
characters enclosed in double quotes (@code{"}). @footnote{Actually, the
current implementation restricts strings to a length of 2^24
characters.}  If you want to insert a double quote character into a
string literal, it must be prefixed with a backslash @code{\} character
(called an @dfn{escape character}).

The following are examples of string literals:

@lisp
"foo"
"bar plonk"
"Hello World"
"\"Hi\", he said."
@end lisp

@c FIXME::martin: What about escape sequences like \r, \n etc.?

@node String Predicates
@subsection String Predicates

The following procedures can be used to check whether a given string
fulfills some specified property.

@rnindex string?
@deffn {Scheme Procedure} string? obj
@deffnx {C Function} scm_string_p (obj)
Return @code{#t} iff @var{obj} is a string, else @code{#f}.
@end deffn

@deffn {Scheme Procedure} string-null? str
@deffnx {C Function} scm_string_null_p (str)
Return @code{#t} if @var{str}'s length is zero, and
@code{#f} otherwise.
@lisp
(string-null? "")  @result{} #t
y                    @result{} "foo"
(string-null? y)     @result{} #f
@end lisp
@end deffn

@node String Constructors
@subsection String Constructors

The string constructor procedures create new string objects, possibly
initializing them with some specified character data.

@c FIXME::martin: list->string belongs into `List/String Conversion'

@rnindex string
@rnindex list->string
@deffn {Scheme Procedure} string . chrs
@deffnx {Scheme Procedure} list->string chrs
@deffnx {C Function} scm_string (chrs)
Return a newly allocated string composed of the arguments,
@var{chrs}.
@end deffn

@rnindex make-string
@deffn {Scheme Procedure} make-string k [chr]
@deffnx {C Function} scm_make_string (k, chr)
Return a newly allocated string of
length @var{k}.  If @var{chr} is given, then all elements of
the string are initialized to @var{chr}, otherwise the contents
of the @var{string} are unspecified.
@end deffn

@node List/String Conversion
@subsection List/String conversion

When processing strings, it is often convenient to first convert them
into a list representation by using the procedure @code{string->list},
work with the resulting list, and then convert it back into a string.
These procedures are useful for similar tasks.

@rnindex string->list
@deffn {Scheme Procedure} string->list str
@deffnx {C Function} scm_string_to_list (str)
Return a newly allocated list of the characters that make up
the given string @var{str}. @code{string->list} and
@code{list->string} are inverses as far as @samp{equal?} is
concerned.
@end deffn

@deffn {Scheme Procedure} string-split str chr
@deffnx {C Function} scm_string_split (str, chr)
Split the string @var{str} into the a list of the substrings delimited
by appearances of the character @var{chr}.  Note that an empty substring
between separator characters will result in an empty string in the
result list.

@lisp
(string-split "root:x:0:0:root:/root:/bin/bash" #\:)
@result{}
("root" "x" "0" "0" "root" "/root" "/bin/bash")

(string-split "::" #\:)
@result{}
("" "" "")

(string-split "" #\:)
@result{}
("")
@end lisp
@end deffn


@node String Selection
@subsection String Selection

Portions of strings can be extracted by these procedures.
@code{string-ref} delivers individual characters whereas
@code{substring} can be used to extract substrings from longer strings.

@rnindex string-length
@deffn {Scheme Procedure} string-length string
@deffnx {C Function} scm_string_length (string)
Return the number of characters in @var{string}.
@end deffn

@rnindex string-ref
@deffn {Scheme Procedure} string-ref str k
@deffnx {C Function} scm_string_ref (str, k)
Return character @var{k} of @var{str} using zero-origin
indexing. @var{k} must be a valid index of @var{str}.
@end deffn

@rnindex string-copy
@deffn {Scheme Procedure} string-copy str
@deffnx {C Function} scm_string_copy (str)
Return a newly allocated copy of the given @var{string}.
@end deffn

@rnindex substring
@deffn {Scheme Procedure} substring str start [end]
@deffnx {C Function} scm_substring (str, start, end)
Return a newly allocated string formed from the characters
of @var{str} beginning with index @var{start} (inclusive) and
ending with index @var{end} (exclusive).
@var{str} must be a string, @var{start} and @var{end} must be
exact integers satisfying:

0 <= @var{start} <= @var{end} <= (string-length @var{str}).
@end deffn

@node String Modification
@subsection String Modification

These procedures are for modifying strings in-place.  This means that the
result of the operation is not a new string; instead, the original string's
memory representation is modified.

@rnindex string-set!
@deffn {Scheme Procedure} string-set! str k chr
@deffnx {C Function} scm_string_set_x (str, k, chr)
Store @var{chr} in element @var{k} of @var{str} and return
an unspecified value. @var{k} must be a valid index of
@var{str}.
@end deffn

@rnindex string-fill!
@deffn {Scheme Procedure} string-fill! str chr
@deffnx {C Function} scm_string_fill_x (str, chr)
Store @var{char} in every element of the given @var{string} and
return an unspecified value.
@end deffn

@deffn {Scheme Procedure} substring-fill! str start end fill
@deffnx {C Function} scm_substring_fill_x (str, start, end, fill)
Change every character in @var{str} between @var{start} and
@var{end} to @var{fill}.

@lisp
(define y "abcdefg")
(substring-fill! y 1 3 #\r)
y
@result{} "arrdefg"
@end lisp
@end deffn

@deffn {Scheme Procedure} substring-move! str1 start1 end1 str2 start2
@deffnx {C Function} scm_substring_move_x (str1, start1, end1, str2, start2)
Copy the substring of @var{str1} bounded by @var{start1} and @var{end1}
into @var{str2} beginning at position @var{start2}.
@var{str1} and @var{str2} can be the same string.
@end deffn


@node String Comparison
@subsection String Comparison

The procedures in this section are similar to the character ordering
predicates (@pxref{Characters}), but are defined on character sequences.
They all return @code{#t} on success and @code{#f} on failure.  The
predicates ending in @code{-ci} ignore the character case when comparing
strings.


@rnindex string=?
@deffn {Scheme Procedure} string=? s1 s2
Lexicographic equality predicate; return @code{#t} if the two
strings are the same length and contain the same characters in
the same positions, otherwise return @code{#f}.

The procedure @code{string-ci=?} treats upper and lower case
letters as though they were the same character, but
@code{string=?} treats upper and lower case as distinct
characters.
@end deffn

@rnindex string<?
@deffn {Scheme Procedure} string<? s1 s2
Lexicographic ordering predicate; return @code{#t} if @var{s1}
is lexicographically less than @var{s2}.
@end deffn

@rnindex string<=?
@deffn {Scheme Procedure} string<=? s1 s2
Lexicographic ordering predicate; return @code{#t} if @var{s1}
is lexicographically less than or equal to @var{s2}.
@end deffn

@rnindex string>?
@deffn {Scheme Procedure} string>? s1 s2
Lexicographic ordering predicate; return @code{#t} if @var{s1}
is lexicographically greater than @var{s2}.
@end deffn

@rnindex string>=?
@deffn {Scheme Procedure} string>=? s1 s2
Lexicographic ordering predicate; return @code{#t} if @var{s1}
is lexicographically greater than or equal to @var{s2}.
@end deffn

@rnindex string-ci=?
@deffn {Scheme Procedure} string-ci=? s1 s2
Case-insensitive string equality predicate; return @code{#t} if
the two strings are the same length and their component
characters match (ignoring case) at each position; otherwise
return @code{#f}.
@end deffn

@rnindex string-ci<
@deffn {Scheme Procedure} string-ci<? s1 s2
Case insensitive lexicographic ordering predicate; return
@code{#t} if @var{s1} is lexicographically less than @var{s2}
regardless of case.
@end deffn

@rnindex string<=?
@deffn {Scheme Procedure} string-ci<=? s1 s2
Case insensitive lexicographic ordering predicate; return
@code{#t} if @var{s1} is lexicographically less than or equal
to @var{s2} regardless of case.
@end deffn

@rnindex string-ci>?
@deffn {Scheme Procedure} string-ci>? s1 s2
Case insensitive lexicographic ordering predicate; return
@code{#t} if @var{s1} is lexicographically greater than
@var{s2} regardless of case.
@end deffn

@rnindex string-ci>=?
@deffn {Scheme Procedure} string-ci>=? s1 s2
Case insensitive lexicographic ordering predicate; return
@code{#t} if @var{s1} is lexicographically greater than or
equal to @var{s2} regardless of case.
@end deffn


@node String Searching
@subsection String Searching

When searching for the index of a character in a string, these
procedures can be used.

@deffn {Scheme Procedure} string-index str chr [frm [to]]
@deffnx {C Function} scm_string_index (str, chr, frm, to)
Return the index of the first occurrence of @var{chr} in
@var{str}.  The optional integer arguments @var{frm} and
@var{to} limit the search to a portion of the string.  This
procedure essentially implements the @code{index} or
@code{strchr} functions from the C library.

@lisp
(string-index "weiner" #\e)
@result{} 1

(string-index "weiner" #\e 2)
@result{} 4

(string-index "weiner" #\e 2 4)
@result{} #f
@end lisp
@end deffn

@deffn {Scheme Procedure} string-rindex str chr [frm [to]]
@deffnx {C Function} scm_string_rindex (str, chr, frm, to)
Like @code{string-index}, but search from the right of the
string rather than from the left.  This procedure essentially
implements the @code{rindex} or @code{strrchr} functions from
the C library.

@lisp
(string-rindex "weiner" #\e)
@result{} 4

(string-rindex "weiner" #\e 2 4)
@result{} #f

(string-rindex "weiner" #\e 2 5)
@result{} 4
@end lisp
@end deffn

@node Alphabetic Case Mapping
@subsection Alphabetic Case Mapping

These are procedures for mapping strings to their upper- or lower-case
equivalents, respectively, or for capitalizing strings.

@deffn {Scheme Procedure} string-upcase str
@deffnx {C Function} scm_string_upcase (str)
Return a freshly allocated string containing the characters of
@var{str} in upper case.
@end deffn

@deffn {Scheme Procedure} string-upcase! str
@deffnx {C Function} scm_string_upcase_x (str)
Destructively upcase every character in @var{str} and return
@var{str}.
@lisp
y                  @result{} "arrdefg"
(string-upcase! y) @result{} "ARRDEFG"
y                  @result{} "ARRDEFG"
@end lisp
@end deffn

@deffn {Scheme Procedure} string-downcase str
@deffnx {C Function} scm_string_downcase (str)
Return a freshly allocation string containing the characters in
@var{str} in lower case.
@end deffn

@deffn {Scheme Procedure} string-downcase! str
@deffnx {C Function} scm_string_downcase_x (str)
Destructively downcase every character in @var{str} and return
@var{str}.
@lisp
y                     @result{} "ARRDEFG"
(string-downcase! y)  @result{} "arrdefg"
y                     @result{} "arrdefg"
@end lisp
@end deffn

@deffn {Scheme Procedure} string-capitalize str
@deffnx {C Function} scm_string_capitalize (str)
Return a freshly allocated string with the characters in
@var{str}, where the first character of every word is
capitalized.
@end deffn

@deffn {Scheme Procedure} string-capitalize! str
@deffnx {C Function} scm_string_capitalize_x (str)
Upcase the first character of every word in @var{str}
destructively and return @var{str}.

@lisp
y                      @result{} "hello world"
(string-capitalize! y) @result{} "Hello World"
y                      @result{} "Hello World"
@end lisp
@end deffn


@node Appending Strings
@subsection Appending Strings

The procedure @code{string-append} appends several strings together to
form a longer result string.

@rnindex string-append
@deffn {Scheme Procedure} string-append . args
@deffnx {C Function} scm_string_append (args)
Return a newly allocated string whose characters form the
concatenation of the given strings, @var{args}.
@end deffn


@node String Miscellanea
@subsection String Miscellanea

This section contains all remaining string procedures.

@deffn {Scheme Procedure} string-ci->symbol str
@deffnx {C Function} scm_string_ci_to_symbol (str)
Return the symbol whose name is @var{str}.  @var{str} is
converted to lowercase before the conversion is done, if Guile
is currently reading symbols case-insensitively.
@end deffn


@node Regular Expressions
@section Regular Expressions
@tpindex Regular expressions

@cindex regular expressions
@cindex regex
@cindex emacs regexp

A @dfn{regular expression} (or @dfn{regexp}) is a pattern that
describes a whole class of strings.  A full description of regular
expressions and their syntax is beyond the scope of this manual;
an introduction can be found in the Emacs manual (@pxref{Regexps,
, Syntax of Regular Expressions, emacs, The GNU Emacs Manual}), or
in many general Unix reference books.

If your system does not include a POSIX regular expression library, and
you have not linked Guile with a third-party regexp library such as Rx,
these functions will not be available.  You can tell whether your Guile
installation includes regular expression support by checking whether the
@code{*features*} list includes the @code{regex} symbol.

@menu
* Regexp Functions::            Functions that create and match regexps.
* Match Structures::            Finding what was matched by a regexp.
* Backslash Escapes::           Removing the special meaning of regexp metacharacters.
* Rx Interface::                Tom Lord's Rx library does things differently.
@end menu

[FIXME: it may be useful to include an Examples section.  Parts of this
interface are bewildering on first glance.]

@node Regexp Functions
@subsection Regexp Functions

By default, Guile supports POSIX extended regular expressions.
That means that the characters @samp{(}, @samp{)}, @samp{+} and
@samp{?} are special, and must be escaped if you wish to match the
literal characters.

This regular expression interface was modeled after that
implemented by SCSH, the Scheme Shell.  It is intended to be
upwardly compatible with SCSH regular expressions.

@c begin (scm-doc-string "regex.scm" "string-match")
@deffn {Scheme Procedure} string-match pattern str [start]
Compile the string @var{pattern} into a regular expression and compare
it with @var{str}.  The optional numeric argument @var{start} specifies
the position of @var{str} at which to begin matching.

@code{string-match} returns a @dfn{match structure} which
describes what, if anything, was matched by the regular
expression.  @xref{Match Structures}.  If @var{str} does not match
@var{pattern} at all, @code{string-match} returns @code{#f}.
@end deffn

Each time @code{string-match} is called, it must compile its
@var{pattern} argument into a regular expression structure.  This
operation is expensive, which makes @code{string-match} inefficient if
the same regular expression is used several times (for example, in a
loop).  For better performance, you can compile a regular expression in
advance and then match strings against the compiled regexp.

@deffn {Scheme Procedure} make-regexp pat . flags
@deffnx {C Function} scm_make_regexp (pat, flags)
Compile the regular expression described by @var{pat}, and
return the compiled regexp structure.  If @var{pat} does not
describe a legal regular expression, @code{make-regexp} throws
a @code{regular-expression-syntax} error.

The @var{flags} arguments change the behavior of the compiled
regular expression.  The following flags may be supplied:

@table @code
@item regexp/icase
Consider uppercase and lowercase letters to be the same when
matching.
@item regexp/newline
If a newline appears in the target string, then permit the
@samp{^} and @samp{$} operators to match immediately after or
immediately before the newline, respectively.  Also, the
@samp{.} and @samp{[^...]} operators will never match a newline
character.  The intent of this flag is to treat the target
string as a buffer containing many lines of text, and the
regular expression as a pattern that may match a single one of
those lines.
@item regexp/basic
Compile a basic (``obsolete'') regexp instead of the extended
(``modern'') regexps that are the default.  Basic regexps do
not consider @samp{|}, @samp{+} or @samp{?} to be special
characters, and require the @samp{@{...@}} and @samp{(...)}
metacharacters to be backslash-escaped (@pxref{Backslash
Escapes}).  There are several other differences between basic
and extended regular expressions, but these are the most
significant.
@item regexp/extended
Compile an extended regular expression rather than a basic
regexp.  This is the default behavior; this flag will not
usually be needed.  If a call to @code{make-regexp} includes
both @code{regexp/basic} and @code{regexp/extended} flags, the
one which comes last will override the earlier one.
@end table
@end deffn

@deffn {Scheme Procedure} regexp-exec rx str [start [flags]]
@deffnx {C Function} scm_regexp_exec (rx, str, start, flags)
Match the compiled regular expression @var{rx} against
@code{str}.  If the optional integer @var{start} argument is
provided, begin matching from that position in the string.
Return a match structure describing the results of the match,
or @code{#f} if no match could be found.

The @var{flags} arguments change the matching behavior.
The following flags may be supplied:

@table @code
@item regexp/notbol
Operator @samp{^} always fails (unless @code{regexp/newline}
is used).  Use this when the beginning of the string should
not be considered the beginning of a line.
@item regexp/noteol
Operator @samp{$} always fails (unless @code{regexp/newline}
is used).  Use this when the end of the string should not be
considered the end of a line.
@end table
@end deffn

@deffn {Scheme Procedure} regexp? obj
@deffnx {C Function} scm_regexp_p (obj)
Return @code{#t} if @var{obj} is a compiled regular expression,
or @code{#f} otherwise.
@end deffn

Regular expressions are commonly used to find patterns in one string and
replace them with the contents of another string.

@c begin (scm-doc-string "regex.scm" "regexp-substitute")
@deffn {Scheme Procedure} regexp-substitute port match [item@dots{}]
Write to the output port @var{port} selected contents of the match
structure @var{match}.  Each @var{item} specifies what should be
written, and may be one of the following arguments:

@itemize @bullet
@item
A string.  String arguments are written out verbatim.

@item
An integer.  The submatch with that number is written.

@item
The symbol @samp{pre}.  The portion of the matched string preceding
the regexp match is written.

@item
The symbol @samp{post}.  The portion of the matched string following
the regexp match is written.
@end itemize

@var{port} may be @code{#f}, in which case nothing is written; instead,
@code{regexp-substitute} constructs a string from the specified
@var{item}s and returns that.
@end deffn

@c begin (scm-doc-string "regex.scm" "regexp-substitute")
@deffn {Scheme Procedure} regexp-substitute/global port regexp target [item@dots{}]
Similar to @code{regexp-substitute}, but can be used to perform global
substitutions on @var{str}.  Instead of taking a match structure as an
argument, @code{regexp-substitute/global} takes two string arguments: a
@var{regexp} string describing a regular expression, and a @var{target}
string which should be matched against this regular expression.

Each @var{item} behaves as in @var{regexp-substitute}, with the
following exceptions:

@itemize @bullet
@item
A function may be supplied.  When this function is called, it will be
passed one argument: a match structure for a given regular expression
match.  It should return a string to be written out to @var{port}.

@item
The @samp{post} symbol causes @code{regexp-substitute/global} to recurse
on the unmatched portion of @var{str}.  This @emph{must} be supplied in
order to perform global search-and-replace on @var{str}; if it is not
present among the @var{item}s, then @code{regexp-substitute/global} will
return after processing a single match.
@end itemize
@end deffn

@node Match Structures
@subsection Match Structures

@cindex match structures

A @dfn{match structure} is the object returned by @code{string-match} and
@code{regexp-exec}.  It describes which portion of a string, if any,
matched the given regular expression.  Match structures include: a
reference to the string that was checked for matches; the starting and
ending positions of the regexp match; and, if the regexp included any
parenthesized subexpressions, the starting and ending positions of each
submatch.

In each of the regexp match functions described below, the @code{match}
argument must be a match structure returned by a previous call to
@code{string-match} or @code{regexp-exec}.  Most of these functions
return some information about the original target string that was
matched against a regular expression; we will call that string
@var{target} for easy reference.

@c begin (scm-doc-string "regex.scm" "regexp-match?")
@deffn {Scheme Procedure} regexp-match? obj
Return @code{#t} if @var{obj} is a match structure returned by a
previous call to @code{regexp-exec}, or @code{#f} otherwise.
@end deffn

@c begin (scm-doc-string "regex.scm" "match:substring")
@deffn {Scheme Procedure} match:substring match [n]
Return the portion of @var{target} matched by subexpression number
@var{n}.  Submatch 0 (the default) represents the entire regexp match.
If the regular expression as a whole matched, but the subexpression
number @var{n} did not match, return @code{#f}.
@end deffn

@c begin (scm-doc-string "regex.scm" "match:start")
@deffn {Scheme Procedure} match:start match [n]
Return the starting position of submatch number @var{n}.
@end deffn

@c begin (scm-doc-string "regex.scm" "match:end")
@deffn {Scheme Procedure} match:end match [n]
Return the ending position of submatch number @var{n}.
@end deffn

@c begin (scm-doc-string "regex.scm" "match:prefix")
@deffn {Scheme Procedure} match:prefix match
Return the unmatched portion of @var{target} preceding the regexp match.
@end deffn

@c begin (scm-doc-string "regex.scm" "match:suffix")
@deffn {Scheme Procedure} match:suffix match
Return the unmatched portion of @var{target} following the regexp match.
@end deffn

@c begin (scm-doc-string "regex.scm" "match:count")
@deffn {Scheme Procedure} match:count match
Return the number of parenthesized subexpressions from @var{match}.
Note that the entire regular expression match itself counts as a
subexpression, and failed submatches are included in the count.
@end deffn

@c begin (scm-doc-string "regex.scm" "match:string")
@deffn {Scheme Procedure} match:string match
Return the original @var{target} string.
@end deffn

@node Backslash Escapes
@subsection Backslash Escapes

Sometimes you will want a regexp to match characters like @samp{*} or
@samp{$} exactly.  For example, to check whether a particular string
represents a menu entry from an Info node, it would be useful to match
it against a regexp like @samp{^* [^:]*::}.  However, this won't work;
because the asterisk is a metacharacter, it won't match the @samp{*} at
the beginning of the string.  In this case, we want to make the first
asterisk un-magic.

You can do this by preceding the metacharacter with a backslash
character @samp{\}.  (This is also called @dfn{quoting} the
metacharacter, and is known as a @dfn{backslash escape}.)  When Guile
sees a backslash in a regular expression, it considers the following
glyph to be an ordinary character, no matter what special meaning it
would ordinarily have.  Therefore, we can make the above example work by
changing the regexp to @samp{^\* [^:]*::}.  The @samp{\*} sequence tells
the regular expression engine to match only a single asterisk in the
target string.

Since the backslash is itself a metacharacter, you may force a regexp to
match a backslash in the target string by preceding the backslash with
itself.  For example, to find variable references in a @TeX{} program,
you might want to find occurrences of the string @samp{\let\} followed
by any number of alphabetic characters.  The regular expression
@samp{\\let\\[A-Za-z]*} would do this: the double backslashes in the
regexp each match a single backslash in the target string.

@c begin (scm-doc-string "regex.scm" "regexp-quote")
@deffn {Scheme Procedure} regexp-quote str
Quote each special character found in @var{str} with a backslash, and
return the resulting string.
@end deffn

@strong{Very important:} Using backslash escapes in Guile source code
(as in Emacs Lisp or C) can be tricky, because the backslash character
has special meaning for the Guile reader.  For example, if Guile
encounters the character sequence @samp{\n} in the middle of a string
while processing Scheme code, it replaces those characters with a
newline character.  Similarly, the character sequence @samp{\t} is
replaced by a horizontal tab.  Several of these @dfn{escape sequences}
are processed by the Guile reader before your code is executed.
Unrecognized escape sequences are ignored: if the characters @samp{\*}
appear in a string, they will be translated to the single character
@samp{*}.

This translation is obviously undesirable for regular expressions, since
we want to be able to include backslashes in a string in order to
escape regexp metacharacters.  Therefore, to make sure that a backslash
is preserved in a string in your Guile program, you must use @emph{two}
consecutive backslashes:

@lisp
(define Info-menu-entry-pattern (make-regexp "^\\* [^:]*"))
@end lisp

The string in this example is preprocessed by the Guile reader before
any code is executed.  The resulting argument to @code{make-regexp} is
the string @samp{^\* [^:]*}, which is what we really want.

This also means that in order to write a regular expression that matches
a single backslash character, the regular expression string in the
source code must include @emph{four} backslashes.  Each consecutive pair
of backslashes gets translated by the Guile reader to a single
backslash, and the resulting double-backslash is interpreted by the
regexp engine as matching a single backslash character.  Hence:

@lisp
(define tex-variable-pattern (make-regexp "\\\\let\\\\=[A-Za-z]*"))
@end lisp

The reason for the unwieldiness of this syntax is historical.  Both
regular expression pattern matchers and Unix string processing systems
have traditionally used backslashes with the special meanings
described above.  The POSIX regular expression specification and ANSI C
standard both require these semantics.  Attempting to abandon either
convention would cause other kinds of compatibility problems, possibly
more severe ones.  Therefore, without extending the Scheme reader to
support strings with different quoting conventions (an ungainly and
confusing extension when implemented in other languages), we must adhere
to this cumbersome escape syntax.

@node Rx Interface
@subsection Rx Interface

@c FIXME::martin:  Shouldn't this be removed or moved to the 
@c ``Guile Modules'' chapter?  The functions are not available in
@c plain Guile...

[FIXME: this is taken from Gary and Mark's quick summaries and should be
reviewed and expanded.  Rx is pretty stable, so could already be done!]

@cindex rx
@cindex finite automaton

Guile includes an interface to Tom Lord's Rx library (currently only to
POSIX regular expressions).  Use of the library requires a two step
process: compile a regular expression into an efficient structure, then
use the structure in any number of string comparisons.

For example, given the
regular expression @samp{abc.} (which matches any string containing
@samp{abc} followed by any single character):

@smalllisp
guile> @kbd{(define r (regcomp "abc."))}
guile> @kbd{r}
#<rgx abc.>
guile> @kbd{(regexec r "abc")}
#f
guile> @kbd{(regexec r "abcd")}
#((0 . 4))
guile>
@end smalllisp

The definitions of @code{regcomp} and @code{regexec} are as follows:

@c NJFIXME not in libguile!
@deffn {Scheme Procedure} regcomp pattern [flags]
Compile the regular expression pattern using POSIX rules.  Flags is
optional and should be specified using symbolic names:
@defvar REG_EXTENDED
use extended POSIX syntax
@end defvar
@defvar REG_ICASE
use case-insensitive matching
@end defvar
@defvar REG_NEWLINE
allow anchors to match after newline characters in the
string and prevents @code{.} or @code{[^...]} from matching newlines.
@end defvar

The @code{logior} procedure can be used to combine multiple flags.
The default is to use
POSIX basic syntax, which makes @code{+} and @code{?}  literals and @code{\+}
and @code{\?}
operators.  Backslashes in @var{pattern} must be escaped if specified in a
literal string e.g., @code{"\\(a\\)\\?"}.
@end deffn

@c NJFIXME not in libguile!
@deffn {Scheme Procedure} regexec regex string [match-pick] [flags]

Match @var{string} against the compiled POSIX regular expression
@var{regex}.
@var{match-pick} and @var{flags} are optional.  Possible flags (which can be
combined using the logior procedure) are:

@defvar REG_NOTBOL
The beginning of line operator won't match the beginning of
@var{string} (presumably because it's not the beginning of a line)
@end defvar

@defvar REG_NOTEOL
Similar to REG_NOTBOL, but prevents the end of line operator
from matching the end of @var{string}.
@end defvar

If no match is possible, regexec returns #f.  Otherwise @var{match-pick}
determines the return value:

@code{#t} or unspecified: a newly-allocated vector is returned,
containing pairs with the indices of the matched part of @var{string} and any
substrings.

@code{""}: a list is returned: the first element contains a nested list
with the matched part of @var{string} surrounded by the the unmatched parts.
Remaining elements are matched substrings (if any).  All returned
substrings share memory with @var{string}.

@code{#f}: regexec returns #t if a match is made, otherwise #f.

vector: the supplied vector is returned, with the first element replaced
by a pair containing the indices of the matched portion of @var{string} and
further elements replaced by pairs containing the indices of matched
substrings (if any).

list: a list will be returned, with each member of the list
specified by a code in the corresponding position of the supplied list:

a number: the numbered matching substring (0 for the entire match).

@code{#\<}: the beginning of @var{string} to the beginning of the part matched
by regex.

@code{#\>}: the end of the matched part of @var{string} to the end of
@var{string}.

@code{#\c}: the "final tag", which seems to be associated with the "cut
operator", which doesn't seem to be available through the posix
interface.

e.g., @code{(list #\< 0 1 #\>)}.  The returned substrings share memory with
@var{string}.
@end deffn

Here are some other procedures that might be used when using regular
expressions:

@c NJFIXME not in libguile!
@deffn {Scheme Procedure} compiled-regexp? obj
Test whether obj is a compiled regular expression.
@end deffn

@c NJFIXME not in libguile!
@deffn {Scheme Procedure} regexp->dfa regex [flags]
@end deffn

@c NJFIXME not in libguile!
@deffn {Scheme Procedure} dfa-fork dfa
@end deffn

@c NJFIXME not in libguile!
@deffn {Scheme Procedure} reset-dfa! dfa
@end deffn

@c NJFIXME not in libguile!
@deffn {Scheme Procedure} dfa-final-tag dfa
@end deffn

@c NJFIXME not in libguile!
@deffn {Scheme Procedure} dfa-continuable? dfa
@end deffn

@c NJFIXME not in libguile!
@deffn {Scheme Procedure} advance-dfa! dfa string
@end deffn


@node Symbols and Variables
@section Symbols and Variables

@c FIXME::martin: Review me!

Symbols are a data type with a special property.  On the one hand,
symbols are used for denoting variables in a Scheme program, on the
other they can be used as literal data as well.

The association between symbols and values is maintained in special data
structures, the symbol tables.

In addition, Guile offers variables as first-class objects.  They can
be used for interacting with the module system.

@menu
* Symbols::                     All about symbols as a data type.
* Symbol Tables::               Tables for mapping symbols to values.
* Variables::                   First-class variables.
@end menu

@node Symbols
@subsection Symbols
@tpindex Symbols

@c FIXME::martin: Review me!

Symbols are especially useful because two symbols which are spelled the
same way are equivalent in the sense of @code{eq?}.  That means that
they are actually the same Scheme object.  The advantage is that symbols
can be compared extremely efficiently, although they carry more
information for the human reader than, say, numbers.

It is very common in Scheme programs to use symbols as keys in
association lists (@pxref{Association Lists}) or hash tables
(@pxref{Hash Tables}), because this usage improves the readability a
lot, and does not cause any performance loss.

The read syntax for symbols is a sequence of letters, digits, and
@dfn{extended alphabetic characters} that begins with a character that
cannot begin a number is an identifier.  In addition, @code{+},
@code{-}, and @code{...} are identifiers.

Extended alphabetic characters may be used within identifiers as if
they were letters.  The following are extended alphabetic characters:

@example
! $ % & * + - . / : < = > ? @@ ^ _ ~ 
@end example

In addition to the read syntax defined above (which is taken from R5RS
(@pxref{Formal syntax,,,r5rs,The Revised^5 Report on Scheme})), Guile
provides a method for writing symbols with unusual characters, such as
space characters.  If you (for whatever reason) need to write a symbol
containing characters not mentioned above, you write symbols as follows:

@itemize @bullet
@item
Begin the symbol with the two character @code{#@{},

@item
write the characters of the symbol and

@item
finish the symbol with the characters @code{@}#}.
@end itemize

Here are a few examples of this form of read syntax; the first
containing a space character, the second containing a line break and the
last one looks like a number.

@lisp
#@{foo bar@}#
#@{what
ever@}#
#@{4242@}#
@end lisp

Usage of this form of read syntax is discouraged, because it is not
portable at all, and is not very readable.

@rnindex symbol?
@deffn {Scheme Procedure} symbol? obj
@deffnx {C Function} scm_symbol_p (obj)
Return @code{#t} if @var{obj} is a symbol, otherwise return
@code{#f}.
@end deffn

@rnindex string->symbol
@deffn {Scheme Procedure} string->symbol string
@deffnx {C Function} scm_string_to_symbol (string)
Return the symbol whose name is @var{string}. This procedure
can create symbols with names containing special characters or
letters in the non-standard case, but it is usually a bad idea
to create such symbols because in some implementations of
Scheme they cannot be read as themselves.  See
@code{symbol->string}.

The following examples assume that the implementation's
standard case is lower case:

@lisp
(eq? 'mISSISSIppi 'mississippi) @result{} #t
(string->symbol "mISSISSIppi") @result{} @r{the symbol with name "mISSISSIppi"}
(eq? 'bitBlt (string->symbol "bitBlt")) @result{} #f
(eq? 'JollyWog
  (string->symbol (symbol->string 'JollyWog))) @result{} #t
(string=? "K. Harper, M.D."
  (symbol->string
    (string->symbol "K. Harper, M.D."))) @result{}#t
@end lisp
@end deffn

@rnindex symbol->string
@deffn {Scheme Procedure} symbol->string s
@deffnx {C Function} scm_symbol_to_string (s)
Return the name of @var{symbol} as a string.  If the symbol was
part of an object returned as the value of a literal expression
(section @pxref{Literal expressions,,,r5rs, The Revised^5
Report on Scheme}) or by a call to the @code{read} procedure,
and its name contains alphabetic characters, then the string
returned will contain characters in the implementation's
preferred standard case---some implementations will prefer
upper case, others lower case.  If the symbol was returned by
@code{string->symbol}, the case of characters in the string
returned will be the same as the case in the string that was
passed to @code{string->symbol}.  It is an error to apply
mutation procedures like @code{string-set!} to strings returned
by this procedure.

The following examples assume that the implementation's
standard case is lower case:

@lisp
(symbol->string 'flying-fish)    @result{} "flying-fish"
(symbol->string 'Martin)         @result{}  "martin"
(symbol->string
   (string->symbol "Malvina")) @result{} "Malvina"
@end lisp
@end deffn

@node Symbol Tables
@subsection Symbol Tables

@c FIXME::martin: Review me!

@c FIXME::martin: Are all these procedures still relevant?

Guile symbol tables are hash tables.  Each hash table, also called an
@dfn{obarray} (for `object array'), is a vector of association lists.
Each entry in the alists is a pair (@var{SYMBOL} . @var{VALUE}).  To
@dfn{intern} a symbol in a symbol table means to return its
(@var{SYMBOL} . @var{VALUE}) pair, adding a new entry to the symbol
table (with an undefined value) if none is yet present.

@deffn {Scheme Procedure} gensym [prefix]
@deffnx {C Function} scm_gensym (prefix)
Create a new symbol with a name constructed from a prefix and
a counter value. The string @var{prefix} can be specified as
an optional argument. Default prefix is @code{g}.  The counter
is increased by 1 at each call. There is no provision for
resetting the counter.
@end deffn

@deffn {Scheme Procedure} gentemp [prefix [obarray]]
Create a new symbol with a name unique in an obarray.
The name is constructed from an optional string @var{prefix}
and a counter value.  The default prefix is @code{t}.  The
@var{obarray} is specified as a second optional argument.
Default is the system obarray where all normal symbols are
interned.  The counter is increased by 1 at each
call.  There is no provision for resetting the counter.
@end deffn

@deffn {Scheme Procedure} intern-symbol obarray string
Add a new symbol to @var{obarray} with name @var{string}, bound to an
unspecified initial value.  The symbol table is not modified if a symbol
with this name is already present.
@end deffn

@deffn {Scheme Procedure} string->obarray-symbol obarray string [soft?]
Intern a new symbol in @var{obarray}, a symbol table, with name
@var{string}.
@end deffn

@deffn {Scheme Procedure} symbol-binding obarray string
Look up in @var{obarray} the symbol whose name is @var{string}, and
return the value to which it is bound.  If @var{obarray} is @code{#f},
use the global symbol table.  If @var{string} is not interned in
@var{obarray}, an error is signalled.
@end deffn

@deffn {Scheme Procedure} symbol-bound? obarray string
Return @code{#t} if @var{obarray} contains a symbol with name
@var{string} bound to a defined value.  This differs from
@var{symbol-interned?} in that the mere mention of a symbol
usually causes it to be interned; @code{symbol-bound?}
determines whether a symbol has been given any meaningful
value.
@end deffn

@deffn {Scheme Procedure} symbol-fref symbol
@deffnx {C Function} scm_symbol_fref (symbol)
Return the contents of @var{symbol}'s @dfn{function slot}.
@end deffn

@deffn {Scheme Procedure} symbol-fset! symbol value
@deffnx {C Function} scm_symbol_fset_x (symbol, value)
Change the binding of @var{symbol}'s function slot.
@end deffn

@deffn {Scheme Procedure} symbol-hash symbol
@deffnx {C Function} scm_symbol_hash (symbol)
Return a hash value for @var{symbol}.
@end deffn

@deffn {Scheme Procedure} symbol-interned? obarray string
Return @code{#t} if @var{obarray} contains a symbol with name
@var{string}, and @code{#f} otherwise.
@end deffn

@deffn {Scheme Procedure} symbol-pref symbol
@deffnx {C Function} scm_symbol_pref (symbol)
Return the @dfn{property list} currently associated with @var{symbol}.
@end deffn

@deffn {Scheme Procedure} symbol-pset! symbol value
@deffnx {C Function} scm_symbol_pset_x (symbol, value)
Change the binding of @var{symbol}'s property slot.
@end deffn

@deffn {Scheme Procedure} symbol-set! obarray string value
Find the symbol in @var{obarray} whose name is @var{string}, and rebind
it to @var{value}.  An error is signalled if @var{string} is not present
in @var{obarray}.
@end deffn

@deffn {Scheme Procedure} unintern-symbol obarray string
Remove the symbol with name @var{string} from @var{obarray}.  This
function returns @code{#t} if the symbol was present and @code{#f}
otherwise.
@end deffn


@node Variables
@subsection Variables
@tpindex Variables

A variable is a box-like object that can hold any Scheme value.  It is
said to be @dfn{undefined} if its box holds a special Scheme value that
denotes undefined-ness (which is different from all other Scheme values,
including for example @code{#f}); otherwise the variable is
@dfn{defined}.

On its own, a variable object is anonymous.  A variable is said to be
@dfn{bound} when it is associated with a name in some way, usually a
symbol in a module obarray.  When this happens, the relationship is
mutual: the variable is bound to the name (in that module), and the name
(in that module) is bound to the variable.

(That's the theory, anyway.  In practice, defined-ness and bound-ness
sometimes get confused, because Lisp and Scheme implementations have
often conflated --- or deliberately drawn no distinction between --- a
name that is unbound and a name that is bound to a variable whose value
is undefined.  We will try to be clear about the difference and explain
any confusion where it is unavoidable.)

Variables do not have a read syntax.  Most commonly they are created and
bound implicitly by @code{define} expressions: a top-level @code{define}
expression of the form

@lisp
(define @var{name} @var{value})
@end lisp

@noindent
creates a variable with initial value @var{value} and binds it to the
name @var{name} in the current module.  But they can also be created
dynamically by calling one of the constructor procedures
@code{make-variable} and @code{make-undefined-variable}.

First-class variables are especially useful for interacting with the
current module system (@pxref{The Guile module system}).

@deffn {Scheme Procedure} make-undefined-variable
@deffnx {C Function} scm_make_undefined_variable ()
Return a variable that is initially unbound.
@end deffn

@deffn {Scheme Procedure} make-variable init
@deffnx {C Function} scm_make_variable (init)
Return a variable initialized to value @var{init}.
@end deffn

@deffn {Scheme Procedure} variable-bound? var
@deffnx {C Function} scm_variable_bound_p (var)
Return @code{#t} iff @var{var} is bound to a value.
Throws an error if @var{var} is not a variable object.
@end deffn

@deffn {Scheme Procedure} variable-ref var
@deffnx {C Function} scm_variable_ref (var)
Dereference @var{var} and return its value.
@var{var} must be a variable object; see @code{make-variable}
and @code{make-undefined-variable}.
@end deffn

@deffn {Scheme Procedure} variable-set! var val
@deffnx {C Function} scm_variable_set_x (var, val)
Set the value of the variable @var{var} to @var{val}.
@var{var} must be a variable object, @var{val} can be any
value. Return an unspecified value.
@end deffn

@deffn {Scheme Procedure} variable? obj
@deffnx {C Function} scm_variable_p (obj)
Return @code{#t} iff @var{obj} is a variable object, else
return @code{#f}.
@end deffn


@node Keywords
@section Keywords
@tpindex Keywords

Keywords are self-evaluating objects with a convenient read syntax that
makes them easy to type.

Guile's keyword support conforms to R5RS, and adds a (switchable) read
syntax extension to permit keywords to begin with @code{:} as well as
@code{#:}.

@menu
* Why Use Keywords?::           Motivation for keyword usage.
* Coding With Keywords::        How to use keywords.
* Keyword Read Syntax::         Read syntax for keywords.
* Keyword Procedures::          Procedures for dealing with keywords.
* Keyword Primitives::          The underlying primitive procedures.
@end menu

@node Why Use Keywords?
@subsection Why Use Keywords?

Keywords are useful in contexts where a program or procedure wants to be
able to accept a large number of optional arguments without making its
interface unmanageable.

To illustrate this, consider a hypothetical @code{make-window}
procedure, which creates a new window on the screen for drawing into
using some graphical toolkit.  There are many parameters that the caller
might like to specify, but which could also be sensibly defaulted, for
example:

@itemize @bullet
@item
colour depth -- Default: the colour depth for the screen

@item
background colour -- Default: white

@item
width -- Default: 600

@item
height -- Default: 400
@end itemize

If @code{make-window} did not use keywords, the caller would have to
pass in a value for each possible argument, remembering the correct
argument order and using a special value to indicate the default value
for that argument:

@lisp
(make-window 'default              ;; Colour depth
             'default              ;; Background colour
             800                   ;; Width
             100                   ;; Height
             @dots{})                  ;; More make-window arguments
@end lisp

With keywords, on the other hand, defaulted arguments are omitted, and
non-default arguments are clearly tagged by the appropriate keyword.  As
a result, the invocation becomes much clearer:

@lisp
(make-window #:width 800 #:height 100)
@end lisp

On the other hand, for a simpler procedure with few arguments, the use
of keywords would be a hindrance rather than a help.  The primitive
procedure @code{cons}, for example, would not be improved if it had to
be invoked as

@lisp
(cons #:car x #:cdr y)
@end lisp

So the decision whether to use keywords or not is purely pragmatic: use
them if they will clarify the procedure invocation at point of call.

@node Coding With Keywords
@subsection Coding With Keywords

If a procedure wants to support keywords, it should take a rest argument
and then use whatever means is convenient to extract keywords and their
corresponding arguments from the contents of that rest argument.

The following example illustrates the principle: the code for
@code{make-window} uses a helper procedure called
@code{get-keyword-value} to extract individual keyword arguments from
the rest argument.

@lisp
(define (get-keyword-value args keyword default)
  (let ((kv (memq keyword args)))
    (if (and kv (>= (length kv) 2))
        (cadr kv)
        default)))

(define (make-window . args)
  (let ((depth  (get-keyword-value args #:depth  screen-depth))
        (bg     (get-keyword-value args #:bg     "white"))
        (width  (get-keyword-value args #:width  800))
        (height (get-keyword-value args #:height 100))
        @dots{})
    @dots{}))
@end lisp

But you don't need to write @code{get-keyword-value}.  The @code{(ice-9
optargs)} module provides a set of powerful macros that you can use to
implement keyword-supporting procedures like this:

@lisp
(use-modules (ice-9 optargs))

(define (make-window . args)
  (let-keywords args #f ((depth  screen-depth)
                         (bg     "white")
                         (width  800)
                         (height 100))
    ...))
@end lisp

@noindent
Or, even more economically, like this:

@lisp
(use-modules (ice-9 optargs))

(define* (make-window #:key (depth  screen-depth)
                            (bg     "white")
                            (width  800)
                            (height 100))
  ...)
@end lisp

For further details on @code{let-keywords}, @code{define*} and other
facilities provided by the @code{(ice-9 optargs)} module, @ref{Optional
Arguments}.


@node Keyword Read Syntax
@subsection Keyword Read Syntax

Guile, by default, only recognizes the keyword syntax specified by R5RS.
A token of the form @code{#:NAME}, where @code{NAME} has the same syntax
as a Scheme symbol, is the external representation of the keyword named
@code{NAME}.  Keyword objects print using this syntax as well, so values
containing keyword objects can be read back into Guile.  When used in an
expression, keywords are self-quoting objects.

If the @code{keyword} read option is set to @code{'prefix}, Guile also
recognizes the alternative read syntax @code{:NAME}.  Otherwise, tokens
of the form @code{:NAME} are read as symbols, as required by R5RS.

To enable and disable the alternative non-R5RS keyword syntax, you use
the @code{read-options} procedure documented in @ref{General option
interface} and @ref{Reader options}.

@smalllisp
(read-set! keywords 'prefix)

#:type
@result{}
#:type

:type
@result{}
#:type

(read-set! keywords #f)

#:type
@result{}
#:type

:type
@result{}
ERROR: In expression :type:
ERROR: Unbound variable: :type
ABORT: (unbound-variable)
@end smalllisp

@node Keyword Procedures
@subsection Keyword Procedures

@c FIXME::martin: Review me!

The following procedures can be used for converting symbols to keywords
and back.

@deffn {Scheme Procedure} symbol->keyword sym
Return a keyword with the same characters as in @var{sym}.
@end deffn

@deffn {Scheme Procedure} keyword->symbol kw
Return a symbol with the same characters as in @var{kw}.
@end deffn


@node Keyword Primitives
@subsection Keyword Primitives

Internally, a keyword is implemented as something like a tagged symbol,
where the tag identifies the keyword as being self-evaluating, and the
symbol, known as the keyword's @dfn{dash symbol} has the same name as
the keyword name but prefixed by a single dash.  For example, the
keyword @code{#:name} has the corresponding dash symbol @code{-name}.

Most keyword objects are constructed automatically by the reader when it
reads a token beginning with @code{#:}.  However, if you need to
construct a keyword object programmatically, you can do so by calling
@code{make-keyword-from-dash-symbol} with the corresponding dash symbol
(as the reader does).  The dash symbol for a keyword object can be
retrieved using the @code{keyword-dash-symbol} procedure.

@deffn {Scheme Procedure} make-keyword-from-dash-symbol symbol
@deffnx {C Function} scm_make_keyword_from_dash_symbol (symbol)
Make a keyword object from a @var{symbol} that starts with a dash.
@end deffn

@deffn {Scheme Procedure} keyword? obj
@deffnx {C Function} scm_keyword_p (obj)
Return @code{#t} if the argument @var{obj} is a keyword, else
@code{#f}.
@end deffn

@deffn {Scheme Procedure} keyword-dash-symbol keyword
@deffnx {C Function} scm_keyword_dash_symbol (keyword)
Return the dash symbol for @var{keyword}.
This is the inverse of @code{make-keyword-from-dash-symbol}.
@end deffn

@node Pairs
@section Pairs
@tpindex Pairs

@c FIXME::martin: Review me!

Pairs are used to combine two Scheme objects into one compound object.
Hence the name: A pair stores a pair of objects.

The data type @dfn{pair} is extremely important in Scheme, just like in
any other Lisp dialect.  The reason is that pairs are not only used to
make two values available as one object, but that pairs are used for
constructing lists of values.  Because lists are so important in Scheme,
they are described in a section of their own (@pxref{Lists}).

Pairs can literally get entered in source code or at the REPL, in the
so-called @dfn{dotted list} syntax.  This syntax consists of an opening
parentheses, the first element of the pair, a dot, the second element
and a closing parentheses.  The following example shows how a pair
consisting of the two numbers 1 and 2, and a pair containing the symbols
@code{foo} and @code{bar} can be entered.  It is very important to write
the whitespace before and after the dot, because otherwise the Scheme
parser whould not be able to figure out where to split the tokens.

@lisp
(1 . 2)
(foo . bar)
@end lisp

But beware, if you want to try out these examples, you have to
@dfn{quote} the expressions.  More information about quotation is
available in the section (REFFIXME).  The correct way to try these
examples is as follows.

@lisp
'(1 . 2)
@result{}
(1 . 2)
'(foo . bar)
@result{}
(foo . bar)
@end lisp

A new pair is made by calling the procedure @code{cons} with two
arguments.  Then the argument values are stored into a newly allocated
pair, and the pair is returned.  The name @code{cons} stands for
"construct".  Use the procedure @code{pair?} to test whether a
given Scheme object is a pair or not.

@rnindex cons
@deffn {Scheme Procedure} cons x y
@deffnx {C Function} scm_cons (x, y)
Return a newly allocated pair whose car is @var{x} and whose
cdr is @var{y}.  The pair is guaranteed to be different (in the
sense of @code{eq?}) from every previously existing object.
@end deffn

@rnindex pair?
@deffn {Scheme Procedure} pair? x
@deffnx {C Function} scm_pair_p (x)
Return @code{#t} if @var{x} is a pair; otherwise return
@code{#f}.
@end deffn

The two parts of a pair are traditionally called @dfn{car} and
@dfn{cdr}.  They can be retrieved with procedures of the same name
(@code{car} and @code{cdr}), and can be modified with the procedures
@code{set-car!} and @code{set-cdr!}.  Since a very common operation in
Scheme programs is to access the car of a pair, or the car of the cdr of
a pair, etc., the procedures called @code{caar}, @code{cadr} and so on
are also predefined.

@rnindex car
@rnindex cdr
@deffn {Scheme Procedure} car pair
@deffnx {Scheme Procedure} cdr pair
Return the car or the cdr of @var{pair}, respectively.
@end deffn

@deffn {Scheme Procedure} caar pair
@deffnx {Scheme Procedure} cadr pair @dots{}
@deffnx {Scheme Procedure} cdddar pair
@deffnx {Scheme Procedure} cddddr pair
These procedures are compositions of @code{car} and @code{cdr}, where
for example @code{caddr} could be defined by

@lisp
(define caddr (lambda (x) (car (cdr (cdr x)))))
@end lisp
@end deffn

@rnindex set-car!
@deffn {Scheme Procedure} set-car! pair value
@deffnx {C Function} scm_set_car_x (pair, value)
Stores @var{value} in the car field of @var{pair}.  The value returned
by @code{set-car!} is unspecified.
@end deffn

@rnindex set-cdr!
@deffn {Scheme Procedure} set-cdr! pair value
@deffnx {C Function} scm_set_cdr_x (pair, value)
Stores @var{value} in the cdr field of @var{pair}.  The value returned
by @code{set-cdr!} is unspecified.
@end deffn


@node Lists
@section Lists
@tpindex Lists

@c FIXME::martin: Review me!

A very important data type in Scheme---as well as in all other Lisp
dialects---is the data type @dfn{list}.@footnote{Strictly speaking,
Scheme does not have a real datatype @dfn{list}.  Lists are made up of
@dfn{chained pairs}, and only exist by definition---a list is a chain
of pairs which looks like a list.}

This is the short definition of what a list is:

@itemize @bullet
@item
Either the empty list @code{()},

@item
or a pair which has a list in its cdr.
@end itemize

@c FIXME::martin: Describe the pair chaining in more detail.

@c FIXME::martin: What is a proper, what an improper list?
@c What is a circular list?

@c FIXME::martin: Maybe steal some graphics from the Elisp reference 
@c manual?

@menu
* List Syntax::                 Writing literal lists.
* List Predicates::             Testing lists.
* List Constructors::           Creating new lists.
* List Selection::              Selecting from lists, getting their length.
* Append/Reverse::              Appending and reversing lists.
* List Modification::           Modifying existing lists.
* List Searching::              Searching for list elements
* List Mapping::                Applying procedures to lists.
@end menu

@node List Syntax
@subsection List Read Syntax

@c FIXME::martin: Review me!

The syntax for lists is an opening parentheses, then all the elements of
the list (separated by whitespace) and finally a closing
parentheses.@footnote{Note that there is no separation character between
the list elements, like a comma or a semicolon.}.

@lisp
(1 2 3)            ; @r{a list of the numbers 1, 2 and 3}
("foo" bar 3.1415) ; @r{a string, a symbol and a real number}
()                 ; @r{the empty list}
@end lisp

The last example needs a bit more explanation.  A list with no elements,
called the @dfn{empty list}, is special in some ways.  It is used for
terminating lists by storing it into the cdr of the last pair that makes
up a list.  An example will clear that up:

@lisp
(car '(1))
@result{}
1
(cdr '(1))
@result{}
()
@end lisp

This example also shows that lists have to be quoted (REFFIXME) when
written, because they would otherwise be mistakingly taken as procedure
applications (@pxref{Simple Invocation}).


@node List Predicates
@subsection List Predicates

@c FIXME::martin: Review me!

Often it is useful to test whether a given Scheme object is a list or
not.  List-processing procedures could use this information to test
whether their input is valid, or they could do different things
depending on the datatype of their arguments.

@rnindex list?
@deffn {Scheme Procedure} list? x
@deffnx {C Function} scm_list_p (x)
Return @code{#t} iff @var{x} is a proper list, else @code{#f}.
@end deffn

The predicate @code{null?} is often used in list-processing code to
tell whether a given list has run out of elements.  That is, a loop
somehow deals with the elements of a list until the list satisfies
@code{null?}.  Then, the algorithm terminates.

@rnindex null?
@deffn {Scheme Procedure} null? x
@deffnx {C Function} scm_null_p (x)
Return @code{#t} iff @var{x} is the empty list, else @code{#f}.
@end deffn

@node List Constructors
@subsection List Constructors

This section describes the procedures for constructing new lists.
@code{list} simply returns a list where the elements are the arguments,
@code{cons*} is similar, but the last argument is stored in the cdr of
the last pair of the list.

@rnindex list
@deffn {Scheme Procedure} list . objs
@deffnx {C Function} scm_list (objs)
Return a list containing @var{objs}, the arguments to
@code{list}.
@end deffn

@deffn {Scheme Procedure} cons* arg1 arg2 @dots{}
@deffnx {C Function} scm_cons_star (arg1, rest)
Like @code{list}, but the last arg provides the tail of the
constructed list, returning @code{(cons @var{arg1} (cons
@var{arg2} (cons @dots{} @var{argn})))}.  Requires at least one
argument.  If given one argument, that argument is returned as
result.  This function is called @code{list*} in some other
Schemes and in Common LISP.
@end deffn

@deffn {Scheme Procedure} list-copy lst
@deffnx {C Function} scm_list_copy (lst)
Return a (newly-created) copy of @var{lst}.
@end deffn

@deffn {Scheme Procedure} make-list n [init]
Create a list containing of @var{n} elements, where each element is
initialized to @var{init}.  @var{init} defaults to the empty list
@code{()} if not given.
@end deffn

Note that @code{list-copy} only makes a copy of the pairs which make up
the spine of the lists.  The list elements are not copied, which means
that modifying the elements of the new list also modyfies the elements
of the old list.  On the other hand, applying procedures like
@code{set-cdr!} or @code{delv!} to the new list will not alter the old
list.  If you also need to copy the list elements (making a deep copy),
use the procedure @code{copy-tree} (@pxref{Copying}).

@node List Selection
@subsection List Selection

@c FIXME::martin: Review me!

These procedures are used to get some information about a list, or to
retrieve one or more elements of a list.

@rnindex length
@deffn {Scheme Procedure} length lst
@deffnx {C Function} scm_length (lst)
Return the number of elements in list @var{lst}.
@end deffn

@deffn {Scheme Procedure} last-pair lst
@deffnx {C Function} scm_last_pair (lst)
Return a pointer to the last pair in @var{lst}, signalling an error if
@var{lst} is circular.
@end deffn

@rnindex list-ref
@deffn {Scheme Procedure} list-ref list k
@deffnx {C Function} scm_list_ref (list, k)
Return the @var{k}th element from @var{list}.
@end deffn

@rnindex list-tail
@deffn {Scheme Procedure} list-tail lst k
@deffnx {Scheme Procedure} list-cdr-ref lst k
@deffnx {C Function} scm_list_tail (lst, k)
Return the "tail" of @var{lst} beginning with its @var{k}th element.
The first element of the list is considered to be element 0.

@code{list-tail} and @code{list-cdr-ref} are identical.  It may help to
think of @code{list-cdr-ref} as accessing the @var{k}th cdr of the list,
or returning the results of cdring @var{k} times down @var{lst}.
@end deffn

@deffn {Scheme Procedure} list-head lst k
@deffnx {C Function} scm_list_head (lst, k)
Copy the first @var{k} elements from @var{lst} into a new list, and
return it.
@end deffn

@node Append/Reverse
@subsection Append and Reverse

@c FIXME::martin: Review me!

@code{append} and @code{append!} are used to concatenate two or more
lists in order to form a new list.  @code{reverse} and @code{reverse!}
return lists with the same elements as their arguments, but in reverse
order.  The procedure variants with an @code{!} directly modify the
pairs which form the list, whereas the other procedures create new
pairs.  This is why you should be careful when using the side-effecting
variants.

@rnindex append
@deffn {Scheme Procedure} append . args
@deffnx {C Function} scm_append (args)
Return a list consisting of the elements the lists passed as
arguments.
@lisp
(append '(x) '(y))          @result{}  (x y)
(append '(a) '(b c d))      @result{}  (a b c d)
(append '(a (b)) '((c)))    @result{}  (a (b) (c))
@end lisp
The resulting list is always newly allocated, except that it
shares structure with the last list argument.  The last
argument may actually be any object; an improper list results
if the last argument is not a proper list.
@lisp
(append '(a b) '(c . d))    @result{}  (a b c . d)
(append '() 'a)             @result{}  a
@end lisp
@end deffn

@deffn {Scheme Procedure} append! . lists
@deffnx {C Function} scm_append_x (lists)
A destructive version of @code{append} (@pxref{Pairs and
Lists,,,r5rs, The Revised^5 Report on Scheme}).  The cdr field
of each list's final pair is changed to point to the head of
the next list, so no consing is performed.  Return a pointer to
the mutated list.
@end deffn

@rnindex reverse
@deffn {Scheme Procedure} reverse lst
@deffnx {C Function} scm_reverse (lst)
Return a new list that contains the elements of @var{lst} but
in reverse order.
@end deffn

@c NJFIXME explain new_tail
@deffn {Scheme Procedure} reverse! lst [new_tail]
@deffnx {C Function} scm_reverse_x (lst, new_tail)
A destructive version of @code{reverse} (@pxref{Pairs and Lists,,,r5rs,
The Revised^5 Report on Scheme}).  The cdr of each cell in @var{lst} is
modified to point to the previous list element.  Return a pointer to the
head of the reversed list.

Caveat: because the list is modified in place, the tail of the original
list now becomes its head, and the head of the original list now becomes
the tail.  Therefore, the @var{lst} symbol to which the head of the
original list was bound now points to the tail.  To ensure that the head
of the modified list is not lost, it is wise to save the return value of
@code{reverse!}
@end deffn

@node List Modification
@subsection List Modification

The following procedures modify an existing list, either by changing
elements of the list, or by changing the list structure itself.

@deffn {Scheme Procedure} list-set! list k val
@deffnx {C Function} scm_list_set_x (list, k, val)
Set the @var{k}th element of @var{list} to @var{val}.
@end deffn

@deffn {Scheme Procedure} list-cdr-set! list k val
@deffnx {C Function} scm_list_cdr_set_x (list, k, val)
Set the @var{k}th cdr of @var{list} to @var{val}.
@end deffn

@deffn {Scheme Procedure} delq item lst
@deffnx {C Function} scm_delq (item, lst)
Return a newly-created copy of @var{lst} with elements
@code{eq?} to @var{item} removed.  This procedure mirrors
@code{memq}: @code{delq} compares elements of @var{lst} against
@var{item} with @code{eq?}.
@end deffn

@deffn {Scheme Procedure} delv item lst
@deffnx {C Function} scm_delv (item, lst)
Return a newly-created copy of @var{lst} with elements
@code{eqv?}  to @var{item} removed.  This procedure mirrors
@code{memv}: @code{delv} compares elements of @var{lst} against
@var{item} with @code{eqv?}.
@end deffn

@deffn {Scheme Procedure} delete item lst
@deffnx {C Function} scm_delete (item, lst)
Return a newly-created copy of @var{lst} with elements
@code{equal?}  to @var{item} removed.  This procedure mirrors
@code{member}: @code{delete} compares elements of @var{lst}
against @var{item} with @code{equal?}.
@end deffn

@deffn {Scheme Procedure} delq! item lst
@deffnx {Scheme Procedure} delv! item lst
@deffnx {Scheme Procedure} delete! item lst
@deffnx {C Function} scm_delq_x (item, lst)
@deffnx {C Function} scm_delv_x (item, lst)
@deffnx {C Function} scm_delete_x (item, lst)
These procedures are destructive versions of @code{delq}, @code{delv}
and @code{delete}: they modify the pointers in the existing @var{lst}
rather than creating a new list.  Caveat evaluator: Like other
destructive list functions, these functions cannot modify the binding of
@var{lst}, and so cannot be used to delete the first element of
@var{lst} destructively.
@end deffn

@deffn {Scheme Procedure} delq1! item lst
@deffnx {C Function} scm_delq1_x (item, lst)
Like @code{delq!}, but only deletes the first occurrence of
@var{item} from @var{lst}.  Tests for equality using
@code{eq?}.  See also @code{delv1!} and @code{delete1!}.
@end deffn

@deffn {Scheme Procedure} delv1! item lst
@deffnx {C Function} scm_delv1_x (item, lst)
Like @code{delv!}, but only deletes the first occurrence of
@var{item} from @var{lst}.  Tests for equality using
@code{eqv?}.  See also @code{delq1!} and @code{delete1!}.
@end deffn

@deffn {Scheme Procedure} delete1! item lst
@deffnx {C Function} scm_delete1_x (item, lst)
Like @code{delete!}, but only deletes the first occurrence of
@var{item} from @var{lst}.  Tests for equality using
@code{equal?}.  See also @code{delq1!} and @code{delv1!}.
@end deffn

@node List Searching
@subsection List Searching

@c FIXME::martin: Review me!

The following procedures search lists for particular elements.  They use
different comparison predicates for comparing list elements with the
object to be searched.  When they fail, they return @code{#f}, otherwise
they return the sublist whose car is equal to the search object, where
equality depends on the equality predicate used.

@rnindex memq
@deffn {Scheme Procedure} memq x lst
@deffnx {C Function} scm_memq (x, lst)
Return the first sublist of @var{lst} whose car is @code{eq?}
to @var{x} where the sublists of @var{lst} are the non-empty
lists returned by @code{(list-tail @var{lst} @var{k})} for
@var{k} less than the length of @var{lst}.  If @var{x} does not
occur in @var{lst}, then @code{#f} (not the empty list) is
returned.
@end deffn

@rnindex memv
@deffn {Scheme Procedure} memv x lst
@deffnx {C Function} scm_memv (x, lst)
Return the first sublist of @var{lst} whose car is @code{eqv?}
to @var{x} where the sublists of @var{lst} are the non-empty
lists returned by @code{(list-tail @var{lst} @var{k})} for
@var{k} less than the length of @var{lst}.  If @var{x} does not
occur in @var{lst}, then @code{#f} (not the empty list) is
returned.
@end deffn

@rnindex member
@deffn {Scheme Procedure} member x lst
@deffnx {C Function} scm_member (x, lst)
Return the first sublist of @var{lst} whose car is
@code{equal?} to @var{x} where the sublists of @var{lst} are
the non-empty lists returned by @code{(list-tail @var{lst}
@var{k})} for @var{k} less than the length of @var{lst}.  If
@var{x} does not occur in @var{lst}, then @code{#f} (not the
empty list) is returned.
@end deffn

[FIXME: Is there any reason to have the `sloppy' functions available at
high level at all?  Maybe these docs should be relegated to a "Guile
Internals" node or something. -twp]

@deffn {Scheme Procedure} sloppy-memq x lst
This procedure behaves like @code{memq}, but does no type or error checking.
Its use is recommended only in writing Guile internals,
not for high-level Scheme programs.
@end deffn

@deffn {Scheme Procedure} sloppy-memv x lst
This procedure behaves like @code{memv}, but does no type or error checking.
Its use is recommended only in writing Guile internals,
not for high-level Scheme programs.
@end deffn

@deffn {Scheme Procedure} sloppy-member x lst
This procedure behaves like @code{member}, but does no type or error checking.
Its use is recommended only in writing Guile internals,
not for high-level Scheme programs.
@end deffn

@node List Mapping
@subsection List Mapping

@c FIXME::martin: Review me!

List processing is very convenient in Scheme because the process of
iterating over the elements of a list can be highly abstracted.  The
procedures in this section are the most basic iterating procedures for
lists.  They take a procedure and one or more lists as arguments, and
apply the procedure to each element of the list.  They differ in their
return value.

@rnindex map
@c begin (texi-doc-string "guile" "map")
@deffn {Scheme Procedure} map proc arg1 arg2 @dots{}
@deffnx {Scheme Procedure} map-in-order proc arg1 arg2 @dots{}
@deffnx {C Function} scm_map (proc, arg1, args)
Apply @var{proc} to each element of the list @var{arg1} (if only two
arguments are given), or to the corresponding elements of the argument
lists (if more than two arguments are given).  The result(s) of the
procedure applications are saved and returned in a list.  For
@code{map}, the order of procedure applications is not specified,
@code{map-in-order} applies the procedure from left to right to the list
elements.
@end deffn

@rnindex for-each
@c begin (texi-doc-string "guile" "for-each")
@deffn {Scheme Procedure} for-each proc arg1 arg2 @dots{}
Like @code{map}, but the procedure is always applied from left to right,
and the result(s) of the procedure applications are thrown away.  The
return value is not specified.
@end deffn


@node Vectors
@section Vectors
@tpindex Vectors

@c FIXME::martin: Review me!

@c FIXME::martin: Should the subsections of this section be nodes
@c of their own, or are the resulting nodes too short, then?

Vectors are sequences of Scheme objects.  Unlike lists, the length of a
vector, once the vector is created, cannot be changed.  The advantage of
vectors over lists is that the time required to access one element of a vector
given its @dfn{position} (synonymous with @dfn{index}), a zero-origin number,
is constant, whereas lists have an access time linear to the position of the
accessed element in the list.

Vectors can contain any kind of Scheme object; it is even possible to have
different types of objects in the same vector.  For vectors containing
vectors, you may wish to use arrays, instead.  Note, too, that some array
procedures operate happily on vectors (@pxref{Arrays}).

@subsection Vector Read Syntax

Vectors can literally be entered in source code, just like strings,
characters or some of the other data types.  The read syntax for vectors
is as follows: A sharp sign (@code{#}), followed by an opening
parentheses, all elements of the vector in their respective read syntax,
and finally a closing parentheses.  The following are examples of the
read syntax for vectors; where the first vector only contains numbers
and the second three different object types: a string, a symbol and a
number in hexadecimal notation.

@lisp
#(1 2 3)
#("Hello" foo #xdeadbeef)
@end lisp

@subsection Vector Predicates

@rnindex vector?
@deffn {Scheme Procedure} vector? obj
@deffnx {C Function} scm_vector_p (obj)
Return @code{#t} if @var{obj} is a vector, otherwise return
@code{#f}.
@end deffn

@subsection Vector Constructors

@rnindex make-vector
@deffn {Scheme Procedure} make-vector k [fill]
@deffnx {C Function} scm_make_vector (k, fill)
Return a newly allocated vector of @var{k} elements.  If a
second argument is given, then each position is initialized to
@var{fill}.  Otherwise the initial contents of each position is
unspecified.
@end deffn

@rnindex vector
@rnindex list->vector
@deffn {Scheme Procedure} vector . l
@deffnx {Scheme Procedure} list->vector l
@deffnx {C Function} scm_vector (l)
Return a newly allocated vector composed of the
given arguments.  Analogous to @code{list}.

@lisp
(vector 'a 'b 'c) @result{} #(a b c)
@end lisp
@end deffn

@rnindex vector->list
@deffn {Scheme Procedure} vector->list v
@deffnx {C Function} scm_vector_to_list (v)
Return a newly allocated list composed of the elements of @var{v}.

@lisp
(vector->list '#(dah dah didah)) @result{}  (dah dah didah)
(list->vector '(dididit dah)) @result{}  #(dididit dah)
@end lisp
@end deffn

@subsection Vector Modification

A vector created by any of the vector constructor procedures
(@pxref{Vectors}) documented above can be modified using the
following procedures.

@emph{NOTE:} According to R5RS, using any of these procedures on
literally entered vectors is an error, because these vectors are
considered to be constant, although Guile currently does not detect this
error.

@rnindex vector-set!
@deffn {Scheme Procedure} vector-set! vector k obj
Store @var{obj} in position @var{k} of @var{vector}.
@var{k} must be a valid index of @var{vector}.
The value returned by @samp{vector-set!} is unspecified.
@lisp
(let ((vec (vector 0 '(2 2 2 2) "Anna")))
  (vector-set! vec 1 '("Sue" "Sue"))
  vec) @result{}  #(0 ("Sue" "Sue") "Anna")
@end lisp
@end deffn

@rnindex vector-fill!
@deffn {Scheme Procedure} vector-fill! v fill
@deffnx {C Function} scm_vector_fill_x (v, fill)
Store @var{fill} in every position of @var{vector}.  The value
returned by @code{vector-fill!} is unspecified.
@end deffn

@deffn {Scheme Procedure} vector-move-left! vec1 start1 end1 vec2 start2
@deffnx {C Function} scm_vector_move_left_x (vec1, start1, end1, vec2, start2)
Copy elements from @var{vec1}, positions @var{start1} to @var{end1},
to @var{vec2} starting at position @var{start2}.  @var{start1} and
@var{start2} are inclusive indices; @var{end1} is exclusive.

@code{vector-move-left!} copies elements in leftmost order.
Therefore, in the case where @var{vec1} and @var{vec2} refer to the
same vector, @code{vector-move-left!} is usually appropriate when
@var{start1} is greater than @var{start2}.
@end deffn

@deffn {Scheme Procedure} vector-move-right! vec1 start1 end1 vec2 start2
@deffnx {C Function} scm_vector_move_right_x (vec1, start1, end1, vec2, start2)
Copy elements from @var{vec1}, positions @var{start1} to @var{end1},
to @var{vec2} starting at position @var{start2}.  @var{start1} and
@var{start2} are inclusive indices; @var{end1} is exclusive.

@code{vector-move-right!} copies elements in rightmost order.
Therefore, in the case where @var{vec1} and @var{vec2} refer to the
same vector, @code{vector-move-right!} is usually appropriate when
@var{start1} is less than @var{start2}.
@end deffn

@subsection Vector Selection

These procedures return information about a given vector, such as the
size or what elements are contained in the vector.

@rnindex vector-length
@deffn {Scheme Procedure} vector-length vector
Return the number of elements in @var{vector} as an exact integer.
@end deffn

@rnindex vector-ref
@deffn {Scheme Procedure} vector-ref vector k
Return the contents of position @var{k} of @var{vector}.
@var{k} must be a valid index of @var{vector}.
@lisp
(vector-ref '#(1 1 2 3 5 8 13 21) 5) @result{} 8
(vector-ref '#(1 1 2 3 5 8 13 21)
    (let ((i (round (* 2 (acos -1)))))
      (if (inexact? i)
        (inexact->exact i)
           i))) @result{} 13
@end lisp
@end deffn


@node Records
@section Records

A @dfn{record type} is a first class object representing a user-defined
data type.  A @dfn{record} is an instance of a record type.

@deffn {Scheme Procedure} record? obj
Return @code{#t} if @var{obj} is a record of any type and @code{#f}
otherwise.

Note that @code{record?} may be true of any Scheme value; there is no
promise that records are disjoint with other Scheme types.
@end deffn

@deffn {Scheme Procedure} make-record-type type-name field-names
Return a @dfn{record-type descriptor}, a value representing a new data
type disjoint from all others.  The @var{type-name} argument must be a
string, but is only used for debugging purposes (such as the printed
representation of a record of the new type).  The @var{field-names}
argument is a list of symbols naming the @dfn{fields} of a record of the
new type.  It is an error if the list contains any duplicates.  It is
unspecified how record-type descriptors are represented.
@end deffn

@deffn {Scheme Procedure} record-constructor rtd [field-names]
Return a procedure for constructing new members of the type represented
by @var{rtd}.  The returned procedure accepts exactly as many arguments
as there are symbols in the given list, @var{field-names}; these are
used, in order, as the initial values of those fields in a new record,
which is returned by the constructor procedure.  The values of any
fields not named in that list are unspecified.  The @var{field-names}
argument defaults to the list of field names in the call to
@code{make-record-type} that created the type represented by @var{rtd};
if the @var{field-names} argument is provided, it is an error if it
contains any duplicates or any symbols not in the default list.
@end deffn

@deffn {Scheme Procedure} record-predicate rtd
Return a procedure for testing membership in the type represented by
@var{rtd}.  The returned procedure accepts exactly one argument and
returns a true value if the argument is a member of the indicated record
type; it returns a false value otherwise.
@end deffn

@deffn {Scheme Procedure} record-accessor rtd field-name
Return a procedure for reading the value of a particular field of a
member of the type represented by @var{rtd}.  The returned procedure
accepts exactly one argument which must be a record of the appropriate
type; it returns the current value of the field named by the symbol
@var{field-name} in that record.  The symbol @var{field-name} must be a
member of the list of field-names in the call to @code{make-record-type}
that created the type represented by @var{rtd}.
@end deffn

@deffn {Scheme Procedure} record-modifier rtd field-name
Return a procedure for writing the value of a particular field of a
member of the type represented by @var{rtd}.  The returned procedure
accepts exactly two arguments: first, a record of the appropriate type,
and second, an arbitrary Scheme value; it modifies the field named by
the symbol @var{field-name} in that record to contain the given value.
The returned value of the modifier procedure is unspecified.  The symbol
@var{field-name} must be a member of the list of field-names in the call
to @code{make-record-type} that created the type represented by
@var{rtd}.
@end deffn

@deffn {Scheme Procedure} record-type-descriptor record
Return a record-type descriptor representing the type of the given
record.  That is, for example, if the returned descriptor were passed to
@code{record-predicate}, the resulting predicate would return a true
value when passed the given record.  Note that it is not necessarily the
case that the returned descriptor is the one that was passed to
@code{record-constructor} in the call that created the constructor
procedure that created the given record.
@end deffn

@deffn {Scheme Procedure} record-type-name rtd
Return the type-name associated with the type represented by rtd.  The
returned value is @code{eqv?} to the @var{type-name} argument given in
the call to @code{make-record-type} that created the type represented by
@var{rtd}.
@end deffn

@deffn {Scheme Procedure} record-type-fields rtd
Return a list of the symbols naming the fields in members of the type
represented by @var{rtd}.  The returned value is @code{equal?} to the
field-names argument given in the call to @code{make-record-type} that
created the type represented by @var{rtd}.
@end deffn


@node Structures
@section Structures
@tpindex Structures

[FIXME: this is pasted in from Tom Lord's original guile.texi and should
be reviewed]

A @dfn{structure type} is a first class user-defined data type.  A
@dfn{structure} is an instance of a structure type.  A structure type is
itself a structure.

Structures are less abstract and more general than traditional records.
In fact, in Guile Scheme, records are implemented using structures.

@menu
* Structure Concepts::          The structure of Structures
* Structure Layout::            Defining the layout of structure types
* Structure Basics::            make-, -ref and -set! procedures for structs
* Vtables::                     Accessing type-specific data
@end menu

@node  Structure Concepts
@subsection Structure Concepts

A structure object consists of a handle, structure data, and a vtable.
The handle is a Scheme value which points to both the vtable and the
structure's data.  Structure data is a dynamically allocated region of
memory, private to the structure, divided up into typed fields.  A
vtable is another structure used to hold type-specific data.  Multiple
structures can share a common vtable.

Three concepts are key to understanding structures.

@itemize @bullet{}
@item @dfn{layout specifications}

Layout specifications determine how memory allocated to structures is
divided up into fields.  Programmers must write a layout specification
whenever a new type of structure is defined.

@item @dfn{structural accessors}

Structure access is by field number.   There is only one set of
accessors common to all structure objects.

@item @dfn{vtables}

Vtables, themselves structures, are first class representations of
disjoint sub-types of structures in general.   In most cases, when a
new structure is created, programmers must specifiy a vtable for the
new structure.   Each vtable has a field describing the layout of its
instances.   Vtables can have additional, user-defined fields as well.
@end itemize



@node  Structure Layout
@subsection Structure Layout

When a structure is created, a region of memory is allocated to hold its
state.  The @dfn{layout} of the structure's type determines how that
memory is divided into fields.

Each field has a specified type.  There are only three types allowed, each
corresponding to a one letter code.  The allowed types are:

@itemize @bullet{}
@item 'u' -- unprotected

The field holds binary data that is not GC protected.

@item 'p' -- protected

The field holds a Scheme value and is GC protected.

@item 's' -- self

The field holds a Scheme value and is GC protected.  When a structure is
created with this type of field, the field is initialized to refer to
the structure's own handle.  This kind of field is mainly useful when
mixing Scheme and C code in which the C code may need to compute a
structure's handle given only the address of its malloced data.
@end itemize


Each field also has an associated access protection.   There are only
three kinds of protection, each corresponding to a one letter code.
The allowed protections are:

@itemize @bullet{}
@item 'w' -- writable

The field can be read and written.

@item 'r' -- readable

The field can be read, but not written.

@item 'o' -- opaque

The field can be neither read nor written.   This kind
of protection is for fields useful only to built-in routines.
@end itemize

A layout specification is described by stringing together pairs
of letters: one to specify a field type and one to specify a field
protection.    For example, a traditional cons pair type object could
be described as:

@example
; cons pairs have two writable fields of Scheme data
"pwpw"
@end example

A pair object in which the first field is held constant could be:

@example
"prpw"
@end example

Binary fields, (fields of type "u"), hold one @dfn{word} each.  The
size of a word is a machine dependent value defined to be equal to the
value of the C expression: @code{sizeof (long)}.

The last field of a structure layout may specify a tail array.
A tail array is indicated by capitalizing the field's protection
code ('W', 'R' or 'O').   A tail-array field is replaced by
a read-only binary data field containing an array size.   The array
size is determined at the time the structure is created.  It is followed
by a corresponding number of fields of the type specified for the
tail array.   For example, a conventional Scheme vector can be
described as:

@example
; A vector is an arbitrary number of writable fields holding Scheme
; values:
"pW"
@end example

In the above example, field 0 contains the size of the vector and
fields beginning at 1 contain the vector elements.

A kind of tagged vector (a constant tag followed by conventioal
vector elements) might be:

@example
"prpW"
@end example


Structure layouts are represented by specially interned symbols whose
name is a string of type and protection codes.  To create a new
structure layout, use this procedure:

@deffn {Scheme Procedure} make-struct-layout fields
@deffnx {C Function} scm_make_struct_layout (fields)
Return a new structure layout object.

@var{fields} must be a string made up of pairs of characters
strung together.  The first character of each pair describes a field
type, the second a field protection.  Allowed types are 'p' for
GC-protected Scheme data, 'u' for unprotected binary data, and 's' for
a field that points to the structure itself.    Allowed protections
are 'w' for mutable fields, 'r' for read-only fields, and 'o' for opaque
fields.  The last field protection specification may be capitalized to
indicate that the field is a tail-array.
@end deffn



@node Structure Basics
@subsection Structure Basics

This section describes the basic procedures for creating and accessing
structures.

@deffn {Scheme Procedure} make-struct vtable tail_array_size . init
@deffnx {C Function} scm_make_struct (vtable, tail_array_size, init)
Create a new structure.

@var{type} must be a vtable structure (@pxref{Vtables}).

@var{tail-elts} must be a non-negative integer.  If the layout
specification indicated by @var{type} includes a tail-array,
this is the number of elements allocated to that array.

The @var{init1}, @dots{} are optional arguments describing how
successive fields of the structure should be initialized.  Only fields
with protection 'r' or 'w' can be initialized, except for fields of
type 's', which are automatically initialized to point to the new
structure itself; fields with protection 'o' can not be initialized by
Scheme programs.

If fewer optional arguments than initializable fields are supplied,
fields of type 'p' get default value #f while fields of type 'u' are
initialized to 0.

Structs are currently the basic representation for record-like data
structures in Guile.  The plan is to eventually replace them with a
new representation which will at the same time be easier to use and
more powerful.

For more information, see the documentation for @code{make-vtable-vtable}.
@end deffn

@deffn {Scheme Procedure} struct? x
@deffnx {C Function} scm_struct_p (x)
Return @code{#t} iff @var{x} is a structure object, else
@code{#f}.
@end deffn


@deffn {Scheme Procedure} struct-ref handle pos
@deffnx {Scheme Procedure} struct-set! struct n value
@deffnx {C Function} scm_struct_ref (handle, pos)
@deffnx {C Function} scm_struct_set_x (struct, n, value)
Access (or modify) the @var{n}th field of @var{struct}.

If the field is of type 'p', then it can be set to an arbitrary value.

If the field is of type 'u', then it can only be set to a non-negative
integer value small enough to fit in one machine word.
@end deffn



@node  Vtables
@subsection Vtables

Vtables are structures that are used to represent structure types.  Each
vtable contains a layout specification in field
@code{vtable-index-layout} -- instances of the type are laid out
according to that specification.  Vtables contain additional fields
which are used only internally to libguile.  The variable
@code{vtable-offset-user} is bound to a field number.  Vtable fields
at that position or greater are user definable.

@deffn {Scheme Procedure} struct-vtable handle
@deffnx {C Function} scm_struct_vtable (handle)
Return the vtable structure that describes the type of @var{struct}.
@end deffn

@deffn {Scheme Procedure} struct-vtable? x
@deffnx {C Function} scm_struct_vtable_p (x)
Return @code{#t} iff @var{x} is a vtable structure.
@end deffn

If you have a vtable structure, @code{V}, you can create an instance of
the type it describes by using @code{(make-struct V ...)}.  But where
does @code{V} itself come from?  One possibility is that @code{V} is an
instance of a user-defined vtable type, @code{V'}, so that @code{V} is
created by using @code{(make-struct V' ...)}.  Another possibility is
that @code{V} is an instance of the type it itself describes.  Vtable
structures of the second sort are created by this procedure:

@deffn {Scheme Procedure} make-vtable-vtable user_fields tail_array_size . init
@deffnx {C Function} scm_make_vtable_vtable (user_fields, tail_array_size, init)
Return a new, self-describing vtable structure.

@var{user-fields} is a string describing user defined fields of the
vtable beginning at index @code{vtable-offset-user}
(see @code{make-struct-layout}).

@var{tail-size} specifies the size of the tail-array (if any) of
this vtable.

@var{init1}, @dots{} are the optional initializers for the fields of
the vtable.

Vtables have one initializable system field---the struct printer.
This field comes before the user fields in the initializers passed
to @code{make-vtable-vtable} and @code{make-struct}, and thus works as
a third optional argument to @code{make-vtable-vtable} and a fourth to
@code{make-struct} when creating vtables:

If the value is a procedure, it will be called instead of the standard
printer whenever a struct described by this vtable is printed.
The procedure will be called with arguments STRUCT and PORT.

The structure of a struct is described by a vtable, so the vtable is
in essence the type of the struct.  The vtable is itself a struct with
a vtable.  This could go on forever if it weren't for the
vtable-vtables which are self-describing vtables, and thus terminate
the chain.

There are several potential ways of using structs, but the standard
one is to use three kinds of structs, together building up a type
sub-system: one vtable-vtable working as the root and one or several
"types", each with a set of "instances".  (The vtable-vtable should be
compared to the class <class> which is the class of itself.)

@lisp
(define ball-root (make-vtable-vtable "pr" 0))

(define (make-ball-type ball-color)
  (make-struct ball-root 0
	       (make-struct-layout "pw")
               (lambda (ball port)
                 (format port "#<a ~A ball owned by ~A>"
                         (color ball)
                         (owner ball)))
               ball-color))
(define (color ball) (struct-ref (struct-vtable ball) vtable-offset-user))
(define (owner ball) (struct-ref ball 0))

(define red (make-ball-type 'red))
(define green (make-ball-type 'green))

(define (make-ball type owner) (make-struct type 0 owner))

(define ball (make-ball green 'Nisse))
ball @result{} #<a green ball owned by Nisse>
@end lisp
@end deffn

@deffn {Scheme Procedure} struct-vtable-name vtable
@deffnx {C Function} scm_struct_vtable_name (vtable)
Return the name of the vtable @var{vtable}.
@end deffn

@deffn {Scheme Procedure} set-struct-vtable-name! vtable name
@deffnx {C Function} scm_set_struct_vtable_name_x (vtable, name)
Set the name of the vtable @var{vtable} to @var{name}.
@end deffn

@deffn {Scheme Procedure} struct-vtable-tag handle
@deffnx {C Function} scm_struct_vtable_tag (handle)
Return the vtable tag of the structure @var{handle}.
@end deffn


@node Arrays
@section Arrays
@tpindex Arrays

@menu
* Conventional Arrays::         Arrays with arbitrary data.
* Array Mapping::               Applying a procedure to the contents of an array.
* Uniform Arrays::              Arrays with data of a single type.
* Bit Vectors::                 Vectors of bits.
@end menu

@node Conventional Arrays
@subsection Conventional Arrays

@dfn{Conventional arrays} are a collection of cells organized into an
arbitrary number of dimensions.  Each cell can hold any kind of Scheme
value and can be accessed in constant time by supplying an index for
each dimension.  This contrasts with uniform arrays, which use memory
more efficiently but can hold data of only a single type, and lists
where inserting and deleting cells is more efficient, but more time
is usually required to access a particular cell.

A conventional array is displayed as @code{#} followed by the @dfn{rank}
(number of dimensions) followed by the cells, organized into dimensions
using parentheses.  The nesting depth of the parentheses is equal to
the rank.

When an array is created, the number of dimensions and range of each
dimension must be specified, e.g., to create a 2x3 array with a
zero-based index:

@example
(make-array 'ho 2 3) @result{}
#2((ho ho ho) (ho ho ho))
@end example

The range of each dimension can also be given explicitly, e.g., another
way to create the same array:

@example
(make-array 'ho '(0 1) '(0 2)) @result{}
#2((ho ho ho) (ho ho ho))
@end example

A conventional array with one dimension based at zero is identical to
a vector:

@example
(make-array 'ho 3) @result{}
#(ho ho ho)
@end example

The following procedures can be used with conventional arrays (or vectors).

@deffn {Scheme Procedure} array? v [prot]
@deffnx {C Function} scm_array_p (v, prot)
Return @code{#t} if the @var{obj} is an array, and @code{#f} if
not.  The @var{prototype} argument is used with uniform arrays
and is described elsewhere.
@end deffn

@deffn {Scheme Procedure} make-array initial-value bound1 bound2 @dots{}
Create and return an array that has as many dimensions as there are
@var{bound}s and fill it with @var{initial-value}.  Each @var{bound}
may be a positive non-zero integer @var{N}, in which case the index for
that dimension can range from 0 through @var{N-1}; or an explicit index
range specifier in the form @code{(LOWER UPPER)}, where both @var{lower}
and @var{upper} are integers, possibly less than zero, and possibly the
same number (however, @var{lower} cannot be greater than @var{upper}).
@end deffn

@c array-ref's type is `compiled-closure'.  There's some weird stuff
@c going on in array.c, too.  Let's call it a primitive. -twp

@deffn {Scheme Procedure} uniform-vector-ref v args
@deffnx {Scheme Procedure} array-ref v . args
@deffnx {C Function} scm_uniform_vector_ref (v, args)
Return the element at the @code{(index1, index2)} element in
@var{array}.
@end deffn

@deffn {Scheme Procedure} array-in-bounds? v . args
@deffnx {C Function} scm_array_in_bounds_p (v, args)
Return @code{#t} if its arguments would be acceptable to
@code{array-ref}.
@end deffn

@c fixme: why do these sigs differ?  -ttn 2001/07/19 01:14:12
@deffn {Scheme Procedure} array-set! v obj . args
@deffnx {Scheme Procedure} uniform-array-set1! v obj args
@deffnx {C Function} scm_array_set_x (v, obj, args)
Set the element at the @code{(index1, index2)} element in @var{array} to
@var{new-value}.  The value returned by array-set! is unspecified.
@end deffn

@deffn {Scheme Procedure} make-shared-array oldra mapfunc . dims
@deffnx {C Function} scm_make_shared_array (oldra, mapfunc, dims)
@code{make-shared-array} can be used to create shared subarrays of other
arrays.  The @var{mapper} is a function that translates coordinates in
the new array into coordinates in the old array.  A @var{mapper} must be
linear, and its range must stay within the bounds of the old array, but
it can be otherwise arbitrary.  A simple example:
@lisp
(define fred (make-array #f 8 8))
(define freds-diagonal
  (make-shared-array fred (lambda (i) (list i i)) 8))
(array-set! freds-diagonal 'foo 3)
(array-ref fred 3 3) @result{} foo
(define freds-center
  (make-shared-array fred (lambda (i j) (list (+ 3 i) (+ 3 j))) 2 2))
(array-ref freds-center 0 0) @result{} foo
@end lisp
@end deffn

@deffn {Scheme Procedure} shared-array-increments ra
@deffnx {C Function} scm_shared_array_increments (ra)
For each dimension, return the distance between elements in the root vector.
@end deffn

@deffn {Scheme Procedure} shared-array-offset ra
@deffnx {C Function} scm_shared_array_offset (ra)
Return the root vector index of the first element in the array.
@end deffn

@deffn {Scheme Procedure} shared-array-root ra
@deffnx {C Function} scm_shared_array_root (ra)
Return the root vector of a shared array.
@end deffn

@deffn {Scheme Procedure} transpose-array ra . args
@deffnx {C Function} scm_transpose_array (ra, args)
Return an array sharing contents with @var{array}, but with
dimensions arranged in a different order.  There must be one
@var{dim} argument for each dimension of @var{array}.
@var{dim0}, @var{dim1}, @dots{} should be integers between 0
and the rank of the array to be returned.  Each integer in that
range must appear at least once in the argument list.

The values of @var{dim0}, @var{dim1}, @dots{} correspond to
dimensions in the array to be returned, their positions in the
argument list to dimensions of @var{array}.  Several @var{dim}s
may have the same value, in which case the returned array will
have smaller rank than @var{array}.

@lisp
(transpose-array '#2((a b) (c d)) 1 0) @result{} #2((a c) (b d))
(transpose-array '#2((a b) (c d)) 0 0) @result{} #1(a d)
(transpose-array '#3(((a b c) (d e f)) ((1 2 3) (4 5 6))) 1 1 0) @result{}
                #2((a 4) (b 5) (c 6))
@end lisp
@end deffn

@deffn {Scheme Procedure} enclose-array ra . axes
@deffnx {C Function} scm_enclose_array (ra, axes)
@var{dim0}, @var{dim1} @dots{} should be nonnegative integers less than
the rank of @var{array}.  @var{enclose-array} returns an array
resembling an array of shared arrays.  The dimensions of each shared
array are the same as the @var{dim}th dimensions of the original array,
the dimensions of the outer array are the same as those of the original
array that did not match a @var{dim}.

An enclosed array is not a general Scheme array.  Its elements may not
be set using @code{array-set!}.  Two references to the same element of
an enclosed array will be @code{equal?} but will not in general be
@code{eq?}.  The value returned by @var{array-prototype} when given an
enclosed array is unspecified.

examples:
@lisp
(enclose-array '#3(((a b c) (d e f)) ((1 2 3) (4 5 6))) 1) @result{}
   #<enclosed-array (#1(a d) #1(b e) #1(c f)) (#1(1 4) #1(2 5) #1(3 6))>

(enclose-array '#3(((a b c) (d e f)) ((1 2 3) (4 5 6))) 1 0) @result{}
   #<enclosed-array #2((a 1) (d 4)) #2((b 2) (e 5)) #2((c 3) (f 6))>
@end lisp
@end deffn

@deffn {Scheme Procedure} array-shape array
Return a list of inclusive bounds of integers.
@example
(array-shape (make-array 'foo '(-1 3) 5)) @result{} ((-1 3) (0 4))
@end example
@end deffn

@deffn {Scheme Procedure} array-dimensions ra
@deffnx {C Function} scm_array_dimensions (ra)
@code{Array-dimensions} is similar to @code{array-shape} but replaces
elements with a @code{0} minimum with one greater than the maximum. So:
@lisp
(array-dimensions (make-array 'foo '(-1 3) 5)) @result{} ((-1 3) 5)
@end lisp
@end deffn

@deffn {Scheme Procedure} array-rank ra
@deffnx {C Function} scm_array_rank (ra)
Return the number of dimensions of @var{obj}.  If @var{obj} is
not an array, @code{0} is returned.
@end deffn

@deffn {Scheme Procedure} array->list v
@deffnx {C Function} scm_t_arrayo_list (v)
Return a list consisting of all the elements, in order, of
@var{array}.
@end deffn

@deffn {Scheme Procedure} array-copy! src dst
@deffnx {Scheme Procedure} array-copy-in-order! src dst
@deffnx {C Function} scm_array_copy_x (src, dst)
Copy every element from vector or array @var{source} to the
corresponding element of @var{destination}.  @var{destination} must have
the same rank as @var{source}, and be at least as large in each
dimension.  The order is unspecified.
@end deffn

@deffn {Scheme Procedure} array-fill! ra fill
@deffnx {C Function} scm_array_fill_x (ra, fill)
Store @var{fill} in every element of @var{array}.  The value returned
is unspecified.
@end deffn

@c begin (texi-doc-string "guile" "array-equal?")
@deffn {Scheme Procedure} array-equal? ra0 ra1
Return @code{#t} iff all arguments are arrays with the same shape, the
same type, and have corresponding elements which are either
@code{equal?}  or @code{array-equal?}.  This function differs from
@code{equal?} in that a one dimensional shared array may be
@var{array-equal?} but not @var{equal?} to a vector or uniform vector.
@end deffn

@deffn {Scheme Procedure} array-contents array [strict]
@deffnx {C Function} scm_array_contents (array, strict)
If @var{array} may be @dfn{unrolled} into a one dimensional shared array
without changing their order (last subscript changing fastest), then
@code{array-contents} returns that shared array, otherwise it returns
@code{#f}.  All arrays made by @var{make-array} and
@var{make-uniform-array} may be unrolled, some arrays made by
@var{make-shared-array} may not be.

If the optional argument @var{strict} is provided, a shared array will
be returned only if its elements are stored internally contiguous in
memory.
@end deffn

@node Array Mapping
@subsection Array Mapping

@deffn {Scheme Procedure} array-map! ra0 proc . lra
@deffnx {Scheme Procedure} array-map-in-order! ra0 proc . lra
@deffnx {C Function} scm_array_map_x (ra0, proc, lra)
@var{array1}, @dots{} must have the same number of dimensions as
@var{array0} and have a range for each index which includes the range
for the corresponding index in @var{array0}.  @var{proc} is applied to
each tuple of elements of @var{array1} @dots{} and the result is stored
as the corresponding element in @var{array0}.  The value returned is
unspecified.  The order of application is unspecified.
@end deffn

@deffn {Scheme Procedure} array-for-each proc ra0 . lra
@deffnx {C Function} scm_array_for_each (proc, ra0, lra)
Apply @var{proc} to each tuple of elements of @var{array0} @dots{}
in row-major order.  The value returned is unspecified.
@end deffn

@deffn {Scheme Procedure} array-index-map! ra proc
@deffnx {C Function} scm_array_index_map_x (ra, proc)
Apply @var{proc} to the indices of each element of @var{array} in
turn, storing the result in the corresponding element.  The value
returned and the order of application are unspecified.

One can implement @var{array-indexes} as
@lisp
(define (array-indexes array)
    (let ((ra (apply make-array #f (array-shape array))))
      (array-index-map! ra (lambda x x))
      ra))
@end lisp
Another example:
@lisp
(define (apl:index-generator n)
    (let ((v (make-uniform-vector n 1)))
      (array-index-map! v (lambda (i) i))
      v))
@end lisp
@end deffn

@node Uniform Arrays
@subsection Uniform Arrays
@tpindex Uniform Arrays

@noindent
@dfn{Uniform arrays} have elements all of the
same type and occupy less storage than conventional
arrays.  Uniform arrays with a single zero-based dimension
are also known as @dfn{uniform vectors}.  The procedures in
this section can also be used on conventional arrays, vectors,
bit-vectors and strings.

@noindent
When creating a uniform array, the type of data to be stored
is indicated with a @var{prototype} argument.  The following table
lists the types available and example prototypes:

@example
prototype           type                       printing character

#t             boolean (bit-vector)                    b
#\a            char (string)                           a
#\nul          byte (integer)                          y
's             short (integer)                         h
1              unsigned long (integer)                 u
-1             signed long (integer)                   e
'l             signed long long (integer)              l
1.0            float (single precision)                s
1/3            double (double precision float)         i
0+i            complex (double precision)              c
()             conventional vector
@end example

@noindent
Unshared uniform arrays of characters with a single zero-based dimension
are identical to strings:

@example
(make-uniform-array #\a 3) @result{}
"aaa"
@end example

@noindent
Unshared uniform arrays of booleans with a single zero-based dimension
are identical to @ref{Bit Vectors, bit-vectors}.

@example
(make-uniform-array #t 3) @result{}
#*111
@end example

@noindent
Other uniform vectors are written in a form similar to that of vectors,
except that a single character from the above table is put between
@code{#} and @code{(}.  For example, a uniform vector of signed
long integers is displayed in the form @code{'#e(3 5 9)}.

@deffn {Scheme Procedure} array? v [prot]
Return @code{#t} if the @var{obj} is an array, and @code{#f} if not.

The @var{prototype} argument is used with uniform arrays and is described
elsewhere.
@end deffn

@deffn {Scheme Procedure} make-uniform-array prototype bound1 bound2 @dots{}
Create and return a uniform array of type corresponding to
@var{prototype} that has as many dimensions as there are @var{bound}s
and fill it with @var{prototype}.
@end deffn

@deffn {Scheme Procedure} array-prototype ra
@deffnx {C Function} scm_array_prototype (ra)
Return an object that would produce an array of the same type
as @var{array}, if used as the @var{prototype} for
@code{make-uniform-array}.
@end deffn

@deffn {Scheme Procedure} list->uniform-array ndim prot lst
@deffnx {Scheme Procedure} list->uniform-vector prot lst
@deffnx {C Function} scm_list_to_uniform_array (ndim, prot, lst)
Return a uniform array of the type indicated by prototype
@var{prot} with elements the same as those of @var{lst}.
Elements must be of the appropriate type, no coercions are
done.
@end deffn

@deffn {Scheme Procedure} uniform-vector-fill! uve fill
Store @var{fill} in every element of @var{uve}.  The value returned is
unspecified.
@end deffn

@deffn {Scheme Procedure} uniform-vector-length v
@deffnx {C Function} scm_uniform_vector_length (v)
Return the number of elements in @var{uve}.
@end deffn

@deffn {Scheme Procedure} dimensions->uniform-array dims prot [fill]
@deffnx {Scheme Procedure} make-uniform-vector length prototype [fill]
@deffnx {C Function} scm_dimensions_to_uniform_array (dims, prot, fill)
Create and return a uniform array or vector of type
corresponding to @var{prototype} with dimensions @var{dims} or
length @var{length}.  If @var{fill} is supplied, it's used to
fill the array, otherwise @var{prototype} is used.
@end deffn

@c Another compiled-closure. -twp

@deffn {Scheme Procedure} uniform-array-read! ra [port_or_fd [start [end]]]
@deffnx {Scheme Procedure} uniform-vector-read! uve [port-or-fdes] [start] [end]
@deffnx {C Function} scm_uniform_array_read_x (ra, port_or_fd, start, end)
Attempt to read all elements of @var{ura}, in lexicographic order, as
binary objects from @var{port-or-fdes}.
If an end of file is encountered,
the objects up to that point are put into @var{ura}
(starting at the beginning) and the remainder of the array is
unchanged.

The optional arguments @var{start} and @var{end} allow
a specified region of a vector (or linearized array) to be read,
leaving the remainder of the vector unchanged.

@code{uniform-array-read!} returns the number of objects read.
@var{port-or-fdes} may be omitted, in which case it defaults to the value
returned by @code{(current-input-port)}.
@end deffn

@deffn {Scheme Procedure} uniform-array-write v [port_or_fd [start [end]]]
@deffnx {Scheme Procedure} uniform-vector-write uve [port-or-fdes] [start] [end]
@deffnx {C Function} scm_uniform_array_write (v, port_or_fd, start, end)
Writes all elements of @var{ura} as binary objects to
@var{port-or-fdes}.

The optional arguments @var{start}
and @var{end} allow
a specified region of a vector (or linearized array) to be written.

The number of objects actually written is returned.
@var{port-or-fdes} may be
omitted, in which case it defaults to the value returned by
@code{(current-output-port)}.
@end deffn

@node Bit Vectors
@subsection Bit Vectors

@noindent
Bit vectors are a specific type of uniform array: an array of booleans
with a single zero-based index.

@noindent
They are displayed as a sequence of @code{0}s and
@code{1}s prefixed by @code{#*}, e.g.,

@example
(make-uniform-vector 8 #t #f) @result{}
#*00000000

#b(#t #f #t) @result{}
#*101
@end example

@deffn {Scheme Procedure} bit-count b bitvector
@deffnx {C Function} scm_bit_count (b, bitvector)
Return the number of occurrences of the boolean @var{b} in
@var{bitvector}.
@end deffn

@deffn {Scheme Procedure} bit-position item v k
@deffnx {C Function} scm_bit_position (item, v, k)
Return the minimum index of an occurrence of @var{bool} in
@var{bv} which is at least @var{k}.  If no @var{bool} occurs
within the specified range @code{#f} is returned.
@end deffn

@deffn {Scheme Procedure} bit-invert! v
@deffnx {C Function} scm_bit_invert_x (v)
Modify @var{bv} by replacing each element with its negation.
@end deffn

@deffn {Scheme Procedure} bit-set*! v kv obj
@deffnx {C Function} scm_bit_set_star_x (v, kv, obj)
If uve is a bit-vector @var{bv} and uve must be of the same
length.  If @var{bool} is @code{#t}, uve is OR'ed into
@var{bv}; If @var{bool} is @code{#f}, the inversion of uve is
AND'ed into @var{bv}.

If uve is a unsigned long integer vector all the elements of uve
must be between 0 and the @code{length} of @var{bv}.  The bits
of @var{bv} corresponding to the indexes in uve are set to
@var{bool}.  The return value is unspecified.
@end deffn

@deffn {Scheme Procedure} bit-count* v kv obj
@deffnx {C Function} scm_bit_count_star (v, kv, obj)
Return
@lisp
(bit-count (bit-set*! (if bool bv (bit-invert! bv)) uve #t) #t).
@end lisp
@var{bv} is not modified.
@end deffn


@node Association Lists and Hash Tables
@section Association Lists and Hash Tables

This chapter discusses dictionary objects: data structures that are
useful for organizing and indexing large bodies of information.

@menu
* Dictionary Types::            About dictionary types; what they're good for.
* Association Lists::           List-based dictionaries.
* Hash Tables::                 Table-based dictionaries.
@end menu

@node Dictionary Types
@subsection Dictionary Types

A @dfn{dictionary} object is a data structure used to index
information in a user-defined way.  In standard Scheme, the main
aggregate data types are lists and vectors.  Lists are not really
indexed at all, and vectors are indexed only by number
(e.g. @code{(vector-ref foo 5)}).  Often you will find it useful
to index your data on some other type; for example, in a library
catalog you might want to look up a book by the name of its
author.  Dictionaries are used to help you organize information in
such a way.

An @dfn{association list} (or @dfn{alist} for short) is a list of
key-value pairs.  Each pair represents a single quantity or
object; the @code{car} of the pair is a key which is used to
identify the object, and the @code{cdr} is the object's value.

A @dfn{hash table} also permits you to index objects with
arbitrary keys, but in a way that makes looking up any one object
extremely fast.  A well-designed hash system makes hash table
lookups almost as fast as conventional array or vector references.

Alists are popular among Lisp programmers because they use only
the language's primitive operations (lists, @dfn{car}, @dfn{cdr}
and the equality primitives).  No changes to the language core are
necessary.  Therefore, with Scheme's built-in list manipulation
facilities, it is very convenient to handle data stored in an
association list.  Also, alists are highly portable and can be
easily implemented on even the most minimal Lisp systems.

However, alists are inefficient, especially for storing large
quantities of data.  Because we want Guile to be useful for large
software systems as well as small ones, Guile provides a rich set
of tools for using either association lists or hash tables.

@node Association Lists
@subsection Association Lists
@tpindex Association Lists
@tpindex Alist

@cindex Association List
@cindex Alist
@cindex Database

An association list is a conventional data structure that is often used
to implement simple key-value databases.  It consists of a list of
entries in which each entry is a pair.  The @dfn{key} of each entry is
the @code{car} of the pair and the @dfn{value} of each entry is the
@code{cdr}.

@example
ASSOCIATION LIST ::=  '( (KEY1 . VALUE1)
                         (KEY2 . VALUE2)
                         (KEY3 . VALUE3)
                         @dots{}
                       )
@end example

@noindent
Association lists are also known, for short, as @dfn{alists}.

The structure of an association list is just one example of the infinite
number of possible structures that can be built using pairs and lists.
As such, the keys and values in an association list can be manipulated
using the general list structure procedures @code{cons}, @code{car},
@code{cdr}, @code{set-car!}, @code{set-cdr!} and so on.  However,
because association lists are so useful, Guile also provides specific
procedures for manipulating them.

@menu
* Alist Key Equality::          
* Adding or Setting Alist Entries::  
* Retrieving Alist Entries::    
* Removing Alist Entries::      
* Sloppy Alist Functions::      
* Alist Example::               
@end menu

@node Alist Key Equality
@subsubsection Alist Key Equality

All of Guile's dedicated association list procedures, apart from
@code{acons}, come in three flavours, depending on the level of equality
that is required to decide whether an existing key in the association
list is the same as the key that the procedure call uses to identify the
required entry.

@itemize @bullet
@item
Procedures with @dfn{assq} in their name use @code{eq?} to determine key
equality.

@item
Procedures with @dfn{assv} in their name use @code{eqv?} to determine
key equality.

@item
Procedures with @dfn{assoc} in their name use @code{equal?} to
determine key equality.
@end itemize

@code{acons} is an exception because it is used to build association
lists which do not require their entries' keys to be unique.

@node Adding or Setting Alist Entries
@subsubsection Adding or Setting Alist Entries

@code{acons} adds a new entry to an association list and returns the
combined association list.  The combined alist is formed by consing the
new entry onto the head of the alist specified in the @code{acons}
procedure call.  So the specified alist is not modified, but its
contents become shared with the tail of the combined alist that
@code{acons} returns.

In the most common usage of @code{acons}, a variable holding the
original association list is updated with the combined alist:

@example
(set! address-list (acons name address address-list))
@end example

In such cases, it doesn't matter that the old and new values of
@code{address-list} share some of their contents, since the old value is
usually no longer independently accessible.

Note that @code{acons} adds the specified new entry regardless of
whether the alist may already contain entries with keys that are, in
some sense, the same as that of the new entry.  Thus @code{acons} is
ideal for building alists where there is no concept of key uniqueness.

@example
(set! task-list (acons 3 "pay gas bill" '()))
task-list
@result{}
((3 . "pay gas bill"))

(set! task-list (acons 3 "tidy bedroom" task-list))
task-list
@result{}
((3 . "tidy bedroom") (3 . "pay gas bill"))
@end example

@code{assq-set!}, @code{assv-set!} and @code{assoc-set!} are used to add
or replace an entry in an association list where there @emph{is} a
concept of key uniqueness.  If the specified association list already
contains an entry whose key is the same as that specified in the
procedure call, the existing entry is replaced by the new one.
Otherwise, the new entry is consed onto the head of the old association
list to create the combined alist.  In all cases, these procedures
return the combined alist.

@code{assq-set!} and friends @emph{may} destructively modify the
structure of the old association list in such a way that an existing
variable is correctly updated without having to @code{set!} it to the
value returned:

@example
address-list
@result{}
(("mary" . "34 Elm Road") ("james" . "16 Bow Street"))

(assoc-set! address-list "james" "1a London Road")
@result{}
(("mary" . "34 Elm Road") ("james" . "1a London Road"))

address-list
@result{}
(("mary" . "34 Elm Road") ("james" . "1a London Road"))
@end example

Or they may not:

@example
(assoc-set! address-list "bob" "11 Newington Avenue")
@result{}
(("bob" . "11 Newington Avenue") ("mary" . "34 Elm Road")
 ("james" . "1a London Road"))

address-list
@result{}
(("mary" . "34 Elm Road") ("james" . "1a London Road"))
@end example

The only safe way to update an association list variable when adding or
replacing an entry like this is to @code{set!} the variable to the
returned value:

@example
(set! address-list
      (assoc-set! address-list "bob" "11 Newington Avenue"))
address-list
@result{}
(("bob" . "11 Newington Avenue") ("mary" . "34 Elm Road")
 ("james" . "1a London Road"))
@end example

Because of this slight inconvenience, you may find it more convenient to
use hash tables to store dictionary data.  If your application will not
be modifying the contents of an alist very often, this may not make much
difference to you.

If you need to keep the old value of an association list in a form
independent from the list that results from modification by
@code{acons}, @code{assq-set!}, @code{assv-set!} or @code{assoc-set!},
use @code{list-copy} to copy the old association list before modifying
it.

@deffn {Scheme Procedure} acons key value alist
@deffnx {C Function} scm_acons (key, value, alist)
Add a new key-value pair to @var{alist}.  A new pair is
created whose car is @var{key} and whose cdr is @var{value}, and the
pair is consed onto @var{alist}, and the new list is returned.  This
function is @emph{not} destructive; @var{alist} is not modified.
@end deffn

@deffn {Scheme Procedure} assq-set! alist key val
@deffnx {Scheme Procedure} assv-set! alist key value
@deffnx {Scheme Procedure} assoc-set! alist key value
@deffnx {C Function} scm_assq_set_x (alist, key, val)
@deffnx {C Function} scm_assv_set_x (alist, key, val)
@deffnx {C Function} scm_assoc_set_x (alist, key, val)
Reassociate @var{key} in @var{alist} with @var{value}: find any existing
@var{alist} entry for @var{key} and associate it with the new
@var{value}.  If @var{alist} does not contain an entry for @var{key},
add a new one.  Return the (possibly new) alist.

These functions do not attempt to verify the structure of @var{alist},
and so may cause unusual results if passed an object that is not an
association list.
@end deffn

@node Retrieving Alist Entries
@subsubsection Retrieving Alist Entries
@rnindex assq
@rnindex assv
@rnindex assoc

@code{assq}, @code{assv} and @code{assoc} take an alist and a key as
arguments and return the entry for that key if an entry exists, or
@code{#f} if there is no entry for that key.  Note that, in the cases
where an entry exists, these procedures return the complete entry, that
is @code{(KEY . VALUE)}, not just the value.

@deffn {Scheme Procedure} assq key alist
@deffnx {Scheme Procedure} assv key alist
@deffnx {Scheme Procedure} assoc key alist
@deffnx {C Function} scm_assq (key, alist)
@deffnx {C Function} scm_assv (key, alist)
@deffnx {C Function} scm_assoc (key, alist)
Fetch the entry in @var{alist} that is associated with @var{key}.  To
decide whether the argument @var{key} matches a particular entry in
@var{alist}, @code{assq} compares keys with @code{eq?}, @code{assv}
uses @code{eqv?} and @code{assoc} uses @code{equal?}.  If @var{key}
cannot be found in @var{alist} (according to whichever equality
predicate is in use), then return @code{#f}.  These functions
return the entire alist entry found (i.e. both the key and the value).
@end deffn

@code{assq-ref}, @code{assv-ref} and @code{assoc-ref}, on the other
hand, take an alist and a key and return @emph{just the value} for that
key, if an entry exists.  If there is no entry for the specified key,
these procedures return @code{#f}.

This creates an ambiguity: if the return value is @code{#f}, it means
either that there is no entry with the specified key, or that there
@emph{is} an entry for the specified key, with value @code{#f}.
Consequently, @code{assq-ref} and friends should only be used where it
is known that an entry exists, or where the ambiguity doesn't matter
for some other reason.

@deffn {Scheme Procedure} assq-ref alist key
@deffnx {Scheme Procedure} assv-ref alist key
@deffnx {Scheme Procedure} assoc-ref alist key
@deffnx {C Function} scm_assq_ref (alist, key)
@deffnx {C Function} scm_assv_ref (alist, key)
@deffnx {C Function} scm_assoc_ref (alist, key)
Like @code{assq}, @code{assv} and @code{assoc}, except that only the
value associated with @var{key} in @var{alist} is returned.  These
functions are equivalent to

@lisp
(let ((ent (@var{associator} @var{key} @var{alist})))
  (and ent (cdr ent)))
@end lisp

where @var{associator} is one of @code{assq}, @code{assv} or @code{assoc}.
@end deffn

@node Removing Alist Entries
@subsubsection Removing Alist Entries

To remove the element from an association list whose key matches a
specified key, use @code{assq-remove!}, @code{assv-remove!} or
@code{assoc-remove!} (depending, as usual, on the level of equality
required between the key that you specify and the keys in the
association list).

As with @code{assq-set!} and friends, the specified alist may or may not
be modified destructively, and the only safe way to update a variable
containing the alist is to @code{set!} it to the value that
@code{assq-remove!} and friends return.

@example
address-list
@result{}
(("bob" . "11 Newington Avenue") ("mary" . "34 Elm Road")
 ("james" . "1a London Road"))

(set! address-list (assoc-remove! address-list "mary"))
address-list
@result{}
(("bob" . "11 Newington Avenue") ("james" . "1a London Road"))
@end example

Note that, when @code{assq/v/oc-remove!} is used to modify an
association list that has been constructed only using the corresponding
@code{assq/v/oc-set!}, there can be at most one matching entry in the
alist, so the question of multiple entries being removed in one go does
not arise.  If @code{assq/v/oc-remove!} is applied to an association
list that has been constructed using @code{acons}, or an
@code{assq/v/oc-set!} with a different level of equality, or any mixture
of these, it removes only the first matching entry from the alist, even
if the alist might contain further matching entries.  For example:

@example
(define address-list '())
(set! address-list (assq-set! address-list "mary" "11 Elm Street"))
(set! address-list (assq-set! address-list "mary" "57 Pine Drive"))
address-list
@result{}
(("mary" . "57 Pine Drive") ("mary" . "11 Elm Street"))

(set! address-list (assoc-remove! address-list "mary"))
address-list
@result{}
(("mary" . "11 Elm Street"))
@end example

In this example, the two instances of the string "mary" are not the same
when compared using @code{eq?}, so the two @code{assq-set!} calls add
two distinct entries to @code{address-list}.  When compared using
@code{equal?}, both "mary"s in @code{address-list} are the same as the
"mary" in the @code{assoc-remove!} call, but @code{assoc-remove!} stops
after removing the first matching entry that it finds, and so one of the
"mary" entries is left in place.

@deffn {Scheme Procedure} assq-remove! alist key
@deffnx {Scheme Procedure} assv-remove! alist key
@deffnx {Scheme Procedure} assoc-remove! alist key
@deffnx {C Function} scm_assq_remove_x (alist, key)
@deffnx {C Function} scm_assv_remove_x (alist, key)
@deffnx {C Function} scm_assoc_remove_x (alist, key)
Delete the first entry in @var{alist} associated with @var{key}, and return
the resulting alist.
@end deffn

@node Sloppy Alist Functions
@subsubsection Sloppy Alist Functions

@code{sloppy-assq}, @code{sloppy-assv} and @code{sloppy-assoc} behave
like the corresponding non-@code{sloppy-} procedures, except that they
return @code{#f} when the specified association list is not well-formed,
where the non-@code{sloppy-} versions would signal an error.

Specifically, there are two conditions for which the non-@code{sloppy-}
procedures signal an error, which the @code{sloppy-} procedures handle
instead by returning @code{#f}.  Firstly, if the specified alist as a
whole is not a proper list:

@example
(assoc "mary" '((1 . 2) ("key" . "door") . "open sesame"))
@result{}
ERROR: In procedure assoc in expression (assoc "mary" (quote #)):
ERROR: Wrong type argument in position 2 (expecting NULLP): "open sesame"
ABORT: (wrong-type-arg)

(sloppy-assoc "mary" '((1 . 2) ("key" . "door") . "open sesame"))
@result{}
#f
@end example

@noindent
Secondly, if one of the entries in the specified alist is not a pair:

@example
(assoc 2 '((1 . 1) 2 (3 . 9)))
@result{}
ERROR: In procedure assoc in expression (assoc 2 (quote #)):
ERROR: Wrong type argument in position 2 (expecting CONSP): 2
ABORT: (wrong-type-arg)

(sloppy-assoc 2 '((1 . 1) 2 (3 . 9)))
@result{}
#f
@end example

Unless you are explicitly working with badly formed association lists,
it is much safer to use the non-@code{sloppy-} procedures, because they
help to highlight coding and data errors that the @code{sloppy-}
versions would silently cover up.

@deffn {Scheme Procedure} sloppy-assq key alist
@deffnx {C Function} scm_sloppy_assq (key, alist)
Behaves like @code{assq} but does not do any error checking.
Recommended only for use in Guile internals.
@end deffn

@deffn {Scheme Procedure} sloppy-assv key alist
@deffnx {C Function} scm_sloppy_assv (key, alist)
Behaves like @code{assv} but does not do any error checking.
Recommended only for use in Guile internals.
@end deffn

@deffn {Scheme Procedure} sloppy-assoc key alist
@deffnx {C Function} scm_sloppy_assoc (key, alist)
Behaves like @code{assoc} but does not do any error checking.
Recommended only for use in Guile internals.
@end deffn

@node Alist Example
@subsubsection Alist Example

Here is a longer example of how alists may be used in practice.

@lisp
(define capitals '(("New York" . "Albany")
                   ("Oregon"   . "Salem")
                   ("Florida"  . "Miami")))

;; What's the capital of Oregon?
(assoc "Oregon" capitals)       @result{} ("Oregon" . "Salem")
(assoc-ref capitals "Oregon")   @result{} "Salem"

;; We left out South Dakota.
(set! capitals
      (assoc-set! capitals "South Dakota" "Bismarck"))
capitals
@result{} (("South Dakota" . "Bismarck")
    ("New York" . "Albany")
    ("Oregon" . "Salem")
    ("Florida" . "Miami"))

;; And we got Florida wrong.
(set! capitals
      (assoc-set! capitals "Florida" "Tallahassee"))
capitals
@result{} (("South Dakota" . "Bismarck")
    ("New York" . "Albany")
    ("Oregon" . "Salem")
    ("Florida" . "Tallahassee"))

;; After Oregon secedes, we can remove it.
(set! capitals
      (assoc-remove! capitals "Oregon"))
capitals
@result{} (("South Dakota" . "Bismarck")
    ("New York" . "Albany")
    ("Florida" . "Tallahassee"))
@end lisp

@node Hash Tables
@subsection Hash Tables
@tpindex Hash Tables

@c FIXME::martin: Review me!

Hash tables are dictionaries which offer similar functionality as
association lists: They provide a mapping from keys to values.  The
difference is that association lists need time linear in the size of
elements when searching for entries, whereas hash tables can normally
search in constant time.  The drawback is that hash tables require a
little bit more memory, and that you can not use the normal list
procedures (@pxref{Lists}) for working with them.

@menu
* Hash Table Examples::         Demonstration of hash table usage.
* Hash Table Reference::        Hash table procedure descriptions.
@end menu


@node Hash Table Examples
@subsubsection Hash Table Examples

@c FIXME::martin: Review me!

For demonstration purposes, this section gives a few usage examples of
some hash table procedures, together with some explanation what they do.

First we start by creating a new hash table with 31 slots, and
populate it with two key/value pairs.

@lisp
(define h (make-hash-table 31))

(hashq-create-handle! h 'foo "bar")
@result{}
(foo . "bar")

(hashq-create-handle! h 'braz "zonk")
@result{}
(braz . "zonk")

(hashq-create-handle! h 'frob #f)
@result{}
(frob . #f)
@end lisp

You can get the value for a given key with the procedure
@code{hashq-ref}, but the problem with this procedure is that you
cannot reliably determine whether a key does exists in the table.  The
reason is that the procedure returns @code{#f} if the key is not in
the table, but it will return the same value if the key is in the
table and just happens to have the value @code{#f}, as you can see in
the following examples.

@lisp
(hashq-ref h 'foo)
@result{}
"bar"

(hashq-ref h 'frob)
@result{}
#f

(hashq-ref h 'not-there)
@result{}
#f
@end lisp

Better is to use the procedure @code{hashq-get-handle}, which makes a
distinction between the two cases.  Just like @code{assq}, this
procedure returns a key/value-pair on success, and @code{#f} if the
key is not found.

@lisp
(hashq-get-handle h 'foo)
@result{}
(foo . "bar")

(hashq-get-handle h 'not-there)
@result{}
#f
@end lisp

There is no procedure for calculating the number of key/value-pairs in
a hash table, but @code{hash-fold} can be used for doing exactly that.

@lisp
(hash-fold (lambda (key value seed) (+ 1 seed)) 0 h)
@result{}
3
@end lisp

@node Hash Table Reference
@subsubsection Hash Table Reference

Like the association list functions, the hash table functions come
in several varieties: @code{hashq}, @code{hashv}, and @code{hash}.
The @code{hashq} functions use @code{eq?} to determine whether two
keys match.  The @code{hashv} functions use @code{eqv?}, and the
@code{hash} functions use @code{equal?}.

In each of the functions that follow, the @var{table} argument
must be a vector.  The @var{key} and @var{value} arguments may be
any Scheme object.

@deffn {Scheme Procedure} make-hash-table size
Create a new hash table of @var{size} slots.  Note that the number of
slots does not limit the size of the table, it just tells how large
the underlying vector will be.  The @var{size} should be similar to
the expected number of elements which will be added to the table, but
they need not match.  For good performance, it might be a good idea to
use a prime number as the @var{size}.
@end deffn

@deffn {Scheme Procedure} hashq-ref table key [dflt]
@deffnx {C Function} scm_hashq_ref (table, key, dflt)
Look up @var{key} in the hash table @var{table}, and return the
value (if any) associated with it.  If @var{key} is not found,
return @var{default} (or @code{#f} if no @var{default} argument
is supplied).  Uses @code{eq?} for equality testing.
@end deffn

@deffn {Scheme Procedure} hashv-ref table key [dflt]
@deffnx {C Function} scm_hashv_ref (table, key, dflt)
Look up @var{key} in the hash table @var{table}, and return the
value (if any) associated with it.  If @var{key} is not found,
return @var{default} (or @code{#f} if no @var{default} argument
is supplied).  Uses @code{eqv?} for equality testing.
@end deffn

@deffn {Scheme Procedure} hash-ref table key [dflt]
@deffnx {C Function} scm_hash_ref (table, key, dflt)
Look up @var{key} in the hash table @var{table}, and return the
value (if any) associated with it.  If @var{key} is not found,
return @var{default} (or @code{#f} if no @var{default} argument
is supplied).  Uses @code{equal?} for equality testing.
@end deffn

@deffn {Scheme Procedure} hashq-set! table key val
@deffnx {C Function} scm_hashq_set_x (table, key, val)
Find the entry in @var{table} associated with @var{key}, and
store @var{value} there. Uses @code{eq?} for equality testing.
@end deffn

@deffn {Scheme Procedure} hashv-set! table key val
@deffnx {C Function} scm_hashv_set_x (table, key, val)
Find the entry in @var{table} associated with @var{key}, and
store @var{value} there. Uses @code{eqv?} for equality testing.
@end deffn

@deffn {Scheme Procedure} hash-set! table key val
@deffnx {C Function} scm_hash_set_x (table, key, val)
Find the entry in @var{table} associated with @var{key}, and
store @var{value} there. Uses @code{equal?} for equality
testing.
@end deffn

@deffn {Scheme Procedure} hashq-remove! table key
@deffnx {C Function} scm_hashq_remove_x (table, key)
Remove @var{key} (and any value associated with it) from
@var{table}.  Uses @code{eq?} for equality tests.
@end deffn

@deffn {Scheme Procedure} hashv-remove! table key
@deffnx {C Function} scm_hashv_remove_x (table, key)
Remove @var{key} (and any value associated with it) from
@var{table}.  Uses @code{eqv?} for equality tests.
@end deffn

@deffn {Scheme Procedure} hash-remove! table key
@deffnx {C Function} scm_hash_remove_x (table, key)
Remove @var{key} (and any value associated with it) from
@var{table}.  Uses @code{equal?} for equality tests.
@end deffn

The standard hash table functions may be too limited for some
applications.  For example, you may want a hash table to store
strings in a case-insensitive manner, so that references to keys
named ``foobar'', ``FOOBAR'' and ``FooBaR'' will all yield the
same item.  Guile provides you with @dfn{extended} hash tables
that permit you to specify a hash function and associator function
of your choosing.  The functions described in the rest of this section
can be used to implement such custom hash table structures.

If you are unfamiliar with the inner workings of hash tables, then
this facility will probably be a little too abstract for you to
use comfortably.  If you are interested in learning more, see an
introductory textbook on data structures or algorithms for an
explanation of how hash tables are implemented.

@deffn {Scheme Procedure} hashq key size
@deffnx {C Function} scm_hashq (key, size)
Determine a hash value for @var{key} that is suitable for
lookups in a hashtable of size @var{size}, where @code{eq?} is
used as the equality predicate.  The function returns an
integer in the range 0 to @var{size} - 1.  Note that
@code{hashq} may use internal addresses.  Thus two calls to
hashq where the keys are @code{eq?} are not guaranteed to
deliver the same value if the key object gets garbage collected
in between.  This can happen, for example with symbols:
@code{(hashq 'foo n) (gc) (hashq 'foo n)} may produce two
different values, since @code{foo} will be garbage collected.
@end deffn

@deffn {Scheme Procedure} hashv key size
@deffnx {C Function} scm_hashv (key, size)
Determine a hash value for @var{key} that is suitable for
lookups in a hashtable of size @var{size}, where @code{eqv?} is
used as the equality predicate.  The function returns an
integer in the range 0 to @var{size} - 1.  Note that
@code{(hashv key)} may use internal addresses.  Thus two calls
to hashv where the keys are @code{eqv?} are not guaranteed to
deliver the same value if the key object gets garbage collected
in between.  This can happen, for example with symbols:
@code{(hashv 'foo n) (gc) (hashv 'foo n)} may produce two
different values, since @code{foo} will be garbage collected.
@end deffn

@deffn {Scheme Procedure} hash key size
@deffnx {C Function} scm_hash (key, size)
Determine a hash value for @var{key} that is suitable for
lookups in a hashtable of size @var{size}, where @code{equal?}
is used as the equality predicate.  The function returns an
integer in the range 0 to @var{size} - 1.
@end deffn

@deffn {Scheme Procedure} hashx-ref hash assoc table key [dflt]
@deffnx {C Function} scm_hashx_ref (hash, assoc, table, key, dflt)
This behaves the same way as the corresponding @code{ref}
function, but uses @var{hash} as a hash function and
@var{assoc} to compare keys.  @code{hash} must be a function
that takes two arguments, a key to be hashed and a table size.
@code{assoc} must be an associator function, like @code{assoc},
@code{assq} or @code{assv}.

By way of illustration, @code{hashq-ref table key} is
equivalent to @code{hashx-ref hashq assq table key}.
@end deffn

@deffn {Scheme Procedure} hashx-set! hash assoc table key val
@deffnx {C Function} scm_hashx_set_x (hash, assoc, table, key, val)
This behaves the same way as the corresponding @code{set!}
function, but uses @var{hash} as a hash function and
@var{assoc} to compare keys.  @code{hash} must be a function
that takes two arguments, a key to be hashed and a table size.
@code{assoc} must be an associator function, like @code{assoc},
@code{assq} or @code{assv}.

 By way of illustration, @code{hashq-set! table key} is
equivalent to @code{hashx-set!  hashq assq table key}.
@end deffn

@deffn {Scheme Procedure} hashq-get-handle table key
@deffnx {C Function} scm_hashq_get_handle (table, key)
This procedure returns the @code{(key . value)} pair from the
hash table @var{table}.  If @var{table} does not hold an
associated value for @var{key}, @code{#f} is returned.
Uses @code{eq?} for equality testing.
@end deffn

@deffn {Scheme Procedure} hashv-get-handle table key
@deffnx {C Function} scm_hashv_get_handle (table, key)
This procedure returns the @code{(key . value)} pair from the
hash table @var{table}.  If @var{table} does not hold an
associated value for @var{key}, @code{#f} is returned.
Uses @code{eqv?} for equality testing.
@end deffn

@deffn {Scheme Procedure} hash-get-handle table key
@deffnx {C Function} scm_hash_get_handle (table, key)
This procedure returns the @code{(key . value)} pair from the
hash table @var{table}.  If @var{table} does not hold an
associated value for @var{key}, @code{#f} is returned.
Uses @code{equal?} for equality testing.
@end deffn

@deffn {Scheme Procedure} hashx-get-handle hash assoc table key
@deffnx {C Function} scm_hashx_get_handle (hash, assoc, table, key)
This behaves the same way as the corresponding
@code{-get-handle} function, but uses @var{hash} as a hash
function and @var{assoc} to compare keys.  @code{hash} must be
a function that takes two arguments, a key to be hashed and a
table size.  @code{assoc} must be an associator function, like
@code{assoc}, @code{assq} or @code{assv}.
@end deffn

@deffn {Scheme Procedure} hashq-create-handle! table key init
@deffnx {C Function} scm_hashq_create_handle_x (table, key, init)
This function looks up @var{key} in @var{table} and returns its handle.
If @var{key} is not already present, a new handle is created which
associates @var{key} with @var{init}.
@end deffn

@deffn {Scheme Procedure} hashv-create-handle! table key init
@deffnx {C Function} scm_hashv_create_handle_x (table, key, init)
This function looks up @var{key} in @var{table} and returns its handle.
If @var{key} is not already present, a new handle is created which
associates @var{key} with @var{init}.
@end deffn

@deffn {Scheme Procedure} hash-create-handle! table key init
@deffnx {C Function} scm_hash_create_handle_x (table, key, init)
This function looks up @var{key} in @var{table} and returns its handle.
If @var{key} is not already present, a new handle is created which
associates @var{key} with @var{init}.
@end deffn

@deffn {Scheme Procedure} hashx-create-handle! hash assoc table key init
@deffnx {C Function} scm_hashx_create_handle_x (hash, assoc, table, key, init)
This behaves the same way as the corresponding
@code{-create-handle} function, but uses @var{hash} as a hash
function and @var{assoc} to compare keys.  @code{hash} must be
a function that takes two arguments, a key to be hashed and a
table size.  @code{assoc} must be an associator function, like
@code{assoc}, @code{assq} or @code{assv}.
@end deffn

@deffn {Scheme Procedure} hash-fold proc init table
@deffnx {C Function} scm_hash_fold (proc, init, table)
An iterator over hash-table elements.
Accumulates and returns a result by applying PROC successively.
The arguments to PROC are "(key value prior-result)" where key
and value are successive pairs from the hash table TABLE, and
prior-result is either INIT (for the first application of PROC)
or the return value of the previous application of PROC.
For example, @code{(hash-fold acons '() tab)} will convert a hash
table into an a-list of key-value pairs.
@end deffn


@node Hooks
@section Hooks
@tpindex Hooks

@c FIXME::martin: Review me!

A hook is basically a list of procedures to be called at well defined
points in time.  Hooks are used internally for several debugging
facilities, but they can be used in user code, too.

Hooks are created with @code{make-hook}, then procedures can be added to
a hook with @code{add-hook!} or removed with @code{remove-hook!} or
@code{reset-hook!}.  The procedures stored in a hook can be invoked with
@code{run-hook}.

@menu
* Hook Examples::               Hook usage by example.
* Hook Reference::              Reference of all hook procedures.
@end menu

@node Hook Examples
@subsection Hook Examples

Hook usage is shown by some examples in this section.  First, we will
define a hook of arity 2 --- that is, the procedures stored in the hook
will have to accept two arguments.

@lisp
(define hook (make-hook 2))
hook
@result{} #<hook 2 40286c90>
@end lisp

Now we are ready to add some procedures to the newly created hook with
@code{add-hook!}.  In the following example, two procedures are added,
which print different messages and do different things with their
arguments.  When the procedures have been added, we can invoke them
using @code{run-hook}.

@lisp
(add-hook! hook (lambda (x y) 
                    (display "Foo: ") 
                    (display (+ x y)) 
                    (newline)))
(add-hook! hook (lambda (x y) 
                    (display "Bar: ") 
                    (display (* x y)) 
                    (newline)))
(run-hook hook 3 4)
@print{} Bar: 12
@print{} Foo: 7
@end lisp

Note that the procedures are called in reverse order than they were
added.  This can be changed by providing the optional third argument
on the second call to @code{add-hook!}.

@lisp
(add-hook! hook (lambda (x y) 
                    (display "Foo: ") 
                    (display (+ x y)) 
                    (newline)))
(add-hook! hook (lambda (x y) 
                    (display "Bar: ") 
                    (display (* x y)) 
                    (newline)) 
                    #t)   ; @r{<- Change here!}
(run-hook hook 3 4)
@print{} Foo: 7
@print{} Bar: 12
@end lisp

@node Hook Reference
@subsection Hook Reference

When a hook is created with @code{make-hook}, you can supply the arity
of the procedures which can be added to the hook.  The arity defaults to
zero.  All procedures of a hook must have the same arity, and when the
procedures are invoked using @code{run-hook}, the number of arguments
must match the arity of the procedures.

The order in which procedures are added to a hook matters.  If the third
parameter to @var{add-hook!} is omitted or is equal to @code{#f}, the
procedure is added in front of the procedures which might already be on
that hook, otherwise the procedure is added at the end.  The procedures
are always called from first to last when they are invoked via
@code{run-hook}.

When calling @code{hook->list}, the procedures in the resulting list are
in the same order as they would have been called by @code{run-hook}.

@deffn {Scheme Procedure} make-hook [n_args]
@deffnx {C Function} scm_make_hook (n_args)
Create a hook for storing procedure of arity @var{n_args}.
@var{n_args} defaults to zero.  The returned value is a hook
object to be used with the other hook procedures.
@end deffn

@deffn {Scheme Procedure} hook? x
@deffnx {C Function} scm_hook_p (x)
Return @code{#t} if @var{x} is a hook, @code{#f} otherwise.
@end deffn

@deffn {Scheme Procedure} hook-empty? hook
@deffnx {C Function} scm_hook_empty_p (hook)
Return @code{#t} if @var{hook} is an empty hook, @code{#f}
otherwise.
@end deffn

@deffn {Scheme Procedure} add-hook! hook proc [append_p]
@deffnx {C Function} scm_add_hook_x (hook, proc, append_p)
Add the procedure @var{proc} to the hook @var{hook}. The
procedure is added to the end if @var{append_p} is true,
otherwise it is added to the front.  The return value of this
procedure is not specified.
@end deffn

@deffn {Scheme Procedure} remove-hook! hook proc
@deffnx {C Function} scm_remove_hook_x (hook, proc)
Remove the procedure @var{proc} from the hook @var{hook}.  The
return value of this procedure is not specified.
@end deffn

@deffn {Scheme Procedure} reset-hook! hook
@deffnx {C Function} scm_reset_hook_x (hook)
Remove all procedures from the hook @var{hook}.  The return
value of this procedure is not specified.
@end deffn

@deffn {Scheme Procedure} run-hook hook . args
@deffnx {C Function} scm_run_hook (hook, args)
Apply all procedures from the hook @var{hook} to the arguments
@var{args}.  The order of the procedure application is first to
last.  The return value of this procedure is not specified.
@end deffn

@deffn {Scheme Procedure} hook->list hook
@deffnx {C Function} scm_hook_to_list (hook)
Convert the procedure list of @var{hook} to a list.
@end deffn


@node Other Data Types
@section Other Core Guile Data Types

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