guile/doc/ref/api-compound.texi

@c -*-texinfo-*-
@c This is part of the GNU Guile Reference Manual.
@c Copyright (C)  1996, 1997, 2000, 2001, 2002, 2003, 2004, 2005, 2006,
@c   2007, 2009, 2010, 2011, 2012  Free Software Foundation, Inc.
@c See the file guile.texi for copying conditions.

@node Compound Data Types
@section Compound Data Types

This chapter describes Guile's compound data types.  By @dfn{compound}
we mean that the primary purpose of these data types is to act as
containers for other kinds of data (including other compound objects).
For instance, a (non-uniform) vector with length 5 is a container that
can hold five arbitrary Scheme objects.

The various kinds of container object differ from each other in how
their memory is allocated, how they are indexed, and how particular
values can be looked up within them.

@menu
* Pairs::                       Scheme's basic building block.
* Lists::                       Special list functions supported by Guile.
* Vectors::                     One-dimensional arrays of Scheme objects.
* Bit Vectors::                 Vectors of bits.
* Generalized Vectors::         Treating all vector-like things uniformly.
* Arrays::                      Matrices, etc.
* VLists::                      Vector-like lists.
* Record Overview::             Walking through the maze of record APIs.
* SRFI-9 Records::              The standard, recommended record API.
* Records::                     Guile's historical record API.
* Structures::                  Low-level record representation.
* Dictionary Types::            About dictionary types in general.
* Association Lists::           List-based dictionaries.
* VHashes::                     VList-based dictionaries.   
* Hash Tables::                 Table-based dictionaries.
@end menu


@node Pairs
@subsection Pairs
@tpindex Pairs

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 would 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 @ref{Expression Syntax}.  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

@deftypefn {C Function} int scm_is_pair (SCM x)
Return 1 when @var{x} is a pair; otherwise return 0.
@end deftypefn

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 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
@deffnx {C Function} scm_car (pair)
@deffnx {C Function} scm_cdr (pair)
Return the car or the cdr of @var{pair}, respectively.
@end deffn

@deftypefn  {C Macro} SCM SCM_CAR (SCM pair)
@deftypefnx {C Macro} SCM SCM_CDR (SCM pair)
These two macros are the fastest way to access the car or cdr of a
pair; they can be thought of as compiling into a single memory
reference.

These macros do no checking at all.  The argument @var{pair} must be a
valid pair.
@end deftypefn

@deffn  {Scheme Procedure} cddr pair
@deffnx {Scheme Procedure} cdar pair
@deffnx {Scheme Procedure} cadr pair
@deffnx {Scheme Procedure} caar pair
@deffnx {Scheme Procedure} cdddr pair
@deffnx {Scheme Procedure} cddar pair
@deffnx {Scheme Procedure} cdadr pair
@deffnx {Scheme Procedure} cdaar pair
@deffnx {Scheme Procedure} caddr pair
@deffnx {Scheme Procedure} cadar pair
@deffnx {Scheme Procedure} caadr pair
@deffnx {Scheme Procedure} caaar pair
@deffnx {Scheme Procedure} cddddr pair
@deffnx {Scheme Procedure} cdddar pair
@deffnx {Scheme Procedure} cddadr pair
@deffnx {Scheme Procedure} cddaar pair
@deffnx {Scheme Procedure} cdaddr pair
@deffnx {Scheme Procedure} cdadar pair
@deffnx {Scheme Procedure} cdaadr pair
@deffnx {Scheme Procedure} cdaaar pair
@deffnx {Scheme Procedure} cadddr pair
@deffnx {Scheme Procedure} caddar pair
@deffnx {Scheme Procedure} cadadr pair
@deffnx {Scheme Procedure} cadaar pair
@deffnx {Scheme Procedure} caaddr pair
@deffnx {Scheme Procedure} caadar pair
@deffnx {Scheme Procedure} caaadr pair
@deffnx {Scheme Procedure} caaaar pair
@deffnx {C Function} scm_cddr (pair)
@deffnx {C Function} scm_cdar (pair)
@deffnx {C Function} scm_cadr (pair)
@deffnx {C Function} scm_caar (pair)
@deffnx {C Function} scm_cdddr (pair)
@deffnx {C Function} scm_cddar (pair)
@deffnx {C Function} scm_cdadr (pair)
@deffnx {C Function} scm_cdaar (pair)
@deffnx {C Function} scm_caddr (pair)
@deffnx {C Function} scm_cadar (pair)
@deffnx {C Function} scm_caadr (pair)
@deffnx {C Function} scm_caaar (pair)
@deffnx {C Function} scm_cddddr (pair)
@deffnx {C Function} scm_cdddar (pair)
@deffnx {C Function} scm_cddadr (pair)
@deffnx {C Function} scm_cddaar (pair)
@deffnx {C Function} scm_cdaddr (pair)
@deffnx {C Function} scm_cdadar (pair)
@deffnx {C Function} scm_cdaadr (pair)
@deffnx {C Function} scm_cdaaar (pair)
@deffnx {C Function} scm_cadddr (pair)
@deffnx {C Function} scm_caddar (pair)
@deffnx {C Function} scm_cadadr (pair)
@deffnx {C Function} scm_cadaar (pair)
@deffnx {C Function} scm_caaddr (pair)
@deffnx {C Function} scm_caadar (pair)
@deffnx {C Function} scm_caaadr (pair)
@deffnx {C Function} scm_caaaar (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

@code{cadr}, @code{caddr} and @code{cadddr} pick out the second, third
or fourth elements of a list, respectively.  SRFI-1 provides the same
under the names @code{second}, @code{third} and @code{fourth}
(@pxref{SRFI-1 Selectors}).
@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
@subsection Lists
@tpindex Lists

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
@subsubsection List Read Syntax

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 when written
(@pxref{Expression Syntax}), because they would otherwise be
mistakingly taken as procedure applications (@pxref{Simple
Invocation}).


@node List Predicates
@subsubsection List Predicates

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

@deftypefn {C Function} int scm_is_null (SCM x)
Return 1 when @var{x} is the empty list; otherwise return 0.
@end deftypefn


@node List Constructors
@subsubsection 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.

@c  C Function scm_list(rest) used to be documented here, but it's a
@c  no-op since it does nothing but return the list the caller must
@c  have already created.
@c
@deffn {Scheme Procedure} list elem @dots{}
@deffnx {C Function} scm_list_1 (elem1)
@deffnx {C Function} scm_list_2 (elem1, elem2)
@deffnx {C Function} scm_list_3 (elem1, elem2, elem3)
@deffnx {C Function} scm_list_4 (elem1, elem2, elem3, elem4)
@deffnx {C Function} scm_list_5 (elem1, elem2, elem3, elem4, elem5)
@deffnx {C Function} scm_list_n (elem1, @dots{}, elemN, @nicode{SCM_UNDEFINED})
@rnindex list
Return a new list containing elements @var{elem} @enddots{}.

@code{scm_list_n} takes a variable number of arguments, terminated by
the special @code{SCM_UNDEFINED}.  That final @code{SCM_UNDEFINED} is
not included in the list.  None of @var{elem} @dots{} can
themselves be @code{SCM_UNDEFINED}, or @code{scm_list_n} will
terminate at that point.
@end deffn

@c  C Function scm_cons_star(arg1,rest) used to be documented here,
@c  but it's not really a useful interface, since it expects the
@c  caller to have already consed up all but the first argument
@c  already.
@c
@deffn {Scheme Procedure} cons* arg1 arg2 @dots{}
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 modifies 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
@subsubsection List Selection

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 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
@subsubsection Append and Reverse

@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 lst @dots{} obj
@deffnx {Scheme Procedure} append
@deffnx {Scheme Procedure} append! lst @dots{} obj
@deffnx {Scheme Procedure} append!
@deffnx {C Function} scm_append (lstlst)
@deffnx {C Function} scm_append_x (lstlst)
Return a list comprising all the elements of lists @var{lst} @dots{}
@var{obj}.  If called with no arguments, return the empty list.

@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 last argument @var{obj} 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

@code{append} doesn't modify the given lists, but the return may share
structure with the final @var{obj}.  @code{append!} modifies the
given lists to form its return.

For @code{scm_append} and @code{scm_append_x}, @var{lstlst} is a list
of the list operands @var{lst} @dots{} @var{obj}.  That @var{lstlst}
itself is not modified or used in the return.
@end deffn

@rnindex reverse
@deffn {Scheme Procedure} reverse lst
@deffnx {Scheme Procedure} reverse! lst [newtail]
@deffnx {C Function} scm_reverse (lst)
@deffnx {C Function} scm_reverse_x (lst, newtail)
Return a list comprising the elements of @var{lst}, in reverse order.

@code{reverse} constructs a new list, @code{reverse!} modifies
@var{lst} in constructing its return.

For @code{reverse!}, the optional @var{newtail} is appended to the
result.  @var{newtail} isn't reversed, it simply becomes the list
tail.  For @code{scm_reverse_x}, the @var{newtail} parameter is
mandatory, but can be @code{SCM_EOL} if no further tail is required.
@end deffn

@node List Modification
@subsubsection 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?}.

See also SRFI-1 which has an extended @code{delete} (@ref{SRFI-1
Deleting}), and also an @code{lset-difference} which can delete
multiple @var{item}s in one call (@ref{SRFI-1 Set Operations}).
@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

@deffn {Scheme Procedure} filter pred lst
@deffnx {Scheme Procedure} filter! pred lst
Return a list containing all elements from @var{lst} which satisfy the
predicate @var{pred}.  The elements in the result list have the same
order as in @var{lst}.  The order in which @var{pred} is applied to
the list elements is not specified.

@code{filter} does not change @var{lst}, but the result may share a
tail with it.  @code{filter!} may modify @var{lst} to construct its
return.
@end deffn

@node List Searching
@subsubsection List Searching

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.

See also SRFI-1 which has an extended @code{member} function
(@ref{SRFI-1 Searching}).
@end deffn


@node List Mapping
@subsubsection List Mapping

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

See also SRFI-1 which extends these functions to take lists of unequal
lengths (@ref{SRFI-1 Fold and Map}).

@node Vectors
@subsection Vectors
@tpindex Vectors

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 vectors are the special case of one dimensional non-uniform arrays
and that most array procedures operate happily on vectors
(@pxref{Arrays}).

@menu
* Vector Syntax::               Read syntax for vectors.
* Vector Creation::             Dynamic vector creation and validation.
* Vector Accessors::            Accessing and modifying vector contents.
* Vector Accessing from C::     Ways to work with vectors from C.
* Uniform Numeric Vectors::     Vectors of unboxed numeric values.
@end menu


@node Vector Syntax
@subsubsection Read Syntax for Vectors

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

Like lists, vectors have to be quoted:

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

@node Vector Creation
@subsubsection Dynamic Vector Creation and Validation

Instead of creating a vector implicitly by using the read syntax just
described, you can create a vector dynamically by calling one of the
@code{vector} and @code{list->vector} primitives with the list of Scheme
values that you want to place into a vector.  The size of the vector
thus created is determined implicitly by the number of arguments given.

@rnindex vector
@rnindex list->vector
@deffn {Scheme Procedure} vector arg @dots{}
@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

The inverse operation is @code{vector->list}:

@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

To allocate a vector with an explicitly specified size, use
@code{make-vector}.  With this primitive you can also specify an initial
value for the vector elements (the same value for all elements, that
is):

@rnindex make-vector
@deffn {Scheme Procedure} make-vector len [fill]
@deffnx {C Function} scm_make_vector (len, fill)
Return a newly allocated vector of @var{len} 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

@deftypefn {C Function} SCM scm_c_make_vector (size_t k, SCM fill)
Like @code{scm_make_vector}, but the length is given as a @code{size_t}.
@end deftypefn

To check whether an arbitrary Scheme value @emph{is} a vector, use the
@code{vector?} primitive:

@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

@deftypefn {C Function} int scm_is_vector (SCM obj)
Return non-zero when @var{obj} is a vector, otherwise return
@code{zero}.
@end deftypefn

@node Vector Accessors
@subsubsection Accessing and Modifying Vector Contents

@code{vector-length} and @code{vector-ref} return information about a
given vector, respectively its size and the elements that are contained
in the vector.

@rnindex vector-length
@deffn {Scheme Procedure} vector-length vector
@deffnx {C Function} scm_vector_length (vector)
Return the number of elements in @var{vector} as an exact integer.
@end deffn

@deftypefn {C Function} size_t scm_c_vector_length (SCM vec)
Return the number of elements in @var{vec} as a @code{size_t}.
@end deftypefn

@rnindex vector-ref
@deffn {Scheme Procedure} vector-ref vec k
@deffnx {C Function} scm_vector_ref (vec, k)
Return the contents of position @var{k} of @var{vec}.
@var{k} must be a valid index of @var{vec}.
@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

@deftypefn {C Function} SCM scm_c_vector_ref (SCM vec, size_t k)
Return the contents of position @var{k} (a @code{size_t}) of
@var{vec}.
@end deftypefn

A vector created by one of the dynamic vector constructor procedures
(@pxref{Vector Creation}) can be modified using the following
procedures.

@emph{NOTE:} According to R5RS, it is an error to use any of these
procedures on a literally read vector, because such vectors should be
considered as constants.  Currently, however, Guile does not detect this
error.

@rnindex vector-set!
@deffn {Scheme Procedure} vector-set! vec k obj
@deffnx {C Function} scm_vector_set_x (vec, k, obj)
Store @var{obj} in position @var{k} of @var{vec}.
@var{k} must be a valid index of @var{vec}.
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

@deftypefn {C Function} void scm_c_vector_set_x (SCM vec, size_t k, SCM obj)
Store @var{obj} in position @var{k} (a @code{size_t}) of @var{vec}.
@end deftypefn

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

@deffn {Scheme Procedure} vector-copy vec
@deffnx {C Function} scm_vector_copy (vec)
Return a copy of @var{vec}.
@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

@node Vector Accessing from C
@subsubsection Vector Accessing from C

A vector can be read and modified from C with the functions
@code{scm_c_vector_ref} and @code{scm_c_vector_set_x}, for example.  In
addition to these functions, there are two more ways to access vectors
from C that might be more efficient in certain situations: you can
restrict yourself to @dfn{simple vectors} and then use the very fast
@emph{simple vector macros}; or you can use the very general framework
for accessing all kinds of arrays (@pxref{Accessing Arrays from C}),
which is more verbose, but can deal efficiently with all kinds of
vectors (and arrays).  For vectors, you can use the
@code{scm_vector_elements} and @code{scm_vector_writable_elements}
functions as shortcuts.

@deftypefn {C Function} int scm_is_simple_vector (SCM obj)
Return non-zero if @var{obj} is a simple vector, else return zero.  A
simple vector is a vector that can be used with the @code{SCM_SIMPLE_*}
macros below.

The following functions are guaranteed to return simple vectors:
@code{scm_make_vector}, @code{scm_c_make_vector}, @code{scm_vector},
@code{scm_list_to_vector}.
@end deftypefn

@deftypefn {C Macro} size_t SCM_SIMPLE_VECTOR_LENGTH (SCM vec)
Evaluates to the length of the simple vector @var{vec}.  No type
checking is done.
@end deftypefn

@deftypefn {C Macro} SCM SCM_SIMPLE_VECTOR_REF (SCM vec, size_t idx)
Evaluates to the element at position @var{idx} in the simple vector
@var{vec}.  No type or range checking is done.
@end deftypefn

@deftypefn {C Macro} void SCM_SIMPLE_VECTOR_SET (SCM vec, size_t idx, SCM val)
Sets the element at position @var{idx} in the simple vector
@var{vec} to @var{val}.  No type or range checking is done.
@end deftypefn

@deftypefn {C Function} {const SCM *} scm_vector_elements (SCM vec, scm_t_array_handle *handle, size_t *lenp, ssize_t *incp)
Acquire a handle for the vector @var{vec} and return a pointer to the
elements of it.  This pointer can only be used to read the elements of
@var{vec}.  When @var{vec} is not a vector, an error is signaled.  The
handle must eventually be released with
@code{scm_array_handle_release}.

The variables pointed to by @var{lenp} and @var{incp} are filled with
the number of elements of the vector and the increment (number of
elements) between successive elements, respectively.  Successive
elements of @var{vec} need not be contiguous in their underlying
``root vector'' returned here; hence the increment is not necessarily
equal to 1 and may well be negative too (@pxref{Shared Arrays}).

The following example shows the typical way to use this function.  It
creates a list of all elements of @var{vec} (in reverse order).

@example
scm_t_array_handle handle;
size_t i, len;
ssize_t inc;
const SCM *elt;
SCM list;

elt = scm_vector_elements (vec, &handle, &len, &inc);
list = SCM_EOL;
for (i = 0; i < len; i++, elt += inc)
  list = scm_cons (*elt, list);
scm_array_handle_release (&handle);
@end example

@end deftypefn

@deftypefn {C Function} {SCM *} scm_vector_writable_elements (SCM vec, scm_t_array_handle *handle, size_t *lenp, ssize_t *incp)
Like @code{scm_vector_elements} but the pointer can be used to modify
the vector.

The following example shows the typical way to use this function.  It
fills a vector with @code{#t}.

@example
scm_t_array_handle handle;
size_t i, len;
ssize_t inc;
SCM *elt;

elt = scm_vector_writable_elements (vec, &handle, &len, &inc);
for (i = 0; i < len; i++, elt += inc)
  *elt = SCM_BOOL_T;
scm_array_handle_release (&handle);
@end example

@end deftypefn

@node Uniform Numeric Vectors
@subsubsection Uniform Numeric Vectors

A uniform numeric vector is a vector whose elements are all of a single
numeric type.  Guile offers uniform numeric vectors for signed and
unsigned 8-bit, 16-bit, 32-bit, and 64-bit integers, two sizes of
floating point values, and complex floating-point numbers of these two
sizes. @xref{SRFI-4}, for more information.

For many purposes, bytevectors work just as well as uniform vectors, and have
the advantage that they integrate well with binary input and output.
@xref{Bytevectors}, for more information on bytevectors.

@node Bit Vectors
@subsection Bit Vectors

@noindent
Bit vectors are zero-origin, one-dimensional arrays of booleans.  They
are displayed as a sequence of @code{0}s and @code{1}s prefixed by
@code{#*}, e.g.,

@example
(make-bitvector 8 #f) @result{}
#*00000000
@end example

Bit vectors are also generalized vectors, @xref{Generalized
Vectors}, and can thus be used with the array procedures, @xref{Arrays}.
Bit vectors are the special case of one dimensional bit arrays.

@deffn {Scheme Procedure} bitvector? obj
@deffnx {C Function} scm_bitvector_p (obj)
Return @code{#t} when @var{obj} is a bitvector, else
return @code{#f}.
@end deffn

@deftypefn {C Function} int scm_is_bitvector (SCM obj)
Return @code{1} when @var{obj} is a bitvector, else return @code{0}.
@end deftypefn

@deffn {Scheme Procedure} make-bitvector len [fill]
@deffnx {C Function} scm_make_bitvector (len, fill)
Create a new bitvector of length @var{len} and
optionally initialize all elements to @var{fill}.
@end deffn

@deftypefn {C Function} SCM scm_c_make_bitvector (size_t len, SCM fill)
Like @code{scm_make_bitvector}, but the length is given as a
@code{size_t}.
@end deftypefn

@deffn {Scheme Procedure} bitvector bit @dots{}
@deffnx {C Function} scm_bitvector (bits)
Create a new bitvector with the arguments as elements.
@end deffn

@deffn {Scheme Procedure} bitvector-length vec
@deffnx {C Function} scm_bitvector_length (vec)
Return the length of the bitvector @var{vec}.
@end deffn

@deftypefn {C Function} size_t scm_c_bitvector_length (SCM vec)
Like @code{scm_bitvector_length}, but the length is returned as a
@code{size_t}.
@end deftypefn

@deffn {Scheme Procedure} bitvector-ref vec idx
@deffnx {C Function} scm_bitvector_ref (vec, idx)
Return the element at index @var{idx} of the bitvector
@var{vec}.
@end deffn

@deftypefn {C Function} SCM scm_c_bitvector_ref (SCM vec, size_t idx)
Return the element at index @var{idx} of the bitvector
@var{vec}.
@end deftypefn

@deffn {Scheme Procedure} bitvector-set! vec idx val
@deffnx {C Function} scm_bitvector_set_x (vec, idx, val)
Set the element at index @var{idx} of the bitvector
@var{vec} when @var{val} is true, else clear it.
@end deffn

@deftypefn {C Function} SCM scm_c_bitvector_set_x (SCM vec, size_t idx, SCM val)
Set the element at index @var{idx} of the bitvector
@var{vec} when @var{val} is true, else clear it.
@end deftypefn

@deffn {Scheme Procedure} bitvector-fill! vec val
@deffnx {C Function} scm_bitvector_fill_x (vec, val)
Set all elements of the bitvector
@var{vec} when @var{val} is true, else clear them.
@end deffn

@deffn {Scheme Procedure} list->bitvector list
@deffnx {C Function} scm_list_to_bitvector (list)
Return a new bitvector initialized with the elements
of @var{list}.
@end deffn

@deffn {Scheme Procedure} bitvector->list vec
@deffnx {C Function} scm_bitvector_to_list (vec)
Return a new list initialized with the elements
of the bitvector @var{vec}.
@end deffn

@deffn {Scheme Procedure} bit-count bool bitvector
@deffnx {C Function} scm_bit_count (bool, bitvector)
Return a count of how many entries in @var{bitvector} are equal to
@var{bool}.  For example,

@example
(bit-count #f #*000111000)  @result{} 6
@end example
@end deffn

@deffn {Scheme Procedure} bit-position bool bitvector start
@deffnx {C Function} scm_bit_position (bool, bitvector, start)
Return the index of the first occurrence of @var{bool} in
@var{bitvector}, starting from @var{start}.  If there is no @var{bool}
entry between @var{start} and the end of @var{bitvector}, then return
@code{#f}.  For example,

@example
(bit-position #t #*000101 0)  @result{} 3
(bit-position #f #*0001111 3) @result{} #f
@end example
@end deffn

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

@deffn {Scheme Procedure} bit-set*! bitvector uvec bool
@deffnx {C Function} scm_bit_set_star_x (bitvector, uvec, bool)
Set entries of @var{bitvector} to @var{bool}, with @var{uvec}
selecting the entries to change.  The return value is unspecified.

If @var{uvec} is a bit vector, then those entries where it has
@code{#t} are the ones in @var{bitvector} which are set to @var{bool}.
@var{uvec} and @var{bitvector} must be the same length.  When
@var{bool} is @code{#t} it's like @var{uvec} is OR'ed into
@var{bitvector}.  Or when @var{bool} is @code{#f} it can be seen as an
ANDNOT.

@example
(define bv #*01000010)
(bit-set*! bv #*10010001 #t)
bv
@result{} #*11010011
@end example

If @var{uvec} is a uniform vector of unsigned long integers, then
they're indexes into @var{bitvector} which are set to @var{bool}.  

@example
(define bv #*01000010)
(bit-set*! bv #u(5 2 7) #t)
bv
@result{} #*01100111
@end example
@end deffn

@deffn {Scheme Procedure} bit-count* bitvector uvec bool
@deffnx {C Function} scm_bit_count_star (bitvector, uvec, bool)
Return a count of how many entries in @var{bitvector} are equal to
@var{bool}, with @var{uvec} selecting the entries to consider.

@var{uvec} is interpreted in the same way as for @code{bit-set*!}
above.  Namely, if @var{uvec} is a bit vector then entries which have
@code{#t} there are considered in @var{bitvector}.  Or if @var{uvec}
is a uniform vector of unsigned long integers then it's the indexes in
@var{bitvector} to consider.

For example,

@example
(bit-count* #*01110111 #*11001101 #t) @result{} 3
(bit-count* #*01110111 #u(7 0 4) #f)  @result{} 2
@end example
@end deffn

@deftypefn {C Function} {const scm_t_uint32 *} scm_bitvector_elements (SCM vec, scm_t_array_handle *handle, size_t *offp, size_t *lenp, ssize_t *incp)
Like @code{scm_vector_elements} (@pxref{Vector Accessing from C}), but
for bitvectors.  The variable pointed to by @var{offp} is set to the
value returned by @code{scm_array_handle_bit_elements_offset}.  See
@code{scm_array_handle_bit_elements} for how to use the returned
pointer and the offset.
@end deftypefn

@deftypefn {C Function} {scm_t_uint32 *} scm_bitvector_writable_elements (SCM vec, scm_t_array_handle *handle, size_t *offp, size_t *lenp, ssize_t *incp)
Like @code{scm_bitvector_elements}, but the pointer is good for reading
and writing.
@end deftypefn

@node Generalized Vectors
@subsection Generalized Vectors

Guile has a number of data types that are generally vector-like:
strings, uniform numeric vectors, bytevectors, bitvectors, and of course
ordinary vectors of arbitrary Scheme values.  These types are disjoint:
a Scheme value belongs to at most one of the five types listed above.

If you want to gloss over this distinction and want to treat all four
types with common code, you can use the procedures in this section.
They work with the @emph{generalized vector} type, which is the union
of the five vector-like types.

@deffn {Scheme Procedure} generalized-vector? obj
@deffnx {C Function} scm_generalized_vector_p (obj)
Return @code{#t} if @var{obj} is a vector, bytevector, string,
bitvector, or uniform numeric vector.
@end deffn

@deffn {Scheme Procedure} generalized-vector-length v
@deffnx {C Function} scm_generalized_vector_length (v)
Return the length of the generalized vector @var{v}.
@end deffn

@deffn {Scheme Procedure} generalized-vector-ref v idx
@deffnx {C Function} scm_generalized_vector_ref (v, idx)
Return the element at index @var{idx} of the
generalized vector @var{v}.
@end deffn

@deffn {Scheme Procedure} generalized-vector-set! v idx val
@deffnx {C Function} scm_generalized_vector_set_x (v, idx, val)
Set the element at index @var{idx} of the
generalized vector @var{v} to @var{val}.
@end deffn

@deffn {Scheme Procedure} generalized-vector->list v
@deffnx {C Function} scm_generalized_vector_to_list (v)
Return a new list whose elements are the elements of the
generalized vector @var{v}.
@end deffn

@deftypefn {C Function} int scm_is_generalized_vector (SCM obj)
Return @code{1} if @var{obj} is a vector, string,
bitvector, or uniform numeric vector; else return @code{0}.
@end deftypefn

@deftypefn {C Function} size_t scm_c_generalized_vector_length (SCM v)
Return the length of the generalized vector @var{v}.
@end deftypefn

@deftypefn {C Function} SCM scm_c_generalized_vector_ref (SCM v, size_t idx)
Return the element at index @var{idx} of the generalized vector @var{v}.
@end deftypefn

@deftypefn {C Function} void scm_c_generalized_vector_set_x (SCM v, size_t idx, SCM val)
Set the element at index @var{idx} of the generalized vector @var{v}
to @var{val}.
@end deftypefn

@deftypefn {C Function} void scm_generalized_vector_get_handle (SCM v, scm_t_array_handle *handle)
Like @code{scm_array_get_handle} but an error is signalled when @var{v}
is not of rank one.  You can use @code{scm_array_handle_ref} and
@code{scm_array_handle_set} to read and write the elements of @var{v},
or you can use functions like @code{scm_array_handle_<foo>_elements} to
deal with specific types of vectors.
@end deftypefn

@node Arrays
@subsection Arrays
@tpindex Arrays

@dfn{Arrays} are a collection of cells organized into an arbitrary
number of dimensions.  Each cell can be accessed in constant time by
supplying an index for each dimension.

In the current implementation, an array uses a generalized vector for
the actual storage of its elements.  Any kind of generalized vector
will do, so you can have arrays of uniform numeric values, arrays of
characters, arrays of bits, and of course, arrays of arbitrary Scheme
values.  For example, arrays with an underlying @code{c64vector} might
be nice for digital signal processing, while arrays made from a
@code{u8vector} might be used to hold gray-scale images.

The number of dimensions of an array is called its @dfn{rank}.  Thus,
a matrix is an array of rank 2, while a vector has rank 1.  When
accessing an array element, you have to specify one exact integer for
each dimension.  These integers are called the @dfn{indices} of the
element.  An array specifies the allowed range of indices for each
dimension via an inclusive lower and upper bound.  These bounds can
well be negative, but the upper bound must be greater than or equal to
the lower bound minus one.  When all lower bounds of an array are
zero, it is called a @dfn{zero-origin} array.

Arrays can be of rank 0, which could be interpreted as a scalar.
Thus, a zero-rank array can store exactly one object and the list of
indices of this element is the empty list.

Arrays contain zero elements when one of their dimensions has a zero
length.  These empty arrays maintain information about their shape: a
matrix with zero columns and 3 rows is different from a matrix with 3
columns and zero rows, which again is different from a vector of
length zero.

Generalized vectors, such as strings, uniform numeric vectors,
bytevectors, bit vectors and ordinary vectors, are the special case of
one dimensional arrays.

@menu
* Array Syntax::                
* Array Procedures::            
* Shared Arrays::               
* Accessing Arrays from C::     
@end menu

@node Array Syntax
@subsubsection Array Syntax

An array is displayed as @code{#} followed by its rank, followed by a
tag that describes the underlying vector, optionally followed by
information about its shape, and finally followed by the cells,
organized into dimensions using parentheses.

In more words, the array tag is of the form

@example
  #<rank><vectag><@@lower><:len><@@lower><:len>...
@end example

where @code{<rank>} is a positive integer in decimal giving the rank of
the array.  It is omitted when the rank is 1 and the array is non-shared
and has zero-origin (see below).  For shared arrays and for a non-zero
origin, the rank is always printed even when it is 1 to distinguish
them from ordinary vectors.

The @code{<vectag>} part is the tag for a uniform numeric vector, like
@code{u8}, @code{s16}, etc, @code{b} for bitvectors, or @code{a} for
strings.  It is empty for ordinary vectors.

The @code{<@@lower>} part is a @samp{@@} character followed by a signed
integer in decimal giving the lower bound of a dimension.  There is one
@code{<@@lower>} for each dimension.  When all lower bounds are zero,
all @code{<@@lower>} parts are omitted.

The @code{<:len>} part is a @samp{:} character followed by an unsigned
integer in decimal giving the length of a dimension.  Like for the lower
bounds, there is one @code{<:len>} for each dimension, and the
@code{<:len>} part always follows the @code{<@@lower>} part for a
dimension.  Lengths are only then printed when they can't be deduced
from the nested lists of elements of the array literal, which can happen
when at least one length is zero.

As a special case, an array of rank 0 is printed as
@code{#0<vectag>(<scalar>)}, where @code{<scalar>} is the result of
printing the single element of the array.

Thus, 

@table @code
@item #(1 2 3)
is an ordinary array of rank 1 with lower bound 0 in dimension 0.
(I.e., a regular vector.)

@item #@@2(1 2 3)
is an ordinary array of rank 1 with lower bound 2 in dimension 0.

@item #2((1 2 3) (4 5 6))
is a non-uniform array of rank 2; a 3@cross{}3 matrix with index ranges 0..2
and 0..2.

@item #u32(0 1 2)
is a uniform u8 array of rank 1.

@item #2u32@@2@@3((1 2) (2 3))
is a uniform u8 array of rank 2 with index ranges 2..3 and 3..4.

@item #2()
is a two-dimensional array with index ranges 0..-1 and 0..-1, i.e.@:
both dimensions have length zero.

@item #2:0:2()
is a two-dimensional array with index ranges 0..-1 and 0..1, i.e.@: the
first dimension has length zero, but the second has length 2.

@item #0(12)
is a rank-zero array with contents 12.

@end table

In addition, bytevectors are also arrays, but use a different syntax
(@pxref{Bytevectors}):

@table @code

@item #vu8(1 2 3)
is a 3-byte long bytevector, with contents 1, 2, 3.

@end table

@node Array Procedures
@subsubsection Array Procedures

When an array is created, the range of each dimension must be
specified, e.g., to create a 2@cross{}3 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

The following procedures can be used with arrays (or vectors).  An
argument shown as @var{idx}@dots{} means one parameter for each
dimension in the array.  A @var{idxlist} argument means a list of such
values, one for each dimension.


@deffn {Scheme Procedure} array? obj
@deffnx {C Function} scm_array_p (obj, unused)
Return @code{#t} if the @var{obj} is an array, and @code{#f} if
not.

The second argument to scm_array_p is there for historical reasons,
but it is not used.  You should always pass @code{SCM_UNDEFINED} as
its value.
@end deffn

@deffn {Scheme Procedure} typed-array? obj type
@deffnx {C Function} scm_typed_array_p (obj, type)
Return @code{#t} if the @var{obj} is an array of type @var{type}, and
@code{#f} if not.
@end deffn

@deftypefn {C Function} int scm_is_array (SCM obj)
Return @code{1} if the @var{obj} is an array and @code{0} if not.
@end deftypefn

@deftypefn {C Function} int scm_is_typed_array (SCM obj, SCM type)
Return @code{0} if the @var{obj} is an array of type @var{type}, and
@code{1} if not.
@end deftypefn

@deffn {Scheme Procedure} make-array fill bound @dots{}
@deffnx {C Function} scm_make_array (fill, bounds)
Equivalent to @code{(make-typed-array #t @var{fill} @var{bound} ...)}.
@end deffn

@deffn {Scheme Procedure} make-typed-array type fill bound @dots{}
@deffnx {C Function} scm_make_typed_array (type, fill, bounds)
Create and return an array that has as many dimensions as there are
@var{bound}s and (maybe) fill it with @var{fill}.

The underlying storage vector is created according to @var{type},
which must be a symbol whose name is the `vectag' of the array as
explained above, or @code{#t} for ordinary, non-specialized arrays.

For example, using the symbol @code{f64} for @var{type} will create an
array that uses a @code{f64vector} for storing its elements, and
@code{a} will use a string.

When @var{fill} is not the special @emph{unspecified} value, the new
array is filled with @var{fill}.  Otherwise, the initial contents of
the array is unspecified.  The special @emph{unspecified} value is
stored in the variable @code{*unspecified*} so that for example
@code{(make-typed-array 'u32 *unspecified* 4)} creates a uninitialized
@code{u32} vector of length 4.

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

@deffn {Scheme Procedure} list->array dimspec list
Equivalent to @code{(list->typed-array #t @var{dimspec}
@var{list})}.
@end deffn

@deffn {Scheme Procedure} list->typed-array type dimspec list
@deffnx {C Function} scm_list_to_typed_array (type, dimspec, list)
Return an array of the type indicated by @var{type} with elements the
same as those of @var{list}.

The argument @var{dimspec} determines the number of dimensions of the
array and their lower bounds.  When @var{dimspec} is an exact integer,
it gives the number of dimensions directly and all lower bounds are
zero.  When it is a list of exact integers, then each element is the
lower index bound of a dimension, and there will be as many dimensions
as elements in the list.
@end deffn

@deffn {Scheme Procedure} array-type array
Return the type of @var{array}.  This is the `vectag' used for
printing @var{array} (or @code{#t} for ordinary arrays) and can be
used with @code{make-typed-array} to create an array of the same kind
as @var{array}.
@end deffn

@deffn {Scheme Procedure} array-ref array idx @dots{}
Return the element at @code{(idx @dots{})} in @var{array}.

@example
(define a (make-array 999 '(1 2) '(3 4)))
(array-ref a 2 4) @result{} 999
@end example
@end deffn

@deffn {Scheme Procedure} array-in-bounds? array idx @dots{}
@deffnx {C Function} scm_array_in_bounds_p (array, idxlist)
Return @code{#t} if the given index would be acceptable to
@code{array-ref}.

@example
(define a (make-array #f '(1 2) '(3 4)))
(array-in-bounds? a 2 3) @result{} #t
(array-in-bounds? a 0 0) @result{} #f
@end example
@end deffn

@deffn {Scheme Procedure} array-set! array obj idx @dots{}
@deffnx {C Function} scm_array_set_x (array, obj, idxlist)
Set the element at @code{(idx @dots{})} in @var{array} to @var{obj}.
The return value is unspecified.

@example
(define a (make-array #f '(0 1) '(0 1)))
(array-set! a #t 1 1)
a @result{} #2((#f #f) (#f #t))
@end example
@end deffn

@deffn {Scheme Procedure} array-shape array
@deffnx {Scheme Procedure} array-dimensions array
@deffnx {C Function} scm_array_dimensions (array)
Return a list of the bounds for each dimension of @var{array}.

@code{array-shape} gives @code{(@var{lower} @var{upper})} for each
dimension.  @code{array-dimensions} instead returns just
@math{@var{upper}+1} for dimensions with a 0 lower bound.  Both are
suitable as input to @code{make-array}.

For example,

@example
(define a (make-array 'foo '(-1 3) 5))
(array-shape a)      @result{} ((-1 3) (0 4))
(array-dimensions a) @result{} ((-1 3) 5)
@end example
@end deffn

@deffn {Scheme Procedure} array-rank array
@deffnx {C Function} scm_array_rank (array)
Return the rank of @var{array}.
@end deffn

@deftypefn {C Function} size_t scm_c_array_rank (SCM array)
Return the rank of @var{array} as a @code{size_t}.
@end deftypefn

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

@c  FIXME: Describe how the order affects the copying (it matters for
@c  shared arrays with the same underlying root vector, presumably).
@c
@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{src} to the corresponding
element of @var{dst}.  @var{dst} must have the same rank as @var{src},
and be at least as large in each dimension.  The return value is
unspecified.
@end deffn

@deffn {Scheme Procedure} array-fill! array fill
@deffnx {C Function} scm_array_fill_x (array, 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? array @dots{}
Return @code{#t} if 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?} (@pxref{Equality}) in that all arguments must be arrays.
@end deffn

@c  FIXME: array-map! accepts no source arrays at all, and in that
@c  case makes calls "(proc)".  Is that meant to be a documented
@c  feature?
@c
@c  FIXME: array-for-each doesn't say what happens if the sources have
@c  different index ranges.  The code currently iterates over the
@c  indices of the first and expects the others to cover those.  That
@c  at least vaguely matches array-map!, but is it meant to be a
@c  documented feature?

@deffn {Scheme Procedure} array-map! dst proc src @dots{}
@deffnx {Scheme Procedure} array-map-in-order! dst proc src1 @dots{} srcN
@deffnx {C Function} scm_array_map_x (dst, proc, srclist)
Set each element of the @var{dst} array to values obtained from calls
to @var{proc}.  The value returned is unspecified.

Each call is @code{(@var{proc} @var{elem1} @dots{} @var{elemN})},
where each @var{elem} is from the corresponding @var{src} array, at
the @var{dst} index.  @code{array-map-in-order!} makes the calls in
row-major order, @code{array-map!} makes them in an unspecified order.

The @var{src} arrays must have the same number of dimensions as
@var{dst}, and must have a range for each dimension which covers the
range in @var{dst}.  This ensures all @var{dst} indices are valid in
each @var{src}.
@end deffn

@deffn {Scheme Procedure} array-for-each proc src1 src2 @dots{}
@deffnx {C Function} scm_array_for_each (proc, src1, srclist)
Apply @var{proc} to each tuple of elements of @var{src1} @var{src2}
@dots{}, in row-major order.  The value returned is unspecified.
@end deffn

@deffn {Scheme Procedure} array-index-map! dst proc
@deffnx {C Function} scm_array_index_map_x (dst, proc)
Set each element of the @var{dst} array to values returned by calls to
@var{proc}.  The value returned is unspecified.

Each call is @code{(@var{proc} @var{i1} @dots{} @var{iN})}, where
@var{i1}@dots{}@var{iN} is the destination index, one parameter for
each dimension.  The order in which the calls are made is unspecified.

For example, to create a @m{4\times4, 4x4} matrix representing a
cyclic group,

@tex
\advance\leftskip by 2\lispnarrowing {
$\left(\matrix{%
0 & 1 & 2 & 3 \cr
1 & 2 & 3 & 0 \cr
2 & 3 & 0 & 1 \cr
3 & 0 & 1 & 2 \cr
}\right)$} \par
@end tex
@ifnottex
@example
    / 0 1 2 3 \
    | 1 2 3 0 |
    | 2 3 0 1 |
    \ 3 0 1 2 /
@end example
@end ifnottex

@example
(define a (make-array #f 4 4))
(array-index-map! a (lambda (i j)
                      (modulo (+ i j) 4)))
@end example
@end deffn

@deffn {Scheme Procedure} uniform-array-read! ra [port_or_fd [start [end]]]
@deffnx {C Function} scm_uniform_array_read_x (ra, port_or_fd, start, end)
Attempt to read all elements of array @var{ra}, in lexicographic order, as
binary objects from @var{port_or_fd}.
If an end of file is encountered,
the objects up to that point are put into @var{ra}
(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_fd} 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 ra [port_or_fd [start [end]]]
@deffnx {C Function} scm_uniform_array_write (ra, port_or_fd, start, end)
Writes all elements of @var{ra} as binary objects to
@var{port_or_fd}.

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_fd} may be
omitted, in which case it defaults to the value returned by
@code{(current-output-port)}.
@end deffn

@node Shared Arrays
@subsubsection Shared Arrays

@deffn {Scheme Procedure} make-shared-array oldarray mapfunc bound @dots{}
@deffnx {C Function} scm_make_shared_array (oldarray, mapfunc, boundlist)
Return a new array which shares the storage of @var{oldarray}.
Changes made through either affect the same underlying storage.  The
@var{bound} @dots{} arguments are the shape of the new array, the same
as @code{make-array} (@pxref{Array Procedures}).

@var{mapfunc} translates coordinates from the new array to the
@var{oldarray}.  It's called as @code{(@var{mapfunc} newidx1 @dots{})}
with one parameter for each dimension of the new array, and should
return a list of indices for @var{oldarray}, one for each dimension of
@var{oldarray}.

@var{mapfunc} must be affine linear, meaning that each @var{oldarray}
index must be formed by adding integer multiples (possibly negative)
of some or all of @var{newidx1} etc, plus a possible integer offset.
The multiples and offset must be the same in each call.

@sp 1
One good use for a shared array is to restrict the range of some
dimensions, so as to apply say @code{array-for-each} or
@code{array-fill!} to only part of an array.  The plain @code{list}
function can be used for @var{mapfunc} in this case, making no changes
to the index values.  For example,

@example
(make-shared-array #2((a b c) (d e f) (g h i)) list 3 2)
@result{} #2((a b) (d e) (g h))
@end example

The new array can have fewer dimensions than @var{oldarray}, for
example to take a column from an array.

@example
(make-shared-array #2((a b c) (d e f) (g h i))
                   (lambda (i) (list i 2))
                   '(0 2))
@result{} #1(c f i)
@end example

A diagonal can be taken by using the single new array index for both
row and column in the old array.  For example,

@example
(make-shared-array #2((a b c) (d e f) (g h i))
                   (lambda (i) (list i i))
                   '(0 2))
@result{} #1(a e i)
@end example

Dimensions can be increased by for instance considering portions of a
one dimensional array as rows in a two dimensional array.
(@code{array-contents} below can do the opposite, flattening an
array.)

@example
(make-shared-array #1(a b c d e f g h i j k l)
                   (lambda (i j) (list (+ (* i 3) j)))
                   4 3)
@result{} #2((a b c) (d e f) (g h i) (j k l))
@end example

By negating an index the order that elements appear can be reversed.
The following just reverses the column order,

@example
(make-shared-array #2((a b c) (d e f) (g h i))
                   (lambda (i j) (list i (- 2 j)))
                   3 3)
@result{} #2((c b a) (f e d) (i h g))
@end example

A fixed offset on indexes allows for instance a change from a 0 based
to a 1 based array,

@example
(define x #2((a b c) (d e f) (g h i)))
(define y (make-shared-array x
                             (lambda (i j) (list (1- i) (1- j)))
                             '(1 3) '(1 3)))
(array-ref x 0 0) @result{} a
(array-ref y 1 1) @result{} a
@end example

A multiple on an index allows every Nth element of an array to be
taken.  The following is every third element,

@example
(make-shared-array #1(a b c d e f g h i j k l)
                   (lambda (i) (list (* i 3)))
                   4)
@result{} #1(a d g j)
@end example

The above examples can be combined to make weird and wonderful
selections from an array, but it's important to note that because
@var{mapfunc} must be affine linear, arbitrary permutations are not
possible.

In the current implementation, @var{mapfunc} is not called for every
access to the new array but only on some sample points to establish a
base and stride for new array indices in @var{oldarray} data.  A few
sample points are enough because @var{mapfunc} is linear.
@end deffn

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

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

@deffn {Scheme Procedure} shared-array-root array
@deffnx {C Function} scm_shared_array_root (array)
Return the root vector of a shared array.
@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 @code{make-array} and
@code{make-typed-array} may be unrolled, some arrays made by
@code{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

@deffn {Scheme Procedure} transpose-array array dim1 dim2 @dots{}
@deffnx {C Function} scm_transpose_array (array, dimlist)
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{dim1}, @var{dim2}, @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{dim1}, @var{dim2}, @dots{} correspond to
dimensions in the array to be returned, and 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

@node Accessing Arrays from C
@subsubsection Accessing Arrays from C

For interworking with external C code, Guile provides an API to allow C
code to access the elements of a Scheme array.  In particular, for
uniform numeric arrays, the API exposes the underlying uniform data as a
C array of numbers of the relevant type.

While pointers to the elements of an array are in use, the array itself
must be protected so that the pointer remains valid.  Such a protected
array is said to be @dfn{reserved}.  A reserved array can be read but
modifications to it that would cause the pointer to its elements to
become invalid are prevented.  When you attempt such a modification, an
error is signalled.

(This is similar to locking the array while it is in use, but without
the danger of a deadlock.  In a multi-threaded program, you will need
additional synchronization to avoid modifying reserved arrays.)

You must take care to always unreserve an array after reserving it,
even in the presence of non-local exits.  If a non-local exit can
happen between these two calls, you should install a dynwind context
that releases the array when it is left (@pxref{Dynamic Wind}).

In addition, array reserving and unreserving must be properly
paired.  For instance, when reserving two or more arrays in a certain
order, you need to unreserve them in the opposite order.

Once you have reserved an array and have retrieved the pointer to its
elements, you must figure out the layout of the elements in memory.
Guile allows slices to be taken out of arrays without actually making a
copy, such as making an alias for the diagonal of a matrix that can be
treated as a vector.  Arrays that result from such an operation are not
stored contiguously in memory and when working with their elements
directly, you need to take this into account.

The layout of array elements in memory can be defined via a
@emph{mapping function} that computes a scalar position from a vector of
indices.  The scalar position then is the offset of the element with the
given indices from the start of the storage block of the array.

In Guile, this mapping function is restricted to be @dfn{affine}: all
mapping functions of Guile arrays can be written as @code{p = b +
c[0]*i[0] + c[1]*i[1] + ... + c[n-1]*i[n-1]} where @code{i[k]} is the
@nicode{k}th index and @code{n} is the rank of the array.  For
example, a matrix of size 3x3 would have @code{b == 0}, @code{c[0] ==
3} and @code{c[1] == 1}.  When you transpose this matrix (with
@code{transpose-array}, say), you will get an array whose mapping
function has @code{b == 0}, @code{c[0] == 1} and @code{c[1] == 3}.

The function @code{scm_array_handle_dims} gives you (indirect) access to
the coefficients @code{c[k]}.

@c XXX
Note that there are no functions for accessing the elements of a
character array yet.  Once the string implementation of Guile has been
changed to use Unicode, we will provide them.

@deftp {C Type} scm_t_array_handle
This is a structure type that holds all information necessary to manage
the reservation of arrays as explained above.  Structures of this type
must be allocated on the stack and must only be accessed by the
functions listed below.
@end deftp

@deftypefn {C Function} void scm_array_get_handle (SCM array, scm_t_array_handle *handle)
Reserve @var{array}, which must be an array, and prepare @var{handle} to
be used with the functions below.  You must eventually call
@code{scm_array_handle_release} on @var{handle}, and do this in a
properly nested fashion, as explained above.  The structure pointed to
by @var{handle} does not need to be initialized before calling this
function.
@end deftypefn

@deftypefn {C Function} void scm_array_handle_release (scm_t_array_handle *handle)
End the array reservation represented by @var{handle}.  After a call to
this function, @var{handle} might be used for another reservation.
@end deftypefn

@deftypefn {C Function} size_t scm_array_handle_rank (scm_t_array_handle *handle)
Return the rank of the array represented by @var{handle}.
@end deftypefn

@deftp {C Type} scm_t_array_dim
This structure type holds information about the layout of one dimension
of an array.  It includes the following fields:

@table @code
@item  ssize_t lbnd
@itemx ssize_t ubnd
The lower and upper bounds (both inclusive) of the permissible index
range for the given dimension.  Both values can be negative, but
@var{lbnd} is always less than or equal to @var{ubnd}.

@item ssize_t inc
The distance from one element of this dimension to the next.  Note, too,
that this can be negative.
@end table
@end deftp

@deftypefn {C Function} {const scm_t_array_dim *} scm_array_handle_dims (scm_t_array_handle *handle)
Return a pointer to a C vector of information about the dimensions of
the array represented by @var{handle}.  This pointer is valid as long as
the array remains reserved.  As explained above, the
@code{scm_t_array_dim} structures returned by this function can be used
calculate the position of an element in the storage block of the array
from its indices.

This position can then be used as an index into the C array pointer
returned by the various @code{scm_array_handle_<foo>_elements}
functions, or with @code{scm_array_handle_ref} and
@code{scm_array_handle_set}.

Here is how one can compute the position @var{pos} of an element given
its indices in the vector @var{indices}:

@example
ssize_t indices[RANK];
scm_t_array_dim *dims;
ssize_t pos;
size_t i;

pos = 0;
for (i = 0; i < RANK; i++)
  @{
    if (indices[i] < dims[i].lbnd || indices[i] > dims[i].ubnd)
      out_of_range ();
    pos += (indices[i] - dims[i].lbnd) * dims[i].inc;
  @}
@end example
@end deftypefn

@deftypefn {C Function} ssize_t scm_array_handle_pos (scm_t_array_handle *handle, SCM indices)
Compute the position corresponding to @var{indices}, a list of
indices.  The position is computed as described above for
@code{scm_array_handle_dims}.  The number of the indices and their
range is checked and an appropriate error is signalled for invalid
indices.
@end deftypefn

@deftypefn {C Function} SCM scm_array_handle_ref (scm_t_array_handle *handle, ssize_t pos)
Return the element at position @var{pos} in the storage block of the
array represented by @var{handle}.  Any kind of array is acceptable.  No
range checking is done on @var{pos}.
@end deftypefn

@deftypefn {C Function} void scm_array_handle_set (scm_t_array_handle *handle, ssize_t pos, SCM val)
Set the element at position @var{pos} in the storage block of the array
represented by @var{handle} to @var{val}.  Any kind of array is
acceptable.  No range checking is done on @var{pos}.  An error is
signalled when the array can not store @var{val}.
@end deftypefn

@deftypefn {C Function} {const SCM *} scm_array_handle_elements (scm_t_array_handle *handle)
Return a pointer to the elements of a ordinary array of general Scheme
values (i.e., a non-uniform array) for reading.  This pointer is valid
as long as the array remains reserved.
@end deftypefn

@deftypefn {C Function} {SCM *} scm_array_handle_writable_elements (scm_t_array_handle *handle)
Like @code{scm_array_handle_elements}, but the pointer is good for
reading and writing.
@end deftypefn

@deftypefn {C Function} {const void *} scm_array_handle_uniform_elements (scm_t_array_handle *handle)
Return a pointer to the elements of a uniform numeric array for reading.
This pointer is valid as long as the array remains reserved.  The size
of each element is given by @code{scm_array_handle_uniform_element_size}.
@end deftypefn

@deftypefn {C Function} {void *} scm_array_handle_uniform_writable_elements (scm_t_array_handle *handle)
Like @code{scm_array_handle_uniform_elements}, but the pointer is good
reading and writing.
@end deftypefn

@deftypefn {C Function} size_t scm_array_handle_uniform_element_size (scm_t_array_handle *handle)
Return the size of one element of the uniform numeric array represented
by @var{handle}.
@end deftypefn

@deftypefn  {C Function} {const scm_t_uint8 *} scm_array_handle_u8_elements (scm_t_array_handle *handle)
@deftypefnx {C Function} {const scm_t_int8 *} scm_array_handle_s8_elements (scm_t_array_handle *handle)
@deftypefnx {C Function} {const scm_t_uint16 *} scm_array_handle_u16_elements (scm_t_array_handle *handle)
@deftypefnx {C Function} {const scm_t_int16 *} scm_array_handle_s16_elements (scm_t_array_handle *handle)
@deftypefnx {C Function} {const scm_t_uint32 *} scm_array_handle_u32_elements (scm_t_array_handle *handle)
@deftypefnx {C Function} {const scm_t_int32 *} scm_array_handle_s32_elements (scm_t_array_handle *handle)
@deftypefnx {C Function} {const scm_t_uint64 *} scm_array_handle_u64_elements (scm_t_array_handle *handle)
@deftypefnx {C Function} {const scm_t_int64 *} scm_array_handle_s64_elements (scm_t_array_handle *handle)
@deftypefnx {C Function} {const float *} scm_array_handle_f32_elements (scm_t_array_handle *handle)
@deftypefnx {C Function} {const double *} scm_array_handle_f64_elements (scm_t_array_handle *handle)
@deftypefnx {C Function} {const float *} scm_array_handle_c32_elements (scm_t_array_handle *handle)
@deftypefnx {C Function} {const double *} scm_array_handle_c64_elements (scm_t_array_handle *handle)
Return a pointer to the elements of a uniform numeric array of the
indicated kind for reading.  This pointer is valid as long as the array
remains reserved.

The pointers for @code{c32} and @code{c64} uniform numeric arrays point
to pairs of floating point numbers.  The even index holds the real part,
the odd index the imaginary part of the complex number.
@end deftypefn

@deftypefn {C Function} {scm_t_uint8 *} scm_array_handle_u8_writable_elements (scm_t_array_handle *handle)
@deftypefnx {C Function} {scm_t_int8 *} scm_array_handle_s8_writable_elements (scm_t_array_handle *handle)
@deftypefnx {C Function} {scm_t_uint16 *} scm_array_handle_u16_writable_elements (scm_t_array_handle *handle)
@deftypefnx {C Function} {scm_t_int16 *} scm_array_handle_s16_writable_elements (scm_t_array_handle *handle)
@deftypefnx {C Function} {scm_t_uint32 *} scm_array_handle_u32_writable_elements (scm_t_array_handle *handle)
@deftypefnx {C Function} {scm_t_int32 *} scm_array_handle_s32_writable_elements (scm_t_array_handle *handle)
@deftypefnx {C Function} {scm_t_uint64 *} scm_array_handle_u64_writable_elements (scm_t_array_handle *handle)
@deftypefnx {C Function} {scm_t_int64 *} scm_array_handle_s64_writable_elements (scm_t_array_handle *handle)
@deftypefnx {C Function} {float *} scm_array_handle_f32_writable_elements (scm_t_array_handle *handle)
@deftypefnx {C Function} {double *} scm_array_handle_f64_writable_elements (scm_t_array_handle *handle)
@deftypefnx {C Function} {float *} scm_array_handle_c32_writable_elements (scm_t_array_handle *handle)
@deftypefnx {C Function} {double *} scm_array_handle_c64_writable_elements (scm_t_array_handle *handle)
Like @code{scm_array_handle_<kind>_elements}, but the pointer is good
for reading and writing.
@end deftypefn

@deftypefn {C Function} {const scm_t_uint32 *} scm_array_handle_bit_elements (scm_t_array_handle *handle)
Return a pointer to the words that store the bits of the represented
array, which must be a bit array.

Unlike other arrays, bit arrays have an additional offset that must be
figured into index calculations.  That offset is returned by
@code{scm_array_handle_bit_elements_offset}.

To find a certain bit you first need to calculate its position as
explained above for @code{scm_array_handle_dims} and then add the
offset.  This gives the absolute position of the bit, which is always a
non-negative integer.

Each word of the bit array storage block contains exactly 32 bits, with
the least significant bit in that word having the lowest absolute
position number.  The next word contains the next 32 bits.

Thus, the following code can be used to access a bit whose position
according to @code{scm_array_handle_dims} is given in @var{pos}:

@example
SCM bit_array;
scm_t_array_handle handle;
scm_t_uint32 *bits;
ssize_t pos;
size_t abs_pos;
size_t word_pos, mask;

scm_array_get_handle (&bit_array, &handle);
bits = scm_array_handle_bit_elements (&handle);

pos = ...
abs_pos = pos + scm_array_handle_bit_elements_offset (&handle);
word_pos = abs_pos / 32;
mask = 1L << (abs_pos % 32);

if (bits[word_pos] & mask)
  /* bit is set. */

scm_array_handle_release (&handle);
@end example

@end deftypefn

@deftypefn {C Function} {scm_t_uint32 *} scm_array_handle_bit_writable_elements (scm_t_array_handle *handle)
Like @code{scm_array_handle_bit_elements} but the pointer is good for
reading and writing.  You must take care not to modify bits outside of
the allowed index range of the array, even for contiguous arrays.
@end deftypefn

@node VLists
@subsection VLists

@cindex vlist

The @code{(ice-9 vlist)} module provides an implementation of the @dfn{VList}
data structure designed by Phil Bagwell in 2002.  VLists are immutable lists,
which can contain any Scheme object.  They improve on standard Scheme linked
lists in several areas:

@itemize
@item
Random access has typically constant-time complexity.

@item
Computing the length of a VList has time complexity logarithmic in the number of
elements.

@item
VLists use less storage space than standard lists.

@item
VList elements are stored in contiguous regions, which improves memory locality
and leads to more efficient use of hardware caches.
@end itemize

The idea behind VLists is to store vlist elements in increasingly large
contiguous blocks (implemented as vectors here).  These blocks are linked to one
another using a pointer to the next block and an offset within that block.  The
size of these blocks form a geometric series with ratio
@code{block-growth-factor} (2 by default).

The VList structure also serves as the basis for the @dfn{VList-based hash
lists} or ``vhashes'', an immutable dictionary type (@pxref{VHashes}).

However, the current implementation in @code{(ice-9 vlist)} has several
noteworthy shortcomings:

@itemize

@item
It is @emph{not} thread-safe.  Although operations on vlists are all
@dfn{referentially transparent} (i.e., purely functional), adding elements to a
vlist with @code{vlist-cons} mutates part of its internal structure, which makes
it non-thread-safe.  This could be fixed, but it would slow down
@code{vlist-cons}.

@item
@code{vlist-cons} always allocates at least as much memory as @code{cons}.
Again, Phil Bagwell describes how to fix it, but that would require tuning the
garbage collector in a way that may not be generally beneficial.

@item
@code{vlist-cons} is a Scheme procedure compiled to bytecode, and it does not
compete with the straightforward C implementation of @code{cons}, and with the
fact that the VM has a special @code{cons} instruction.

@end itemize

We hope to address these in the future.

The programming interface exported by @code{(ice-9 vlist)} is defined below.
Most of it is the same as SRFI-1 with an added @code{vlist-} prefix to function
names.

@deffn {Scheme Procedure} vlist? obj
Return true if @var{obj} is a VList.
@end deffn

@defvr {Scheme Variable} vlist-null
The empty VList.  Note that it's possible to create an empty VList not
@code{eq?} to @code{vlist-null}; thus, callers should always use
@code{vlist-null?} when testing whether a VList is empty.
@end defvr

@deffn {Scheme Procedure} vlist-null? vlist
Return true if @var{vlist} is empty.
@end deffn

@deffn {Scheme Procedure} vlist-cons item vlist
Return a new vlist with @var{item} as its head and @var{vlist} as its tail.
@end deffn

@deffn {Scheme Procedure} vlist-head vlist
Return the head of @var{vlist}.
@end deffn

@deffn {Scheme Procedure} vlist-tail vlist
Return the tail of @var{vlist}.
@end deffn

@defvr {Scheme Variable} block-growth-factor
A fluid that defines the growth factor of VList blocks, 2 by default.
@end defvr

The functions below provide the usual set of higher-level list operations.

@deffn {Scheme Procedure} vlist-fold proc init vlist
@deffnx {Scheme Procedure} vlist-fold-right proc init vlist
Fold over @var{vlist}, calling @var{proc} for each element, as for SRFI-1
@code{fold} and @code{fold-right} (@pxref{SRFI-1, @code{fold}}).
@end deffn

@deffn {Scheme Procedure} vlist-ref vlist index
Return the element at index @var{index} in @var{vlist}.  This is typically a
constant-time operation.
@end deffn

@deffn {Scheme Procedure} vlist-length vlist
Return the length of @var{vlist}.  This is typically logarithmic in the number
of elements in @var{vlist}.
@end deffn

@deffn {Scheme Procedure} vlist-reverse vlist
Return a new @var{vlist} whose content are those of @var{vlist} in reverse
order.
@end deffn

@deffn {Scheme Procedure} vlist-map proc vlist
Map @var{proc} over the elements of @var{vlist} and return a new vlist.
@end deffn

@deffn {Scheme Procedure} vlist-for-each proc vlist
Call @var{proc} on each element of @var{vlist}.  The result is unspecified.
@end deffn

@deffn {Scheme Procedure} vlist-drop vlist count
Return a new vlist that does not contain the @var{count} first elements of
@var{vlist}.  This is typically a constant-time operation.
@end deffn

@deffn {Scheme Procedure} vlist-take vlist count
Return a new vlist that contains only the @var{count} first elements of
@var{vlist}.
@end deffn

@deffn {Scheme Procedure} vlist-filter pred vlist
Return a new vlist containing all the elements from @var{vlist} that satisfy
@var{pred}.
@end deffn

@deffn {Scheme Procedure} vlist-delete x vlist [equal?]
Return a new vlist corresponding to @var{vlist} without the elements
@var{equal?} to @var{x}.
@end deffn

@deffn {Scheme Procedure} vlist-unfold p f g seed [tail-gen]
@deffnx {Scheme Procedure} vlist-unfold-right p f g seed [tail]
Return a new vlist, as for SRFI-1 @code{unfold} and @code{unfold-right}
(@pxref{SRFI-1, @code{unfold}}).
@end deffn

@deffn {Scheme Procedure} vlist-append vlist @dots{}
Append the given vlists and return the resulting vlist.
@end deffn

@deffn {Scheme Procedure} list->vlist lst
Return a new vlist whose contents correspond to @var{lst}.
@end deffn

@deffn {Scheme Procedure} vlist->list vlist
Return a new list whose contents match those of @var{vlist}.
@end deffn

@node Record Overview
@subsection Record Overview

@cindex record
@cindex structure

@dfn{Records}, also called @dfn{structures}, are Scheme's primary
mechanism to define new disjoint types.  A @dfn{record type} defines a
list of @dfn{fields} that instances of the type consist of.  This is like
C's @code{struct}.

Historically, Guile has offered several different ways to define record
types and to create records, offering different features, and making
different trade-offs.  Over the years, each ``standard'' has also come
with its own new record interface, leading to a maze of record APIs.

At the highest level is SRFI-9, a high-level record interface
implemented by most Scheme implementations (@pxref{SRFI-9}).  It defines
a simple and efficient syntactic abstraction of record types and their
associated type predicate, fields, and field accessors.  SRFI-9 is
suitable for most uses, and this is the recommended way to create record
types in Guile.  Similar high-level record APIs include SRFI-35
(@pxref{SRFI-35}) and R6RS records (@pxref{rnrs records syntactic}).

Then comes Guile's historical ``records'' API (@pxref{Records}).  Record
types defined this way are first-class objects.  Introspection
facilities are available, allowing users to query the list of fields or
the value of a specific field at run-time, without prior knowledge of
the type.

Finally, the common denominator of these interfaces is Guile's
@dfn{structure} API (@pxref{Structures}).  Guile's structures are the
low-level building block for all other record APIs.  Application writers
will normally not need to use it.

Records created with these APIs may all be pattern-matched using Guile's
standard pattern matcher (@pxref{Pattern Matching}).


@node SRFI-9 Records
@subsection SRFI-9 Records

@cindex SRFI-9
@cindex record

SRFI-9 standardizes a syntax for defining new record types and creating
predicate, constructor, and field getter and setter functions.  In Guile
this is the recommended option to create new record types (@pxref{Record
Overview}).  It can be used with:

@example
(use-modules (srfi srfi-9))
@end example

@deffn {library syntax} define-record-type type @* (constructor fieldname @dots{}) @* predicate @* (fieldname accessor [modifier]) @dots{}
@sp 1
Create a new record type, and make various @code{define}s for using
it.  This syntax can only occur at the top-level, not nested within
some other form.

@var{type} is bound to the record type, which is as per the return
from the core @code{make-record-type}.  @var{type} also provides the
name for the record, as per @code{record-type-name}.

@var{constructor} is bound to a function to be called as
@code{(@var{constructor} fieldval @dots{})} to create a new record of
this type.  The arguments are initial values for the fields, one
argument for each field, in the order they appear in the
@code{define-record-type} form.

The @var{fieldname}s provide the names for the record fields, as per
the core @code{record-type-fields} etc, and are referred to in the
subsequent accessor/modifier forms.

@var{predicate} is bound to a function to be called as
@code{(@var{predicate} obj)}.  It returns @code{#t} or @code{#f}
according to whether @var{obj} is a record of this type.

Each @var{accessor} is bound to a function to be called
@code{(@var{accessor} record)} to retrieve the respective field from a
@var{record}.  Similarly each @var{modifier} is bound to a function to
be called @code{(@var{modifier} record val)} to set the respective
field in a @var{record}.
@end deffn

@noindent
An example will illustrate typical usage,

@example
(define-record-type employee-type
  (make-employee name age salary)
  employee?
  (name    get-employee-name)
  (age     get-employee-age    set-employee-age)
  (salary  get-employee-salary set-employee-salary))
@end example

This creates a new employee data type, with name, age and salary
fields.  Accessor functions are created for each field, but no
modifier function for the name (the intention in this example being
that it's established only when an employee object is created).  These
can all then be used as for example,

@example
employee-type @result{} #<record-type employee-type>

(define fred (make-employee "Fred" 45 20000.00))

(employee? fred)        @result{} #t
(get-employee-age fred) @result{} 45
(set-employee-salary fred 25000.00)  ;; pay rise
@end example

The functions created by @code{define-record-type} are ordinary
top-level @code{define}s.  They can be redefined or @code{set!} as
desired, exported from a module, etc.

@unnumberedsubsubsec Non-toplevel Record Definitions

The SRFI-9 specification explicitly disallows record definitions in a
non-toplevel context, such as inside @code{lambda} body or inside a
@var{let} block.  However, Guile's implementation does not enforce that
restriction.

@unnumberedsubsubsec Custom Printers

You may use @code{set-record-type-printer!} to customize the default printing
behavior of records.  This is a Guile extension and is not part of SRFI-9.  It
is located in the @nicode{(srfi srfi-9 gnu)} module.

@deffn {Scheme Syntax} set-record-type-printer! name proc
Where @var{type} corresponds to the first argument of @code{define-record-type},
and @var{proc} is a procedure accepting two arguments, the record to print, and
an output port.
@end deffn

@noindent
This example prints the employee's name in brackets, for instance @code{[Fred]}.

@example
(set-record-type-printer! employee-type
  (lambda (record port)
    (write-char #\[ port)
    (display (get-employee-name record) port)
    (write-char #\] port)))
@end example

@unnumberedsubsubsec Functional ``Setters''

@cindex functional setters

When writing code in a functional style, it is desirable to never alter
the contents of records.  For such code, a simple way to return new
record instances based on existing ones is highly desirable.

The @code{(srfi srfi-9 gnu)} module extends SRFI-9 with facilities to
return new record instances based on existing ones, only with one or
more field values changed---@dfn{functional setters}.  First, the
@code{define-immutable-record-type} works like
@code{define-record-type}, except that fields are immutable and setters
are defined as functional setters.

@deffn {Scheme Syntax} define-immutable-record-type type @* (constructor fieldname @dots{}) @* predicate @* (fieldname accessor [modifier]) @dots{}
Define @var{type} as a new record type, like @code{define-record-type}.
However, the record type is made @emph{immutable} (records may not be
mutated, even with @code{struct-set!}), and any @var{modifier} is
defined to be a functional setter---a procedure that returns a new
record instance with the specified field changed, and leaves the
original unchanged (see example below.)
@end deffn

@noindent
In addition, the generic @code{set-field} and @code{set-fields} macros
may be applied to any SRFI-9 record.

@deffn {Scheme Syntax} set-field record (field sub-fields ...) value
Return a new record of @var{record}'s type whose fields are equal to
the corresponding fields of @var{record} except for the one specified by
@var{field}.

@var{field} must be the name of the getter corresponding to the field of
@var{record} being ``set''.  Subsequent @var{sub-fields} must be record
getters designating sub-fields within that field value to be set (see
example below.)
@end deffn

@deffn {Scheme Syntax} set-fields record ((field sub-fields ...) value) ...
Like @code{set-field}, but can be used to set more than one field at a
time.  This expands to code that is more efficient than a series of
single @code{set-field} calls.
@end deffn

To illustrate the use of functional setters, let's assume these two
record type definitions:

@example
(define-record-type <address>
  (address street city country)
  address?
  (street  address-street)
  (city    address-city)
  (country address-country))

(define-immutable-record-type <person>
  (person age email address)
  person?
  (age     person-age set-person-age)
  (email   person-email set-person-email)
  (address person-address set-person-address))
@end example

@noindent
First, note that the @code{<person>} record type definition introduces
named functional setters.  These may be used like this:

@example
(define fsf-address
  (address "Franklin Street" "Boston" "USA"))

(define rms
  (person 30 "rms@@gnu.org" fsf-address))

(and (equal? (set-person-age rms 60)
             (person 60 "rms@@gnu.org" fsf-address))
     (= (person-age rms) 30))
@result{} #t
@end example

@noindent
Here, the original @code{<person>} record, to which @var{rms} is bound,
is left unchanged.

Now, suppose we want to change both the street and age of @var{rms}.
This can be achieved using @code{set-fields}:

@example
(set-fields rms
  ((person-age) 60)
  ((person-address address-street) "Temple Place"))
@result{} #<<person> age: 60 email: "rms@@gnu.org"
  address: #<<address> street: "Temple Place" city: "Boston" country: "USA">>
@end example

@noindent
Notice how the above changed two fields of @var{rms}, including the
@code{street} field of its @code{address} field, in a concise way.  Also
note that @code{set-fields} works equally well for types defined with
just @code{define-record-type}.

@node Records
@subsection 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.

Note that in many ways, this interface is too low-level for every-day
use.  Most uses of records are better served by SRFI-9 records.
@xref{SRFI-9}.

@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 [print]
Create and return a new @dfn{record-type descriptor}.

@var{type-name} is a string naming the type.  Currently it's only used
in the printed representation of records, and in diagnostics.
@var{field-names} is a list of symbols naming the fields of a record
of the type.  Duplicates are not allowed among these symbols.

@example
(make-record-type "employee" '(name age salary))
@end example

The optional @var{print} argument is a function used by
@code{display}, @code{write}, etc, for printing a record of the new
type.  It's called as @code{(@var{print} record port)} and should look
at @var{record} and write to @var{port}.
@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
@subsection Structures
@tpindex Structures

A @dfn{structure} is a first class data type which holds Scheme values
or C words in fields numbered 0 upwards.  A @dfn{vtable} is a structure
that represents a structure type, giving field types and permissions,
and an optional print function for @code{write} etc.

Structures are lower level than records (@pxref{Records}).  Usually,
when you need to represent structured data, you just want to use
records.  But sometimes you need to implement new kinds of structured
data abstractions, and for that purpose structures are useful.  Indeed,
records in Guile are implemented with structures.

@menu
* Vtables::
* Structure Basics::
* Vtable Contents::
* Meta-Vtables::
* Vtable Example::
* Tail Arrays::
@end menu

@node Vtables
@subsubsection Vtables

A vtable is a structure type, specifying its layout, and other
information.  A vtable is actually itself a structure, but there's no
need to worry about that initially (@pxref{Vtable Contents}.)

@deffn {Scheme Procedure} make-vtable fields [print]
Create a new vtable.

@var{fields} is a string describing the fields in the structures to be
created.  Each field is represented by two characters, a type letter
and a permissions letter, for example @code{"pw"}.  The types are as
follows.

@itemize @bullet{}
@item
@code{p} -- a Scheme value.  ``p'' stands for ``protected'' meaning
it's protected against garbage collection.

@item
@code{u} -- an arbitrary word of data (an @code{scm_t_bits}).  At the
Scheme level it's read and written as an unsigned integer.  ``u''
stands for ``uninterpreted'' (it's not treated as a Scheme value), or
``unprotected'' (it's not marked during GC), or ``unsigned long'' (its
size), or all of these things.

@item
@code{s} -- a self-reference.  Such a field holds the @code{SCM} value
of the structure itself (a circular reference).  This can be useful in
C code where you might have a pointer to the data array, and want to
get the Scheme @code{SCM} handle for the structure.  In Scheme code it
has no use.
@end itemize

The second letter for each field is a permission code,

@itemize @bullet{}
@item
@code{w} -- writable, the field can be read and written.
@item
@code{r} -- read-only, the field can be read but not written.
@item
@code{o} -- opaque, the field can be neither read nor written at the
Scheme level.  This can be used for fields which should only be used
from C code.
@end itemize

Here are some examples.  @xref{Tail Arrays}, for information on the
legacy tail array facility.

@example
(make-vtable "pw")      ;; one writable field
(make-vtable "prpw")    ;; one read-only and one writable
(make-vtable "pwuwuw")  ;; one scheme and two uninterpreted
@end example

The optional @var{print} argument is a function called by
@code{display} and @code{write} (etc) to give a printed representation
of a structure created from this vtable.  It's called
@code{(@var{print} struct port)} and should look at @var{struct} and
write to @var{port}.  The default print merely gives a form like
@samp{#<struct ADDR:ADDR>} with a pair of machine addresses.

The following print function for example shows the two fields of its
structure.

@example
(make-vtable "prpw"
             (lambda (struct port)
               (format port "#<~a and ~a>"
                       (struct-ref struct 0)
                       (struct-ref struct 1))))
@end example
@end deffn


@node Structure Basics
@subsubsection Structure Basics

This section describes the basic procedures for working with
structures.  @code{make-struct} creates a structure, and
@code{struct-ref} and @code{struct-set!} access its fields.

@deffn {Scheme Procedure} make-struct vtable tail-size init @dots{}
@deffnx {Scheme Procedure} make-struct/no-tail vtable init @dots{}
Create a new structure, with layout per the given @var{vtable}
(@pxref{Vtables}).

The optional @var{init}@dots{} arguments are initial values for the
fields of the structure.  This is the only way to
put values in read-only fields.  If there are fewer @var{init}
arguments than fields then the defaults are @code{#f} for a Scheme
field (type @code{p}) or 0 for an uninterpreted field (type @code{u}).

Structures also have the ability to allocate a variable number of
additional cells at the end, at their tails.  However, this legacy
@dfn{tail array} facilty is confusing and inefficient, and so we do not
recommend it.  @xref{Tail Arrays}, for more on the legacy tail array
interface.

Type @code{s} self-reference fields, permission @code{o} opaque
fields, and the count field of a tail array are all ignored for the
@var{init} arguments, ie.@: an argument is not consumed by such a
field.  An @code{s} is always set to the structure itself, an @code{o}
is always set to @code{#f} or 0 (with the intention that C code will
do something to it later), and the tail count is always the given
@var{tail-size}.

For example,

@example
(define v (make-vtable "prpwpw"))
(define s (make-struct v 0 123 "abc" 456))
(struct-ref s 0) @result{} 123
(struct-ref s 1) @result{} "abc"
@end example
@end deffn

@deftypefn {C Function} SCM scm_make_struct (SCM vtable, SCM tail_size, SCM init_list)
@deftypefnx {C Function} SCM scm_c_make_struct (SCM vtable, SCM tail_size, SCM init, ...)
@deftypefnx {C Function} SCM scm_c_make_structv (SCM vtable, SCM tail_size, size_t n_inits, scm_t_bits init[])
There are a few ways to make structures from C.  @code{scm_make_struct}
takes a list, @code{scm_c_make_struct} takes variable arguments
terminated with SCM_UNDEFINED, and @code{scm_c_make_structv} takes a
packed array.
@end deftypefn

@deffn {Scheme Procedure} struct? obj
@deffnx {C Function} scm_struct_p (obj)
Return @code{#t} if @var{obj} is a structure, or @code{#f} if not.
@end deffn

@deffn {Scheme Procedure} struct-ref struct n
@deffnx {C Function} scm_struct_ref (struct, n)
Return the contents of field number @var{n} in @var{struct}.  The
first field is number 0.

An error is thrown if @var{n} is out of range, or if the field cannot
be read because it's @code{o} opaque.
@end deffn

@deffn {Scheme Procedure} struct-set! struct n value
@deffnx {C Function} scm_struct_set_x (struct, n, value)
Set field number @var{n} in @var{struct} to @var{value}.  The first
field is number 0.

An error is thrown if @var{n} is out of range, or if the field cannot
be written because it's @code{r} read-only or @code{o} opaque.  
@end deffn

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

The vtable is effectively the type of the structure.  See @ref{Vtable
Contents}, for more on vtables.
@end deffn


@node Vtable Contents
@subsubsection Vtable Contents

A vtable is itself a structure.  It has a specific set of fields
describing various aspects of its @dfn{instances}: the structures
created from a vtable.  Some of the fields are internal to Guile, some
of them are part of the public interface, and there may be additional
fields added on by the user.

Every vtable has a field for the layout of their instances, a field for
the procedure used to print its instances, and a field for the name of
the vtable itself.  Access to the layout and printer is exposed directly
via field indexes.  Access to the vtable name is exposed via accessor
procedures.

@defvr {Scheme Variable} vtable-index-layout
@defvrx {C Macro} scm_vtable_index_layout
The field number of the layout specification in a vtable.  The layout
specification is a symbol like @code{pwpw} formed from the fields
string passed to @code{make-vtable}, or created by
@code{make-struct-layout} (@pxref{Meta-Vtables}).

@example
(define v (make-vtable "pwpw" 0))
(struct-ref v vtable-index-layout) @result{} pwpw
@end example

This field is read-only, since the layout of structures using a vtable
cannot be changed.
@end defvr

@defvr {Scheme Variable} vtable-index-printer
@defvrx {C Macro} scm_vtable_index_printer
The field number of the printer function.  This field contains @code{#f}
if the default print function should be used.

@example
(define (my-print-func struct port)
  ...)
(define v (make-vtable "pwpw" my-print-func))
(struct-ref v vtable-index-printer) @result{} my-print-func
@end example

This field is writable, allowing the print function to be changed
dynamically.
@end defvr

@deffn {Scheme Procedure} struct-vtable-name vtable
@deffnx {Scheme Procedure} set-struct-vtable-name! vtable name
@deffnx {C Function} scm_struct_vtable_name (vtable)
@deffnx {C Function} scm_set_struct_vtable_name_x (vtable, name)
Get or set the name of @var{vtable}.  @var{name} is a symbol and is
used in the default print function when printing structures created
from @var{vtable}.

@example
(define v (make-vtable "pw"))
(set-struct-vtable-name! v 'my-name)

(define s (make-struct v 0))
(display s) @print{} #<my-name b7ab3ae0:b7ab3730>
@end example
@end deffn


@node Meta-Vtables
@subsubsection Meta-Vtables

As a structure, a vtable also has a vtable, which is also a structure.
Structures, their vtables, the vtables of the vtables, and so on form a
tree of structures.  Making a new structure adds a leaf to the tree, and
if that structure is a vtable, it may be used to create other leaves.

If you traverse up the tree of vtables, via calling
@code{struct-vtable}, eventually you reach a root which is the vtable of
itself:

@example
scheme@@(guile-user)> (current-module)
$1 = #<directory (guile-user) 221b090>
scheme@@(guile-user)> (struct-vtable $1)
$2 = #<record-type module>
scheme@@(guile-user)> (struct-vtable $2)
$3 = #<<standard-vtable> 12c30a0>
scheme@@(guile-user)> (struct-vtable $3)
$4 = #<<standard-vtable> 12c3fa0>
scheme@@(guile-user)> (struct-vtable $4)
$5 = #<<standard-vtable> 12c3fa0>
scheme@@(guile-user)> <standard-vtable>
$6 = #<<standard-vtable> 12c3fa0>
@end example

In this example, we can say that @code{$1} is an instance of @code{$2},
@code{$2} is an instance of @code{$3}, @code{$3} is an instance of
@code{$4}, and @code{$4}, strangely enough, is an instance of itself.
The value bound to @code{$4} in this console session also bound to
@code{<standard-vtable>} in the default environment.

@defvr {Scheme Variable} <standard-vtable>
A meta-vtable, useful for making new vtables.
@end defvr

All of these values are structures.  All but @code{$1} are vtables.  As
@code{$2} is an instance of @code{$3}, and @code{$3} is a vtable, we can
say that @code{$3} is a @dfn{meta-vtable}: a vtable that can create
vtables.

With this definition, we can specify more precisely what a vtable is: a
vtable is a structure made from a meta-vtable.  Making a structure from
a meta-vtable runs some special checks to ensure that the first field of
the structure is a valid layout.  Additionally, if these checks see that
the layout of the child vtable contains all the required fields of a
vtable, in the correct order, then the child vtable will also be a
meta-table, inheriting a magical bit from the parent.

@deffn {Scheme Procedure} struct-vtable? obj
@deffnx {C Function} scm_struct_vtable_p (obj)
Return @code{#t} if @var{obj} is a vtable structure: an instance of a
meta-vtable.
@end deffn

@code{<standard-vtable>} is a root of the vtable tree.  (Normally there
is only one root in a given Guile process, but due to some legacy
interfaces there may be more than one.)

The set of required fields of a vtable is the set of fields in the
@code{<standard-vtable>}, and is bound to @code{standard-vtable-fields}
in the default environment.  It is possible to create a meta-vtable that
with additional fields in its layout, which can be used to create
vtables with additional data:

@example
scheme@@(guile-user)> (struct-ref $3 vtable-index-layout)
$6 = pruhsruhpwphuhuhprprpw
scheme@@(guile-user)> (struct-ref $4 vtable-index-layout)
$7 = pruhsruhpwphuhuh
scheme@@(guile-user)> standard-vtable-fields 
$8 = "pruhsruhpwphuhuh"
scheme@@(guile-user)> (struct-ref $2 vtable-offset-user)
$9 = module
@end example

In this continuation of our earlier example, @code{$2} is a vtable that
has extra fields, because its vtable, @code{$3}, was made from a
meta-vtable with an extended layout.  @code{vtable-offset-user} is a
convenient definition that indicates the number of fields in
@code{standard-vtable-fields}.

@defvr {Scheme Variable} standard-vtable-fields
A string containing the orderedq set of fields that a vtable must have.
@end defvr

@defvr {Scheme Variable} vtable-offset-user
The first index in a vtable that is available for a user.
@end defvr

@deffn {Scheme Procedure} make-struct-layout fields
@deffnx {C Function} scm_make_struct_layout (fields)
Return a structure layout symbol, from a @var{fields} string.
@var{fields} is as described under @code{make-vtable}
(@pxref{Vtables}).  An invalid @var{fields} string is an error.
@end deffn

With these definitions, one can define @code{make-vtable} in this way:

@example
(define* (make-vtable fields #:optional printer)
  (make-struct/no-tail <standard-vtable>
    (make-struct-layout fields)
    printer))
@end example


@node Vtable Example
@subsubsection Vtable Example

Let us bring these points together with an example.  Consider a simple
object system with single inheritance.  Objects will be normal
structures, and classes will be vtables with three extra class fields:
the name of the class, the parent class, and the list of fields.

So, first we need a meta-vtable that allocates instances with these
extra class fields.

@example
(define <class>
  (make-vtable
   (string-append standard-vtable-fields "pwpwpw")
   (lambda (x port)
     (format port "<<class> ~a>" (class-name x)))))

(define (class? x)
  (and (struct? x)
       (eq? (struct-vtable x) <class>)))
@end example

To make a structure with a specific meta-vtable, we will use
@code{make-struct/no-tail}, passing it the computed instance layout and
printer, as with @code{make-vtable}, and additionally the extra three
class fields.

@example
(define (make-class name parent fields)
  (let* ((fields (compute-fields parent fields))
         (layout (compute-layout fields)))
    (make-struct/no-tail <class>
      layout 
      (lambda (x port)
        (print-instance x port))
      name
      parent
      fields)))
@end example

Instances will store their associated data in slots in the structure: as
many slots as there are fields.  The @code{compute-layout} procedure
below can compute a layout, and @code{field-index} returns the slot
corresponding to a field.

@example
(define-syntax-rule (define-accessor name n)
  (define (name obj)
    (struct-ref obj n)))

;; Accessors for classes
(define-accessor class-name (+ vtable-offset-user 0))
(define-accessor class-parent (+ vtable-offset-user 1))
(define-accessor class-fields (+ vtable-offset-user 2))

(define (compute-fields parent fields)
  (if parent
      (append (class-fields parent) fields)
      fields))

(define (compute-layout fields)
  (make-struct-layout
   (string-concatenate (make-list (length fields) "pw"))))

(define (field-index class field)
  (list-index (class-fields class) field))

(define (print-instance x port)
  (format port "<~a" (class-name (struct-vtable x)))
  (for-each (lambda (field idx)
              (format port " ~a: ~a" field (struct-ref x idx)))
            (class-fields (struct-vtable x))
            (iota (length (class-fields (struct-vtable x)))))
  (format port ">"))
@end example

So, at this point we can actually make a few classes:

@example
(define-syntax-rule (define-class name parent field ...)
  (define name (make-class 'name parent '(field ...))))

(define-class <surface> #f
  width height)

(define-class <window> <surface>
  x y)
@end example

And finally, make an instance:

@example
(make-struct/no-tail <window> 400 300 10 20)
@result{} <<window> width: 400 height: 300 x: 10 y: 20>
@end example

And that's that.  Note that there are many possible optimizations and
feature enhancements that can be made to this object system, and the
included GOOPS system does make most of them.  For more simple use
cases, the records facility is usually sufficient.  But sometimes you
need to make new kinds of data abstractions, and for that purpose,
structs are here.

@node Tail Arrays
@subsubsection Tail Arrays

Guile's structures have a facility whereby each instance of a vtable can
contain a variable-length tail array of values.  The length of the tail
array is stored in the structure.  This facility was originally intended
to allow C code to expose raw C structures with word-sized tail arrays
to Scheme.

However, the tail array facility is confusing and doesn't work very
well.  It is very rarely used, but it insinuates itself into all
invocations of @code{make-struct}.  For this reason the clumsily-named
@code{make-struct/no-tail} procedure can actually be more elegant in
actual use, because it doesn't have a random @code{0} argument stuck in
the middle.

Tail arrays also inhibit optimization by allowing instances to affect
their shapes.  In the absence of tail arrays, all instances of a given
vtable have the same number and kinds of fields.  This uniformity can be
exploited by the runtime and the optimizer.  The presence of tail arrays
make some of these optimizations more difficult.

Finally, the tail array facility is ad-hoc and does not compose with the
rest of Guile.  If a Guile user wants an array with user-specified
length, it's best to use a vector.  It is more clear in the code, and
the standard optimization techniques will do a good job with it.

That said, we should mention some details about the interface.  A vtable
that has tail array has upper-case permission descriptors: @code{W},
@code{R} or @code{O}, correspoding to tail arrays of writable,
read-only, or opaque elements.  A tail array permission descriptor may
only appear in the last element of a vtable layout.

For exampple, @samp{pW} indicates a tail of writable Scheme-valued
fields.  The @samp{pW} field itself holds the tail size, and the tail
fields come after it.

@example
(define v (make-vtable "prpW")) ;; one fixed then a tail array
(define s (make-struct v 6 "fixed field" 'x 'y))
(struct-ref s 0) @result{} "fixed field"
(struct-ref s 1) @result{} 2    ;; tail size
(struct-ref s 2) @result{} x    ;; tail array ...
(struct-ref s 3) @result{} y
(struct-ref s 4) @result{} #f
@end example


@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} find the entry in an alist
for a given key, and return the @code{(@var{key} . @var{value})} pair.
@code{assq-ref}, @code{assv-ref} and @code{assoc-ref} do a similar
lookup, but return just the @var{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)
Return the first entry in @var{alist} with the given @var{key}.  The
return is the pair @code{(KEY . VALUE)} from @var{alist}.  If there's
no matching entry the return is @code{#f}.

@code{assq} compares keys with @code{eq?}, @code{assv} uses
@code{eqv?} and @code{assoc} uses @code{equal?}.  See also SRFI-1
which has an extended @code{assoc} (@ref{SRFI-1 Association Lists}).
@end deffn

@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)
Return the value from the first entry in @var{alist} with the given
@var{key}, or @code{#f} if there's no such entry.

@code{assq-ref} compares keys with @code{eq?}, @code{assv-ref} uses
@code{eqv?} and @code{assoc-ref} uses @code{equal?}.

Notice these functions have the @var{key} argument last, like other
@code{-ref} functions, but this is opposite to what @code{assq}
etc above use.

When the return is @code{#f} it can be either @var{key} not found, or
an entry which happens to have value @code{#f} in the @code{cdr}.  Use
@code{assq} etc above if you need to differentiate these cases.
@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
   association list): ((1 . 2) ("key" . "door") . "open sesame")

(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
   association list): ((1 . 1) 2 (3 . 9))

(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" "Pierre"))
capitals
@result{} (("South Dakota" . "Pierre")
    ("New York" . "Albany")
    ("Oregon" . "Salem")
    ("Florida" . "Miami"))

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

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

@node VHashes
@subsection VList-Based Hash Lists or ``VHashes''

@cindex VList-based hash lists
@cindex VHash

The @code{(ice-9 vlist)} module provides an implementation of @dfn{VList-based
hash lists} (@pxref{VLists}).  VList-based hash lists, or @dfn{vhashes}, are an
immutable dictionary type similar to association lists that maps @dfn{keys} to
@dfn{values}.  However, unlike association lists, accessing a value given its
key is typically a constant-time operation.

The VHash programming interface of @code{(ice-9 vlist)} is mostly the same as
that of association lists found in SRFI-1, with procedure names prefixed by
@code{vhash-} instead of @code{alist-} (@pxref{SRFI-1 Association Lists}).

In addition, vhashes can be manipulated using VList operations:

@example
(vlist-head (vhash-consq 'a 1 vlist-null))
@result{} (a . 1)

(define vh1 (vhash-consq 'b 2 (vhash-consq 'a 1 vlist-null)))
(define vh2 (vhash-consq 'c 3 (vlist-tail vh1)))

(vhash-assq 'a vh2)
@result{} (a . 1)
(vhash-assq 'b vh2)
@result{} #f
(vhash-assq 'c vh2)
@result{} (c . 3)
(vlist->list vh2)
@result{} ((c . 3) (a . 1))
@end example

However, keep in mind that procedures that construct new VLists
(@code{vlist-map}, @code{vlist-filter}, etc.) return raw VLists, not vhashes:

@example
(define vh (alist->vhash '((a . 1) (b . 2) (c . 3)) hashq))
(vhash-assq 'a vh)
@result{} (a . 1)

(define vl
  ;; This will create a raw vlist.
  (vlist-filter (lambda (key+value) (odd? (cdr key+value))) vh))
(vhash-assq 'a vl)
@result{} ERROR: Wrong type argument in position 2

(vlist->list vl)
@result{} ((a . 1) (c . 3))
@end example

@deffn {Scheme Procedure} vhash? obj
Return true if @var{obj} is a vhash.
@end deffn

@deffn {Scheme Procedure} vhash-cons key value vhash [hash-proc]
@deffnx {Scheme Procedure} vhash-consq key value vhash
@deffnx {Scheme Procedure} vhash-consv key value vhash
Return a new hash list based on @var{vhash} where @var{key} is associated with
@var{value}, using @var{hash-proc} to compute the hash of @var{key}.
@var{vhash} must be either @code{vlist-null} or a vhash returned by a previous
call to @code{vhash-cons}.  @var{hash-proc} defaults to @code{hash} (@pxref{Hash
Table Reference, @code{hash} procedure}).  With @code{vhash-consq}, the
@code{hashq} hash function is used; with @code{vhash-consv} the @code{hashv}
hash function is used.

All @code{vhash-cons} calls made to construct a vhash should use the same
@var{hash-proc}.  Failing to do that, the result is undefined.
@end deffn

@deffn {Scheme Procedure} vhash-assoc key vhash [equal? [hash-proc]]
@deffnx {Scheme Procedure} vhash-assq key vhash
@deffnx {Scheme Procedure} vhash-assv key vhash
Return the first key/value pair from @var{vhash} whose key is equal to @var{key}
according to the @var{equal?} equality predicate (which defaults to
@code{equal?}), and using @var{hash-proc} (which defaults to @code{hash}) to
compute the hash of @var{key}.  The second form uses @code{eq?} as the equality
predicate and @code{hashq} as the hash function; the last form uses @code{eqv?}
and @code{hashv}.

Note that it is important to consistently use the same hash function for
@var{hash-proc} as was passed to @code{vhash-cons}.  Failing to do that, the
result is unpredictable.
@end deffn

@deffn {Scheme Procedure} vhash-delete key vhash [equal? [hash-proc]]
@deffnx {Scheme Procedure} vhash-delq key vhash
@deffnx {Scheme Procedure} vhash-delv key vhash
Remove all associations from @var{vhash} with @var{key}, comparing keys with
@var{equal?} (which defaults to @code{equal?}), and computing the hash of
@var{key} using @var{hash-proc} (which defaults to @code{hash}).  The second
form uses @code{eq?} as the equality predicate and @code{hashq} as the hash
function; the last one uses @code{eqv?} and @code{hashv}.

Again the choice of @var{hash-proc} must be consistent with previous calls to
@code{vhash-cons}.
@end deffn

@deffn {Scheme Procedure} vhash-fold proc init vhash
@deffnx {Scheme Procedure} vhash-fold-right proc init vhash
Fold over the key/value elements of @var{vhash} in the given direction,
with each call to @var{proc} having the form @code{(@var{proc} key value
result)}, where @var{result} is the result of the previous call to
@var{proc} and @var{init} the value of @var{result} for the first call
to @var{proc}.
@end deffn

@deffn {Scheme Procedure} vhash-fold* proc init key vhash [equal? [hash]]
@deffnx {Scheme Procedure} vhash-foldq* proc init key vhash
@deffnx {Scheme Procedure} vhash-foldv* proc init key vhash
Fold over all the values associated with @var{key} in @var{vhash}, with each
call to @var{proc} having the form @code{(proc value result)}, where
@var{result} is the result of the previous call to @var{proc} and @var{init} the
value of @var{result} for the first call to @var{proc}.

Keys in @var{vhash} are hashed using @var{hash} are compared using @var{equal?}.
The second form uses @code{eq?} as the equality predicate and @code{hashq} as
the hash function; the third one uses @code{eqv?} and @code{hashv}.

Example:

@example
(define vh
  (alist->vhash '((a . 1) (a . 2) (z . 0) (a . 3))))

(vhash-fold* cons '() 'a vh)
@result{} (3 2 1)

(vhash-fold* cons '() 'z vh)
@result{} (0)
@end example
@end deffn

@deffn {Scheme Procedure} alist->vhash alist [hash-proc]
Return the vhash corresponding to @var{alist}, an association list, using
@var{hash-proc} to compute key hashes.  When omitted, @var{hash-proc} defaults
to @code{hash}.
@end deffn


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

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.

Guile provides two types of hashtables.  One is an abstract data type
that can only be manipulated with the functions in this section.  The
other type is concrete: it uses a normal vector with alists as
elements.  The advantage of the abstract hash tables is that they will
be automatically resized when they become too full or too empty.

@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

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))

;; This is an opaque object
h
@result{}
#<hash-table 0/31>

;; We can also use a vector of alists.
(define h (make-vector 7 '()))

h
@result{}
#(() () () () () () ())

;; Inserting into a hash table can be done with hashq-set!
(hashq-set! h 'foo "bar")
@result{}
"bar"

(hashq-set! h 'braz "zonk")
@result{}
"zonk"

;; Or with hash-create-handle!
(hashq-create-handle! h 'frob #f)
@result{}
(frob . #f)

;; The vector now contains three elements in the alists and the frob
;; entry is at index (hashq 'frob).
h
@result{}
#(((braz . "zonk")) ((foo . "bar")) () () () () ((frob . #f)))

(hashq 'frob 7)
@result{}
6

@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

@c  FIXME: Describe in broad terms what happens for resizing, and what
@c  the initial size means for this.

Like the association list functions, the hash table functions come in
several varieties, according to the equality test used for the keys.
Plain @code{hash-} functions use @code{equal?}, @code{hashq-}
functions use @code{eq?}, @code{hashv-} functions use @code{eqv?}, and
the @code{hashx-} functions use an application supplied test.

A single @code{make-hash-table} creates a hash table suitable for use
with any set of functions, but it's imperative that just one set is
then used consistently, or results will be unpredictable.

Hash tables are implemented as a vector indexed by a hash value formed
from the key, with an association list of key/value pairs for each
bucket in case distinct keys hash together.  Direct access to the
pairs in those lists is provided by the @code{-handle-} functions.
The abstract kind of hash tables hide the vector in an opaque object
that represents the hash table, while for the concrete kind the vector
@emph{is} the hashtable.

When the number of table entries in an abstract hash table goes above
a threshold, the vector is made larger and the entries are rehashed,
to prevent the bucket lists from becoming too long and slowing down
accesses.  When the number of entries goes below a threshold, the
vector is shrunk to save space.

A abstract hash table is created with @code{make-hash-table}.  To
create a vector that is suitable as a hash table, use
@code{(make-vector @var{size} '())}, for example.

For the @code{hashx-} ``extended'' routines, an application supplies a
@var{hash} function producing an integer index like @code{hashq} etc
below, and an @var{assoc} alist search function like @code{assq} etc
(@pxref{Retrieving Alist Entries}).  Here's an example of such
functions implementing case-insensitive hashing of string keys,

@example
(use-modules (srfi srfi-1)
             (srfi srfi-13))

(define (my-hash str size)
  (remainder (string-hash-ci str) size))
(define (my-assoc str alist)
  (find (lambda (pair) (string-ci=? str (car pair))) alist))

(define my-table (make-hash-table))
(hashx-set! my-hash my-assoc my-table "foo" 123)

(hashx-ref my-hash my-assoc my-table "FOO")
@result{} 123
@end example

In a @code{hashx-} @var{hash} function the aim is to spread keys
across the vector, so bucket lists don't become long.  But the actual
values are arbitrary as long as they're in the range 0 to
@math{@var{size}-1}.  Helpful functions for forming a hash value, in
addition to @code{hashq} etc below, include @code{symbol-hash}
(@pxref{Symbol Keys}), @code{string-hash} and @code{string-hash-ci}
(@pxref{String Comparison}), and @code{char-set-hash}
(@pxref{Character Set Predicates/Comparison}).

@sp 1
@deffn {Scheme Procedure} make-hash-table [size]
Create a new abstract hash table object, with an optional minimum
vector @var{size}.

When @var{size} is given, the table vector will still grow and shrink
automatically, as described above, but with @var{size} as a minimum.
If an application knows roughly how many entries the table will hold
then it can use @var{size} to avoid rehashing when initial entries are
added.
@end deffn

@deffn {Scheme Procedure} hash-table? obj
@deffnx {C Function} scm_hash_table_p (obj)
Return @code{#t} if @var{obj} is a abstract hash table object.
@end deffn

@deffn {Scheme Procedure} hash-clear! table
@deffnx {C Function} scm_hash_clear_x (table)
Remove all items from @var{table} (without triggering a resize).
@end deffn

@deffn {Scheme Procedure} hash-ref table key [dflt]
@deffnx {Scheme Procedure} hashq-ref table key [dflt]
@deffnx {Scheme Procedure} hashv-ref table key [dflt]
@deffnx {Scheme Procedure} hashx-ref hash assoc table key [dflt]
@deffnx {C Function} scm_hash_ref (table, key, dflt)
@deffnx {C Function} scm_hashq_ref (table, key, dflt)
@deffnx {C Function} scm_hashv_ref (table, key, dflt)
@deffnx {C Function} scm_hashx_ref (hash, assoc, table, key, dflt)
Lookup @var{key} in the given hash @var{table}, and return the
associated value.  If @var{key} is not found, return @var{dflt}, or
@code{#f} if @var{dflt} is not given.
@end deffn

@deffn {Scheme Procedure} hash-set! table key val
@deffnx {Scheme Procedure} hashq-set! table key val
@deffnx {Scheme Procedure} hashv-set! table key val
@deffnx {Scheme Procedure} hashx-set! hash assoc table key val
@deffnx {C Function} scm_hash_set_x (table, key, val)
@deffnx {C Function} scm_hashq_set_x (table, key, val)
@deffnx {C Function} scm_hashv_set_x (table, key, val)
@deffnx {C Function} scm_hashx_set_x (hash, assoc, table, key, val)
Associate @var{val} with @var{key} in the given hash @var{table}.  If
@var{key} is already present then it's associated value is changed.
If it's not present then a new entry is created.
@end deffn

@deffn {Scheme Procedure} hash-remove! table key
@deffnx {Scheme Procedure} hashq-remove! table key
@deffnx {Scheme Procedure} hashv-remove! table key
@deffnx {Scheme Procedure} hashx-remove! hash assoc table key
@deffnx {C Function} scm_hash_remove_x (table, key)
@deffnx {C Function} scm_hashq_remove_x (table, key)
@deffnx {C Function} scm_hashv_remove_x (table, key)
@deffnx {C Function} scm_hashx_remove_x (hash, assoc, table, key)
Remove any association for @var{key} in the given hash @var{table}.
If @var{key} is not in @var{table} then nothing is done.
@end deffn

@deffn {Scheme Procedure} hash key size
@deffnx {Scheme Procedure} hashq key size
@deffnx {Scheme Procedure} hashv key size
@deffnx {C Function} scm_hash (key, size)
@deffnx {C Function} scm_hashq (key, size)
@deffnx {C Function} scm_hashv (key, size)
Return a hash value for @var{key}.  This is a number in the range
@math{0} to @math{@var{size}-1}, which is suitable for use in a hash
table of the given @var{size}.

Note that @code{hashq} and @code{hashv} may use internal addresses of
objects, so if an object is garbage collected and re-created it can
have a different hash value, even when the two are notionally
@code{eq?}.  For instance with symbols,

@example
(hashq 'something 123)   @result{} 19
(gc)
(hashq 'something 123)   @result{} 62
@end example

In normal use this is not a problem, since an object entered into a
hash table won't be garbage collected until removed.  It's only if
hashing calculations are somehow separated from normal references that
its lifetime needs to be considered.
@end deffn

@deffn {Scheme Procedure} hash-get-handle table key
@deffnx {Scheme Procedure} hashq-get-handle table key
@deffnx {Scheme Procedure} hashv-get-handle table key
@deffnx {Scheme Procedure} hashx-get-handle hash assoc table key
@deffnx {C Function} scm_hash_get_handle (table, key)
@deffnx {C Function} scm_hashq_get_handle (table, key)
@deffnx {C Function} scm_hashv_get_handle (table, key)
@deffnx {C Function} scm_hashx_get_handle (hash, assoc, table, key)
Return the @code{(@var{key} . @var{value})} pair for @var{key} in the
given hash @var{table}, or @code{#f} if @var{key} is not in
@var{table}.
@end deffn

@deffn {Scheme Procedure} hash-create-handle! table key init
@deffnx {Scheme Procedure} hashq-create-handle! table key init
@deffnx {Scheme Procedure} hashv-create-handle! table key init
@deffnx {Scheme Procedure} hashx-create-handle! hash assoc table key init
@deffnx {C Function} scm_hash_create_handle_x (table, key, init)
@deffnx {C Function} scm_hashq_create_handle_x (table, key, init)
@deffnx {C Function} scm_hashv_create_handle_x (table, key, init)
@deffnx {C Function} scm_hashx_create_handle_x (hash, assoc, table, key, init)
Return the @code{(@var{key} . @var{value})} pair for @var{key} in the
given hash @var{table}.  If @var{key} is not in @var{table} then
create an entry for it with @var{init} as the value, and return that
pair.
@end deffn

@deffn {Scheme Procedure} hash-map->list proc table
@deffnx {Scheme Procedure} hash-for-each proc table
@deffnx {C Function} scm_hash_map_to_list (proc, table)
@deffnx {C Function} scm_hash_for_each (proc, table)
Apply @var{proc} to the entries in the given hash @var{table}.  Each
call is @code{(@var{proc} @var{key} @var{value})}.  @code{hash-map->list}
returns a list of the results from these calls, @code{hash-for-each}
discards the results and returns an unspecified value.

Calls are made over the table entries in an unspecified order, and for
@code{hash-map->list} the order of the values in the returned list is
unspecified.  Results will be unpredictable if @var{table} is modified
while iterating.

For example the following returns a new alist comprising all the
entries from @code{mytable}, in no particular order.

@example
(hash-map->list cons mytable)
@end example
@end deffn

@deffn {Scheme Procedure} hash-for-each-handle proc table
@deffnx {C Function} scm_hash_for_each_handle (proc, table)
Apply @var{proc} to the entries in the given hash @var{table}.  Each
call is @code{(@var{proc} @var{handle})}, where @var{handle} is a
@code{(@var{key} . @var{value})} pair. Return an unspecified value.

@code{hash-for-each-handle} differs from @code{hash-for-each} only in
the argument list of @var{proc}.
@end deffn

@deffn {Scheme Procedure} hash-fold proc init table
@deffnx {C Function} scm_hash_fold (proc, init, table)
Accumulate a result by applying @var{proc} to the elements of the
given hash @var{table}.  Each call is @code{(@var{proc} @var{key}
@var{value} @var{prior-result})}, where @var{key} and @var{value} are
from the @var{table} and @var{prior-result} is the return from the
previous @var{proc} call.  For the first call, @var{prior-result} is
the given @var{init} value.

Calls are made over the table entries in an unspecified order.
Results will be unpredictable if @var{table} is modified while
@code{hash-fold} is running.

For example, the following returns a count of how many keys in
@code{mytable} are strings.

@example
(hash-fold (lambda (key value prior)
             (if (string? key) (1+ prior) prior))
           0 mytable)
@end example
@end deffn


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