guile/doc/ref/scheme-options.texi

@page
@node Options and Config
@chapter Runtime Options and Configuration

Guile's behaviour can be modified by setting options.  For example, is
the language that Guile accepts case sensitive, or should the debugger
automatically show a backtrace on error?

Guile has two levels of interface for managing options: a low-level
control interface, and a user-level interface which allows the enabling
or disabling of options.

Moreover, the options are classified in groups according to whether they
configure @emph{reading}, @emph{printing}, @emph{debugging} or
@emph{evaluating}.

@menu
* General option interface::
* Reader options::
* Printing options::
* Debugger options::
* Evaluator options::
* Evaluator trap options::
* Examples of option use::
* Install Config::              Installation and configuration data.
* Feature Tracking::            Available features in the Guile process.
@end menu

@node General option interface
@section General option interface

We will use the expression @code{<group>} to represent @code{read},
@code{print}, @code{debug} or @code{evaluator}.

@subheading Low level

@c NJFIXME
@deffn {Scheme Procedure} <group>-options-interface
@deffnx {Scheme Procedure} read-options-interface [SOME-INT]
@deffnx {Scheme Procedure} print-options-interface [SOME-INT]
@deffnx {Scheme Procedure} evaluator-traps-interface [SOME-INT]
@deffnx {Scheme Procedure} read-options-interface [SOME-INT]
[FIXME: I have just taken the comments for C routine scm_options that
implements all of these.  It needs to be presented better.]

If scm_options is called without arguments, the current option setting
is returned.  If the argument is an option setting, options are altered
and the old setting is returned.  If the argument isn't a list, a list
of sublists is returned, where each sublist contains option name, value
and documentation string.
@end deffn


@subheading User level

@c @deftp {Data type} scm_option
@c @code{scm_option} is used to represent run time options.  It can be a
@c @emph{boolean} type, in which case the option will be set by the strings
@c @code{"yes"} and @code{"no"}.  It can be a
@c @end deftp

@c NJFIXME
@deffn {Scheme Procedure} <group>-options [arg]
@deffnx {Scheme Procedure} read-options [arg]
@deffnx {Scheme Procedure} print-options [arg]
@deffnx {Scheme Procedure} debug-options [arg]
@deffnx {Scheme Procedure} traps [arg]
These functions list the options in their group.  The optional argument
@var{arg} is a symbol which modifies the form in which the options are
presented.

With no arguments, @code{<group>-options} returns the values of the
options in that particular group.  If @var{arg} is @code{'help}, a
description of each option is given.  If @var{arg} is @code{'full},
programmers' options are also shown.

@var{arg} can also be a list representing the state of all options.  In
this case, the list contains single symbols (for enabled boolean
options) and symbols followed by values.
@end deffn
[FIXME: I don't think 'full is ever any different from 'help.  What's
up?]

@c NJFIXME
@deffn {Scheme Procedure} <group>-enable option-symbol
@deffnx {Scheme Procedure} read-enable option-symbol
@deffnx {Scheme Procedure} print-enable option-symbol
@deffnx {Scheme Procedure} debug-enable option-symbol
@deffnx {Scheme Procedure} trap-enable option-symbol
These functions set the specified @var{option-symbol} in their options
group.  They only work if the option is boolean, and throw an error
otherwise.
@end deffn

@c NJFIXME
@deffn {Scheme Procedure} <group>-disable option-symbol
@deffnx {Scheme Procedure} read-disable option-symbol
@deffnx {Scheme Procedure} print-disable option-symbol
@deffnx {Scheme Procedure} debug-disable option-symbol
@deffnx {Scheme Procedure} trap-disable option-symbol
These functions turn off the specified @var{option-symbol} in their
options group.  They only work if the option is boolean, and throw an
error otherwise.
@end deffn

@c NJFIXME
@deffn syntax <group>-set! option-symbol value
@deffnx syntax read-set! option-symbol value
@deffnx syntax print-set! option-symbol value
@deffnx syntax debug-set! option-symbol value
@deffnx syntax trap-set! option-symbol value
These functions set a non-boolean @var{option-symbol} to the specified
@var{value}.
@end deffn


@node Reader options
@section Reader options
@cindex options - read
@cindex read options

Here is the list of reader options generated by typing
@code{(read-options 'full)} in Guile.  You can also see the default
values.

@smalllisp
keywords         #f      Style of keyword recognition: #f or 'prefix
case-insensitive no      Convert symbols to lower case.
positions        yes     Record positions of source code expressions.
copy             no      Copy source code expressions.
@end smalllisp

Notice that while Standard Scheme is case insensitive, to ease
translation of other Lisp dialects, notably Emacs Lisp, into Guile,
Guile is case-sensitive by default.

To make Guile case insensitive, you can type

@smalllisp
(read-enable 'case-insensitive)
@end smalllisp

@node Printing options
@section Printing options

Here is the list of print options generated by typing
@code{(print-options 'full)} in Guile.  You can also see the default
values.

@smallexample
source          no      Print closures with source.
closure-hook    #f      Hook for printing closures.
@end smallexample


@node Evaluator options
@section Evaluator options

These are the evaluator options with their default values, as they are
printed by typing @code{(eval-options 'full)} in Guile.

@smallexample
stack           22000   Size of thread stacks (in machine words).
@end smallexample

@node Evaluator trap options
@section Evaluator trap options
[FIXME: These flags, together with their corresponding handlers, are not
user level options.  Probably this entire section should be moved to the
documentation about the low-level programmer debugging interface.]

Here is the list of evaluator trap options generated by typing
@code{(traps 'full)} in Guile.  You can also see the default values.

@smallexample
exit-frame      no      Trap when exiting eval or apply.
apply-frame     no      Trap when entering apply.
enter-frame     no      Trap when eval enters new frame.
traps		yes	Enable evaluator traps.
@end smallexample

@deffn apply-frame-handler key cont tailp
Called when a procedure is being applied.

Called if:

@itemize @bullet
@item
evaluator traps are enabled [traps interface], and
@item
either
@itemize @minus
@item
@code{apply-frame} is enabled [traps interface], or
@item
trace mode is on [debug-options interface], and the procedure being
called has the trace property enabled.
@end itemize
@end itemize

If cheap traps are enabled [debug-options interface], @var{cont} is a
debug object, otherwise it is a restartable continuation.

@var{tailp} is true if this is a tail call
@end deffn

@deffn exit-frame-handler key cont retval
Called when a value is returned from a procedure.

Called if:

@itemize @bullet
@item
evaluator traps are enabled [traps interface], and
@item
either
@itemize @minus
@item
 @code{exit-frame} is enabled [traps interface], or
@item
trace mode is on [debug-options interface], and the procedure being
called has the trace property enabled.
@end itemize
@end itemize

If cheap traps are enabled [debug-options interface], @var{cont} is a
debug object, otherwise it is a restartable continuation.

@var{retval} is the return value.
@end deffn

@node Debugger options
@section Debugger options

Here is the list of print options generated by typing
@code{(debug-options 'full)} in Guile.  You can also see the default
values.

@smallexample
stack           20000   Stack size limit (0 = no check).
debug           yes     Use the debugging evaluator.
backtrace       no      Show backtrace on error.
depth           20      Maximal length of printed backtrace.
maxdepth        1000    Maximal number of stored backtrace frames.
frames          3       Maximum number of tail-recursive frames in backtrace.
indent          10      Maximal indentation in backtrace.
backwards       no      Display backtrace in anti-chronological order.
procnames       yes     Record procedure names at definition.
trace           no      *Trace mode.
breakpoints     no      *Check for breakpoints.
cheap           yes     *Flyweight representation of the stack at traps.
@end smallexample

@subsection Stack overflow

@cindex overflow, stack
@cindex stack overflow
Stack overflow errors are caused by a computation trying to use more
stack space than has been enabled by the @code{stack} option.  They are
reported like this:

@lisp
(non-tail-recursive-factorial 500)
@print{}
ERROR: Stack overflow
ABORT: (stack-overflow)
@end lisp

If you get an error like this, you can either try rewriting your code to
use less stack space, or increase the maximum stack size.  To increase
the maximum stack size, use @code{debug-set!}, for example:

@lisp
(debug-set! stack 200000)
@result{}
(show-file-name #t stack 200000 debug backtrace depth 20 maxdepth 1000 frames 3 indent 10 width 79 procnames cheap)

(non-tail-recursive-factorial 500)
@result{}
122013682599111006870123878542304692625357434@dots{}
@end lisp

If you prefer to try rewriting your code, you may be able to save stack
space by making some of your procedures @dfn{tail recursive}.  For a
description of what this means, see @ref{Proper tail
recursion,,,r5rs,The Revised^5 Report on Scheme}.


@node Examples of option use
@section Examples of option use

Here is an example of a session in which some read and debug option
handling procedures are used.  In this example, the user

@enumerate
@item
Notices that the symbols @code{abc} and @code{aBc} are not the same
@item
Examines the @code{read-options}, and sees that @code{case-insensitive}
is set to ``no''.
@item
Enables @code{case-insensitive}
@item
Verifies that now @code{aBc} and @code{abc} are the same
@item
Disables @code{case-insensitive} and enables debugging @code{backtrace}
@item
Reproduces the error of displaying @code{aBc} with backtracing enabled
[FIXME: this last example is lame because there is no depth in the
backtrace.  Need to give a better example, possibly putting debugging
option examples in a separate session.]
@end enumerate


@smalllisp
guile> (define abc "hello")
guile> abc
"hello"
guile> aBc
ERROR: In expression aBc:
ERROR: Unbound variable: aBc
ABORT: (misc-error)

Type "(backtrace)" to get more information.
guile> (read-options 'help)
keywords	#f	Style of keyword recognition: #f or 'prefix
case-insensitive	no	Convert symbols to lower case.
positions	yes	Record positions of source code expressions.
copy		no	Copy source code expressions.
guile> (debug-options 'help)
stack		20000	Stack size limit (0 = no check).
debug		yes	Use the debugging evaluator.
backtrace	no	Show backtrace on error.
depth		20	Maximal length of printed backtrace.
maxdepth	1000	Maximal number of stored backtrace frames.
frames		3	Maximum number of tail-recursive frames in backtrace.
indent		10	Maximal indentation in backtrace.
backwards	no	Display backtrace in anti-chronological order.
procnames	yes	Record procedure names at definition.
trace		no	*Trace mode.
breakpoints	no	*Check for breakpoints.
cheap		yes	*Flyweight representation of the stack at traps.
guile> (read-enable 'case-insensitive)
(keywords #f case-insensitive positions)
guile> aBc
"hello"
guile> (read-disable 'case-insensitive)
(keywords #f positions)
guile> (debug-enable 'backtrace)
(stack 20000 debug backtrace depth 20 maxdepth 1000 frames 3 indent 10 procnames cheap)
guile> aBc

Backtrace:
0* aBc

ERROR: In expression aBc:
ERROR: Unbound variable: aBc
ABORT: (misc-error)
guile>
@end smalllisp


@node Install Config
@section Installation and Configuration Data

It is often useful to have site-specific information about the current
Guile installation.  This chapter describes how to find out about
Guile's configuration at run time.

@deffn {Scheme Procedure} version
@deffnx {Scheme Procedure} major-version
@deffnx {Scheme Procedure} minor-version
@deffnx {Scheme Procedure} micro-version
@deffnx {C Function} scm_version ()
@deffnx {C Function} scm_major_version ()
@deffnx {C Function} scm_minor_version ()
@deffnx {C Function} scm_micro_version ()
Return a string describing Guile's version number, or its major, minor
or micro version number, respectively.

@lisp
(version) @result{} "1.6.0"
(major-version) @result{} "1"
(minor-version) @result{} "6"
(micro-version) @result{} "0"
@end lisp
@end deffn

@c NJFIXME not in libguile!
@deffn {Scheme Procedure} libguile-config-stamp
Return a string describing the date on which @code{libguile} was
configured.  This is used to determine whether the Guile core
interpreter and the ice-9 runtime have grown out of date with one
another.
@end deffn

@deffn {Scheme Procedure} %package-data-dir
@deffnx {C Function} scm_sys_package_data_dir ()
Return the name of the directory where Scheme packages, modules and
libraries are kept.  On most Unix systems, this will be
@samp{/usr/local/share/guile}.
@end deffn

@deffn {Scheme Procedure} %library-dir
@deffnx {C Function} scm_sys_library_dir ()
Return the directory where the Guile Scheme library files are installed.
E.g., may return "/usr/share/guile/1.3.5".
@end deffn

@deffn {Scheme Procedure} %site-dir
@deffnx {C Function} scm_sys_site_dir ()
Return the directory where the Guile site files are installed.
E.g., may return "/usr/share/guile/site".
@end deffn

@deffn {Scheme Procedure} parse-path path [tail]
@deffnx {C Function} scm_parse_path (path, tail)
Parse @var{path}, which is expected to be a colon-separated
string, into a list and return the resulting list with
@var{tail} appended. If @var{path} is @code{#f}, @var{tail}
is returned.
@end deffn

@deffn {Scheme Procedure} search-path path filename [extensions]
@deffnx {C Function} scm_search_path (path, filename, extensions)
Search @var{path} for a directory containing a file named
@var{filename}. The file must be readable, and not a directory.
If we find one, return its full filename; otherwise, return
@code{#f}.  If @var{filename} is absolute, return it unchanged.
If given, @var{extensions} is a list of strings; for each
directory in @var{path}, we search for @var{filename}
concatenated with each @var{extension}.
@end deffn

@defvar %load-path
List of directories which should be searched for Scheme
modules and libraries.
@end defvar

@defvar %guile-build-info
Alist of information collected during the building of a particular
@code{guile} program.  Entries can be grouped into one of several
categories: directories, env vars, and versioning info.

Briefly, here are the keys in @code{%guile-build-info}, by group:
@itemize @bullet
@item directories
srcdir, top_srcdir, prefix, exec_prefix, bindir, sbindir, libexecdir,
datadir, sysconfdir, sharedstatedir, localstatedir, libdir, infodir,
mandir, includedir, pkgdatadir, pkglibdir, pkgincludedir
@item env vars
LIBS
@item versioning info
guileversion, libguileinterface, buildstamp
@end itemize

Values are all strings.  The value for @code{LIBS} is typically found also as
a part of "guile-config link" output.  The value for @code{guileversion} has
form X.Y.Z, and should be the same as returned by @code{version}.  The value
for @code{libguileinterface} is libtool compatible and has form
CURRENT:REVISION:AGE.  The value for @code{buildstamp} is the output of the
date(1) command.

In the source, @code{%guile-build-info} is initialized from
libguile/libpath.h, which is completely generated, so deleting this file
before a build guarantees up-to-date values for that build.
@end defvar


@node Feature Tracking
@section Feature Tracking

Guile has a Scheme level variable @code{*features*} that keeps track to
some extent of the features that are available in a running Guile.
@code{*features*} is a list of symbols, for example @code{threads}, each
of which describes a feature of the running Guile process.

@defvar *features*
A list of symbols describing available features of the Guile process.
@end defvar

You shouldn't modify the @code{*features*} variable directly using
@code{set!}.  Instead, see the procedures that are provided for this
purpose in the following subsection.

@menu
* Feature Manipulation::        Checking for and advertising features.
* Common Feature Symbols::      Commonly available features.
@end menu


@node Feature Manipulation
@subsection Feature Manipulation

To check whether a particular feature is available, use the
@code{provided?} procedure:

@deffn {Scheme Procedure} provided? feature
@deffnx {Deprecated Scheme Procedure} feature? feature
Return @code{#t} if the specified @var{feature} is available, otherwise
@code{#f}.
@end deffn

To advertise a feature from your own Scheme code, you can use the
@code{provide} procedure:

@deffn {Scheme Procedure} provide feature
Add @var{feature} to the list of available features in this Guile
process.
@end deffn

For C code, the equivalent function takes its feature name as a
@code{char *} argument for convenience:

@deftypefn {C Function} void scm_add_feature (const char *str)
Add a symbol with name @var{str} to the list of available features in
this Guile process.
@end deftypefn


@node Common Feature Symbols
@subsection Common Feature Symbols

In general, a particular feature may be available for one of two
reasons.  Either because the Guile library was configured and compiled
with that feature enabled --- i.e. the feature is built into the library
on your system.  Or because some C or Scheme code that was dynamically
loaded by Guile has added that feature to the list.

In the first category, here are the features that the current version of
Guile may define (depending on how it is built), and what they mean.

@table @code
@item array
Indicates support for arrays (@pxref{Arrays}).

@item array-for-each
Indicates availability of @code{array-for-each} and other array mapping
procedures (@pxref{Array Mapping}).

@item char-ready?
Indicates that the @code{char-ready?} function is available
(@pxref{Reading}).

@item complex
Indicates support for complex numbers.

@item current-time
Indicates availability of time-related functions: @code{times},
@code{get-internal-run-time} and so on (@pxref{Time}).

@item debug-extensions
Indicates that the debugging evaluator is available, together with the
options for controlling it.

@item delay
Indicates support for promises (@pxref{Delayed Evaluation}).

@item EIDs
Indicates that the @code{geteuid} and @code{getegid} really return
effective user and group IDs (@pxref{Processes}).

@item inexact
Indicates support for inexact numbers.

@item i/o-extensions
Indicates availability of the following extended I/O procedures:
@code{ftell}, @code{redirect-port}, @code{dup->fdes}, @code{dup2},
@code{fileno}, @code{isatty?}, @code{fdopen},
@code{primitive-move->fdes} and @code{fdes->ports} (@pxref{Ports and
File Descriptors}).

@item net-db
Indicates availability of network database functions:
@code{scm_gethost}, @code{scm_getnet}, @code{scm_getproto},
@code{scm_getserv}, @code{scm_sethost}, @code{scm_setnet}, @code{scm_setproto},
@code{scm_setserv}, and their `byXXX' variants (@pxref{Network
Databases}).

@item posix
Indicates support for POSIX functions: @code{pipe}, @code{getgroups},
@code{kill}, @code{execl} and so on (@pxref{POSIX}).

@item random
Indicates availability of random number generation functions:
@code{random}, @code{copy-random-state}, @code{random-uniform} and so on
(@pxref{Random}).

@item reckless
Indicates that Guile was built with important checks omitted --- you
should never see this!

@item regex
Indicates support for POSIX regular expressions using
@code{make-regexp}, @code{regexp-exec} and friends (@pxref{Regexp
Functions}).

@item socket
Indicates availability of socket-related functions: @code{socket},
@code{bind}, @code{connect} and so on (@pxref{Network Sockets and
Communication}).

@item sort
Indicates availability of sorting and merging functions
(@pxref{Sorting}).

@item system
Indicates that the @code{system} function is available
(@pxref{Processes}).

@item threads
Indicates support for multithreading (@pxref{Threads}).

@item values
Indicates support for multiple return values using @code{values} and
@code{call-with-values} (@pxref{Multiple Values}).
@end table

Available features in the second category depend, by definition, on what
additional code your Guile process has loaded in.  The following table
lists features that you might encounter for this reason.

@table @code
@item defmacro
Indicates that the @code{defmacro} macro is available (@pxref{Macros}).

@item describe
Indicates that the @code{(oop goops describe)} module has been loaded,
which provides a procedure for describing the contents of GOOPS
instances.

@item readline
Indicates that Guile has loaded in Readline support, for command line
editing (@pxref{Readline Support}).

@item record
Indicates support for record definition using @code{make-record-type}
and friends (@pxref{Records}).
@end table

Although these tables may seem exhaustive, it is probably unwise in
practice to rely on them, as the correspondences between feature symbols
and available procedures/behaviour are not strictly defined.  If you are
writing code that needs to check for the existence of some procedure, it
is probably safer to do so directly using the @code{defined?} procedure
than to test for the corresponding feature using @code{feature?}.


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