guile/doc/ref/api-options.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, 2008, 2009, 2010
@c   Free Software Foundation, Inc.
@c See the file guile.texi for copying conditions.

@node Options and Config
@section Configuration, Features and Runtime Options

Why is my Guile different from your Guile?  There are three kinds of
possible variation:

@itemize @bullet
@item
build differences --- different versions of the Guile source code,
installation directories, configuration flags that control pieces of
functionality being included or left out, etc.

@item
differences in dynamically loaded code --- behaviour and features
provided by modules that can be dynamically loaded into a running Guile

@item
different runtime options --- some of the options that are provided for
controlling Guile's behaviour may be set differently.
@end itemize

Guile provides ``introspective'' variables and procedures to query all
of these possible variations at runtime.  For runtime options, it also
provides procedures to change the settings of options and to obtain
documentation on what the options mean.

@menu
* Build Config::                Build and installation configuration.
* Feature Tracking::            Available features in the Guile process.
* Runtime Options::             Controlling Guile's runtime behaviour.
@end menu


@node Build Config
@subsection Configuration, Build and Installation

The following procedures and variables provide information about how
Guile was configured, built and installed on your system.

@deffn {Scheme Procedure} version
@deffnx {Scheme Procedure} effective-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_effective_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 full version number, effective
version number, major, minor or micro version number, respectively.
The @code{effective-version} function returns the version name that
should remain unchanged during a stable series.  Currently that means
that it omits the micro version.  The effective version should be used
for items like the versioned share directory name
i.e. @file{/usr/share/guile/1.6/}

@lisp
(version) @result{} "1.6.0"
(effective-version) @result{} "1.6"
(major-version) @result{} "1"
(minor-version) @result{} "6"
(micro-version) @result{} "0"
@end lisp
@end deffn

@deffn {Scheme Procedure} %package-data-dir
@deffnx {C Function} scm_sys_package_data_dir ()
Return the name of the directory under which Guile Scheme files in
general are stored.  On Unix-like systems, this is usually
@file{/usr/local/share/guile} or @file{/usr/share/guile}.
@end deffn

@deffn {Scheme Procedure} %library-dir
@deffnx {C Function} scm_sys_library_dir ()
Return the name of the directory where the Guile Scheme files that
belong to the core Guile installation (as opposed to files from a 3rd
party package) are installed.  On Unix-like systems this is usually
@file{/usr/local/share/guile/<GUILE_EFFECTIVE_VERSION>} or
@file{/usr/share/guile/<GUILE_EFFECTIVE_VERSION>};

@noindent for example @file{/usr/local/share/guile/1.6}.
@end deffn

@deffn {Scheme Procedure} %site-dir
@deffnx {C Function} scm_sys_site_dir ()
Return the name of the directory where Guile Scheme files specific to
your site should be installed.  On Unix-like systems, this is usually
@file{/usr/local/share/guile/site} or @file{/usr/share/guile/site}.
@end deffn

@cindex GUILE_LOAD_PATH
@defvar %load-path
List of directories which should be searched for Scheme modules and
libraries.  @code{%load-path} is initialized when Guile starts up to
@code{(list (%site-dir) (%library-dir) (%package-data-dir))},
prepended with the contents of the GUILE_LOAD_PATH environment variable,
if it is set.
@end defvar

@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 [require-exts?]]
@deffnx {C Function} scm_search_path (path, filename, rest)
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}.  If @var{require-exts?}
is true, require that the returned file name have one of the
given extensions; if @var{require-exts?} is not given, it
defaults to @code{#f}.

For compatibility with Guile 1.8 and earlier, the C function takes only
three arguments
@end deffn

@defvar %guile-build-info
Alist of information collected during the building of a particular
Guile.  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:

@cindex @code{srcdir}
@cindex @code{top_srcdir}
@cindex @code{prefix}
@cindex @code{exec_prefix}
@cindex @code{bindir}
@cindex @code{sbindir}
@cindex @code{libexecdir}
@cindex @code{datadir}
@cindex @code{sysconfdir}
@cindex @code{sharedstatedir}
@cindex @code{localstatedir}
@cindex @code{libdir}
@cindex @code{infodir}
@cindex @code{mandir}
@cindex @code{includedir}
@cindex @code{pkgdatadir}
@cindex @code{pkglibdir}
@cindex @code{pkgincludedir}
@table @asis
@item   directories
srcdir, top_srcdir, prefix, exec_prefix, bindir, sbindir, libexecdir,
datadir, sysconfdir, sharedstatedir, localstatedir, libdir, infodir,
mandir, includedir, pkgdatadir, pkglibdir, pkgincludedir
@cindex @code{LIBS}
@item   env vars
LIBS
@cindex @code{guileversion}
@cindex @code{libguileinterface}
@cindex @code{buildstamp}
@item   versioning info
guileversion, libguileinterface, buildstamp
@end table

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
(@pxref{Versioning,, Library interface versions, libtool, GNU
Libtool}).  The value for @code{buildstamp} is the output of the
command @samp{date -u +'%Y-%m-%d %T'} (UTC).

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

@cindex GNU triplet
@cindex canonical host type

@defvar %host-type
The canonical host type (GNU triplet) of the host Guile was configured
for, e.g., @code{"x86_64-unknown-linux-gnu"} (@pxref{Canonicalizing,,,
autoconf, The GNU Autoconf Manual}).
@end defvar

@node Feature Tracking
@subsection 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
@subsubsection 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
@subsubsection 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{Arrays}).

@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{provided?}.


@node Runtime Options
@subsection Runtime Options

Guile's runtime 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
* Low level options interfaces::
* User level options interfaces::
* Reader options::
* Printing options::
* Debugger options::
* Examples of option use::
@end menu


@node Low level options interfaces
@subsubsection Low Level Options Interfaces

@deffn {Scheme Procedure} read-options-interface [setting]
@deffnx {Scheme Procedure} print-options-interface [setting]
@deffnx {Scheme Procedure} debug-options-interface [setting]
@deffnx {C Function} scm_read_options (setting)
@deffnx {C Function} scm_print_options (setting)
@deffnx {C Function} scm_debug_options (setting)
If one of these procedures is called with no arguments (or with
@code{setting == SCM_UNDEFINED} in C code), it returns a list describing
the current setting of the read, eval, print, debug or evaluator traps
options respectively.  The setting of a boolean option is indicated
simply by the presence or absence of the option symbol in the list.  The
setting of a non-boolean option is indicated by the presence of the
option symbol immediately followed by the option's current value.

If called with a list argument, these procedures interpret the list as
an option setting and modify the relevant options accordingly.  [FIXME
--- this glosses over a lot of details!]

If called with any other argument, such as @code{'help}, these
procedures return a list of entries like @code{(@var{OPTION-SYMBOL}
@var{DEFAULT-VALUE} @var{DOC-STRING})}, with each entry giving the
default value and documentation for each option symbol in the relevant
set of options.
@end deffn


@node User level options interfaces
@subsubsection User Level Options Interfaces

@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]
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
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
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
These functions set a non-boolean @var{option-symbol} to the specified
@var{value}.
@end deffn


@node Reader options
@subsubsection 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
copy              no    Copy source code expressions.
positions         yes   Record positions of source code expressions.
case-insensitive  no    Convert symbols to lower case.
keywords          #f    Style of keyword recognition: #f, 'prefix or 'postfix.
r6rs-hex-escapes  no    Use R6RS variable-length character and string hex escapes.
square-brackets   yes   Treat `[' and `]' as parentheses, for R6RS compatibility.
@end smalllisp

Historically, many Scheme implementations have been case-insensitive,
treating @code{foo} and @code{FOO} as the same symbol. Guile has always
defaulted to case-sensitivity, as allowed since the R4RS and codified in
the R6RS.

Guile also has a reader option to fold all symbols to their lower
case. To enable this option, perhaps for compatibility with older Scheme
code, you can enter

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

For more information on the effect of the @code{r6rs-hex-escapes} option, see
(@pxref{String Syntax}).

@node Printing options
@subsubsection 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
quote-keywordish-symbols reader How to print symbols that have a colon
                                as their first or last character. The
                                value '#f' does not quote the colons;
                                '#t' quotes them; 'reader' quotes
                                them when the reader option
                                'keywords' is not '#f'.

highlight-prefix         @{      The string to print before highlighted values.
highlight-suffix         @}      The string to print after highlighted values.
@end smallexample


@node Debugger options
@subsubsection 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
warn-deprecated	no	Warn when deprecated features are used.
show-file-name	#t	Show file names and line numbers in backtraces
                        when not `#f'.  A value of `base' displays only
                        base names, while `#t' displays full names.
stack		1048576	Stack size limit (measured in words; 0 = no check).
backtrace	yes	Show backtrace on error.
depth		20	Maximal length of printed backtrace.
width		79	Maximal width of backtrace.
backwards	no	Display backtrace in anti-chronological order.
@end smallexample

@subsubheading 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}
(@pxref{Tail Calls}).


@node Examples of option use
@subsubsection 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, 'prefix or 'postfix
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).
backtrace	no	Show backtrace on error.
depth		20	Maximal length of printed backtrace.
backwards	no	Display backtrace in anti-chronological order.
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


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