@page
@node Read/Load/Eval
@chapter Reading and Evaluating Scheme Code

This chapter describes Guile functions that are concerned with reading,
loading and evaluating Scheme code at run time.

@menu
* Scheme Syntax::               Standard and extended Scheme syntax.
* Scheme Read::                 Reading Scheme code.
* Fly Evaluation::              Procedures for on the fly evaluation.
* Loading::                     Loading Scheme code from file.
* Delayed Evaluation::          Postponing evaluation until it is needed.
* Local Evaluation::            Evaluation in a local environment.
* Evaluator Behaviour::         Modifying Guile's evaluator.
@end menu


@node Scheme Syntax
@section Scheme Syntax: Standard and Guile Extensions

@menu
* Expression Syntax::
* Comments::
* Block Comments::
* Case Sensitivity::
* Keyword Syntax::
* Reader Extensions::
@end menu


@node Expression Syntax
@subsection Expression Syntax


@node Comments
@subsection Comments

@c FIXME::martin: Review me!

Comments in Scheme source files are written by starting them with a
semicolon character (@code{;}).  The comment then reaches up to the end
of the line.  Comments can begin at any column, and the may be inserted
on the same line as Scheme code.

@lisp
; Comment
;; Comment too
(define x 1)        ; Comment after expression
(let ((y 1))
  ;; Display something.
  (display y)
;;; Comment at left margin.
  (display (+ y 1)))
@end lisp

It is common to use a single semicolon for comments following
expressions on a line, to use two semicolons for comments which are
indented like code, and three semicolons for comments which start at
column 0, even if they are inside an indented code block.  This
convention is used when indenting code in Emacs' Scheme mode.


@node Block Comments
@subsection Block Comments

@c FIXME::martin: Review me!

@cindex multiline comments
In addition to the standard line comments defined by R5RS, Guile has
another comment type for multiline comments, called @dfn{block
comments}.  This type of comment begins with the character sequence
@code{#!} and ends with the characters @code{!#}, which must appear on a
line of their own.  These comments are compatible with the block
comments in the Scheme Shell @file{scsh} (@pxref{The Scheme shell
(scsh)}).  The characters @code{#!} were chosen because they are the
magic characters used in shell scripts for indicating that the name of
the program for executing the script follows on the same line.

Thus a Guile script often starts like this.

@lisp
#! /usr/local/bin/guile -s
!#
@end lisp

More details on Guile scripting can be found in the scripting section
(@pxref{Guile Scripting}).


@node Case Sensitivity
@subsection Case Sensitivity

@c FIXME::martin: Review me!

Scheme as defined in R5RS is not case sensitive when reading symbols.
Guile, on the contrary is case sensitive by default, so the identifiers

@lisp
guile-whuzzy
Guile-Whuzzy
@end lisp

are the same in R5RS Scheme, but are different in Guile.

It is possible to turn off case sensitivity in Guile by setting the
reader option @code{case-insensitive}.  More on reader options can be
found at (@pxref{Reader options}).

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

Note that this is seldom a problem, because Scheme programmers tend not
to use uppercase letters in their identifiers anyway.


@node Keyword Syntax
@subsection Keyword Syntax


@node Reader Extensions
@subsection Reader Extensions

@deffn {Scheme Procedure} read-hash-extend chr proc
@deffnx {C Function} scm_read_hash_extend (chr, proc)
Install the procedure @var{proc} for reading expressions
starting with the character sequence @code{#} and @var{chr}.
@var{proc} will be called with two arguments:  the character
@var{chr} and the port to read further data from. The object
returned will be the return value of @code{read}.
@end deffn


@node Scheme Read
@section Reading Scheme Code

@rnindex read
@deffn {Scheme Procedure} read [port]
@deffnx {C Function} scm_read (port)
Read an s-expression from the input port @var{port}, or from
the current input port if @var{port} is not specified.
Any whitespace before the next token is discarded.
@end deffn

The behaviour of Guile's Scheme reader can be modified by manipulating
its read options.  For more information about options, @xref{User level
options interfaces}.  If you want to know which reader options are
available, @xref{Reader options}.

@c FIXME::martin: This is taken from libguile/options.c.  Is there 
@c actually a difference between 'help and 'full?

@deffn {Scheme Procedure} read-options [setting]
Display the current settings of the read options.  If @var{setting} is
omitted, only a short form of the current read options is printed.
Otherwise, @var{setting} should be one of the following symbols:
@table @code
@item help
Display the complete option settings.
@item full
Like @code{help}, but also print programmer options.
@end table
@end deffn

@deffn {Scheme Procedure} read-enable option-name
@deffnx {Scheme Procedure} read-disable option-name
@deffnx {Scheme Procedure} read-set! option-name value
Modify the read options.  @code{read-enable} should be used with boolean
options and switches them on, @code{read-disable} switches them off.
@code{read-set!} can be used to set an option to a specific value.
@end deffn

@deffn {Scheme Procedure} read-options-interface [setting]
@deffnx {C Function} scm_read_options (setting)
Option interface for the read options. Instead of using
this procedure directly, use the procedures @code{read-enable},
@code{read-disable}, @code{read-set!} and @code{read-options}.
@end deffn


@node Fly Evaluation
@section Procedures for On the Fly Evaluation

@xref{Environments}.

@rnindex eval
@c ARGFIXME environment/environment specifier
@deffn {Scheme Procedure} eval exp module
@deffnx {C Function} scm_eval (exp, module)
Evaluate @var{exp}, a list representing a Scheme expression,
in the top-level environment specified by @var{module}.
While @var{exp} is evaluated (using @code{primitive-eval}),
@var{module} is made the current module.  The current module
is reset to its previous value when @var{eval} returns.
@end deffn

@rnindex interaction-environment
@deffn {Scheme Procedure} interaction-environment
@deffnx {C Function} scm_interaction_environment ()
Return a specifier for the environment that contains
implementation--defined bindings, typically a superset of those
listed in the report.  The intent is that this procedure will
return the environment in which the implementation would
evaluate expressions dynamically typed by the user.
@end deffn

@deffn {Scheme Procedure} eval-string string
@deffnx {C Function} scm_eval_string (string)
Evaluate @var{string} as the text representation of a Scheme
form or forms, and return whatever value they produce.
Evaluation takes place in the environment returned by the
procedure @code{interaction-environment}.
@end deffn

@deffn {Scheme Procedure} apply:nconc2last lst
@deffnx {C Function} scm_nconc2last (lst)
Given a list (@var{arg1} @dots{} @var{args}), this function
conses the @var{arg1} @dots{} arguments onto the front of
@var{args}, and returns the resulting list. Note that
@var{args} is a list; thus, the argument to this function is
a list whose last element is a list.
Note: Rather than do new consing, @code{apply:nconc2last}
destroys its argument, so use with care.
@end deffn

@rnindex apply
@deffn {Scheme Procedure} apply proc arg1 @dots{} args
@var{proc} must be a procedure and @var{args} must be a list.  Call
@var{proc} with the elements of the list @code{(append (list @var{arg1}
@dots{}) @var{args})} as the actual arguments.
@end deffn

@deffn {Scheme Procedure} primitive-eval exp
@deffnx {C Function} scm_primitive_eval (exp)
Evaluate @var{exp} in the top-level environment specified by
the current module.
@end deffn


@node Loading
@section Loading Scheme Code from File

@rnindex load
@deffn {Scheme Procedure} load filename
Load @var{filename} and evaluate its contents in the top-level
environment.  The load paths are not searched.  If the variable
@code{%load-hook} is defined, it should be bound to a procedure that
will be called before any code is loaded.  See documentation for
@code{%load-hook} later in this section.
@end deffn

@deffn {Scheme Procedure} load-from-path filename
Similar to @code{load}, but searches for @var{filename} in the load
paths.
@end deffn

@deffn {Scheme Procedure} primitive-load filename
@deffnx {C Function} scm_primitive_load (filename)
Load the file named @var{filename} and evaluate its contents in
the top-level environment. The load paths are not searched;
@var{filename} must either be a full pathname or be a pathname
relative to the current directory.  If the  variable
@code{%load-hook} is defined, it should be bound to a procedure
that will be called before any code is loaded.  See the
documentation for @code{%load-hook} later in this section.
@end deffn

@deffn {Scheme Procedure} primitive-load-path filename
@deffnx {C Function} scm_primitive_load_path (filename)
Search @var{%load-path} for the file named @var{filename} and
load it into the top-level environment.  If @var{filename} is a
relative pathname and is not found in the list of search paths,
an error is signalled.
@end deffn

@deffn {Scheme Procedure} %search-load-path filename
@deffnx {C Function} scm_sys_search_load_path (filename)
Search @var{%load-path} for the file named @var{filename},
which must be readable by the current user.  If @var{filename}
is found in the list of paths to search or is an absolute
pathname, return its full pathname.  Otherwise, return
@code{#f}.  Filenames may have any of the optional extensions
in the @code{%load-extensions} list; @code{%search-load-path}
will try each extension automatically.
@end deffn

@defvar %load-hook
A procedure to be run whenever @code{primitive-load} is called.  If this
procedure is defined, it will be called with the filename argument that
was passed to @code{primitive-load}.

@example
(define %load-hook (lambda (file)
                     (display "Loading ")
                     (display file)
                     (write-line "...."))) @result{} undefined
(load-from-path "foo.scm")
@print{} Loading /usr/local/share/guile/site/foo.scm....
@end example

@end defvar

@deffn {Scheme Procedure} current-load-port
@deffnx {C Function} scm_current_load_port ()
Return the current-load-port.
The load port is used internally by @code{primitive-load}.
@end deffn

@defvar %load-extensions
A list of default file extensions for files containing Scheme code.
@code{%search-load-path} tries each of these extensions when looking for
a file to load.  By default, @code{%load-extensions} is bound to the
list @code{("" ".scm")}.
@end defvar


@node Delayed Evaluation
@section Delayed Evaluation

[delay]

@deffn {Scheme Procedure} promise? obj
@deffnx {C Function} scm_promise_p (obj)
Return true if @var{obj} is a promise, i.e. a delayed computation
(@pxref{Delayed evaluation,,,r5rs.info,The Revised^5 Report on Scheme}).
@end deffn

@rnindex force
@deffn {Scheme Procedure} force x
@deffnx {C Function} scm_force (x)
If the promise @var{x} has not been computed yet, compute and
return @var{x}, otherwise just return the previously computed
value.
@end deffn


@node Local Evaluation
@section Local Evaluation

[the-environment]

@deffn {Scheme Procedure} local-eval exp [env]
@deffnx {C Function} scm_local_eval (exp, env)
Evaluate @var{exp} in its environment.  If @var{env} is supplied,
it is the environment in which to evaluate @var{exp}.  Otherwise,
@var{exp} must be a memoized code object (in which case, its environment
is implicit).
@end deffn


@node Evaluator Behaviour
@section Evaluator Behaviour

@c FIXME::martin: Maybe this node name is bad, but the old name clashed with
@c `Evaluator options' under `Options and Config'.

The behaviour of Guile's evaluator can be modified by manipulating the
evaluator options.  For more information about options, @xref{User level
options interfaces}.  If you want to know which evaluator options are
available, @xref{Evaluator options}.

@c FIXME::martin: This is taken from libguile/options.c.  Is there 
@c actually a difference between 'help and 'full?

@deffn {Scheme Procedure} eval-options [setting]
Display the current settings of the evaluator options.  If @var{setting}
is omitted, only a short form of the current evaluator options is
printed.  Otherwise, @var{setting} should be one of the following
symbols:
@table @code
@item help
Display the complete option settings.
@item full
Like @code{help}, but also print programmer options.
@end table
@end deffn

@deffn {Scheme Procedure} eval-enable option-name
@deffnx {Scheme Procedure} eval-disable option-name
@deffnx {Scheme Procedure} eval-set! option-name value
Modify the evaluator options.  @code{eval-enable} should be used with boolean
options and switches them on, @code{eval-disable} switches them off.
@code{eval-set!} can be used to set an option to a specific value.
@end deffn

@deffn {Scheme Procedure} eval-options-interface [setting]
@deffnx {C Function} scm_eval_options_interface (setting)
Option interface for the evaluation options. Instead of using
this procedure directly, use the procedures @code{eval-enable},
@code{eval-disable}, @code{eval-set!} and @code{eval-options}.
@end deffn

@c FIXME::martin: Why aren't these procedure named like the other options
@c procedures?

@deffn {Scheme Procedure} traps [setting]
Display the current settings of the evaluator traps options.  If
@var{setting} is omitted, only a short form of the current evaluator
traps options is printed.  Otherwise, @var{setting} should be one of the
following symbols:
@table @code
@item help
Display the complete option settings.
@item full
Like @code{help}, but also print programmer options.
@end table
@end deffn

@deffn {Scheme Procedure} trap-enable option-name
@deffnx {Scheme Procedure} trap-disable option-name
@deffnx {Scheme Procedure} trap-set! option-name value
Modify the evaluator options.  @code{trap-enable} should be used with boolean
options and switches them on, @code{trap-disable} switches them off.
@code{trap-set!} can be used to set an option to a specific value.
@end deffn

@deffn {Scheme Procedure} evaluator-traps-interface [setting]
@deffnx {C Function} scm_evaluator_traps (setting)
Option interface for the evaluator trap options.
@end deffn


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