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

@page
@node Read/Load/Eval
@section 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
@subsection Scheme Syntax: Standard and Guile Extensions

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


@node Expression Syntax
@subsubsection Expression Syntax

An expression to be evaluated takes one of the following forms.

@table @nicode

@item @var{symbol}
A symbol is evaluated by dereferencing.  A binding of that symbol is
sought and the value there used.  For example,

@example
(define x 123)
x @result{} 123
@end example

@item (@var{proc} @var{args}@dots{})
A parenthesised expression is a function call.  @var{proc} and each
argument are evaluated, then the function (which @var{proc} evaluated
to) is called with those arguments.

The order in which @var{proc} and the arguments are evaluated is
unspecified, so be careful when using expressions with side effects.

@example
(max 1 2 3) @result{} 3

(define (get-some-proc)  min)
((get-some-proc) 1 2 3) @result{} 1
@end example

The same sort of parenthesised form is used for a macro invocation,
but in that case the arguments are not evaluated.  See the
descriptions of macros for more on this (@pxref{Macros}, and
@pxref{Syntax Rules}).

@item @var{constant}
Number, string, character and boolean constants evaluate ``to
themselves'', so can appear as literals.

@example
123     @result{} 123
99.9    @result{} 99.9
"hello" @result{} "hello"
#\z     @result{} #\z
#t      @result{} #t
@end example

Note that an application must not attempt to modify literal strings,
since they may be in read-only memory.

@item (quote @var{data})
@itemx '@var{data}
@findex quote
@findex '
Quoting is used to obtain a literal symbol (instead of a variable
reference), a literal list (instead of a function call), or a literal
vector.  @nicode{'} is simply a shorthand for a @code{quote} form.
For example,

@example
'x                   @result{} x
'(1 2 3)             @result{} (1 2 3)
'#(1 (2 3) 4)        @result{} #(1 (2 3) 4)
(quote x)            @result{} x
(quote (1 2 3))      @result{} (1 2 3)
(quote #(1 (2 3) 4)) @result{} #(1 (2 3) 4)
@end example

Note that an application must not attempt to modify literal lists or
vectors obtained from a @code{quote} form, since they may be in
read-only memory.

@item (quasiquote @var{data})
@itemx `@var{data}
@findex quasiquote
@findex `
Backquote quasi-quotation is like @code{quote}, but selected
sub-expressions are evaluated.  This is a convenient way to construct
a list or vector structure most of which is constant, but at certain
points should have expressions substituted.

The same effect can always be had with suitable @code{list},
@code{cons} or @code{vector} calls, but quasi-quoting is often easier.

@table @nicode

@item (unquote @var{expr})
@itemx ,@var{expr}
@findex unquote
@findex ,
Within the quasiquote @var{data}, @code{unquote} or @code{,} indicates
an expression to be evaluated and inserted.  The comma syntax @code{,}
is simply a shorthand for an @code{unquote} form.  For example,

@example
`(1 2 ,(* 9 9) 3 4)      @result{} (1 2 81 3 4)
`(1 (unquote (+ 1 1)) 3) @result{} (1 2 3)
`#(1 ,(/ 12 2))          @result{} #(1 6)
@end example

@item (unquote-splicing @var{expr})
@itemx ,@@@var{expr}
@findex unquote-splicing
@findex ,@@
Within the quasiquote @var{data}, @code{unquote-splicing} or
@code{,@@} indicates an expression to be evaluated and the elements of
the returned list inserted.  @var{expr} must evaluate to a list.  The
``comma-at'' syntax @code{,@@} is simply a shorthand for an
@code{unquote-splicing} form.

@example
(define x '(2 3))
`(1 ,@@x 4)                         @result{} (1 2 3 4)
`(1 (unquote-splicing (map 1+ x))) @result{} (1 3 4)
`#(9 ,@@x 9)                        @result{} #(9 2 3 9)
@end example

Notice @code{,@@} differs from plain @code{,} in the way one level of
nesting is stripped.  For @code{,@@} the elements of a returned list
are inserted, whereas with @code{,} it would be the list itself
inserted.
@end table

@c
@c  FIXME: What can we say about the mutability of a quasiquote
@c  result?  R5RS doesn't seem to specify anything, though where it
@c  says backquote without commas is the same as plain quote then
@c  presumably the "fixed" portions of a quasiquote expression must be
@c  treated as immutable.
@c

@end table


@node Comments
@subsubsection 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
@subsubsection Block Comments
@cindex multiline comments
@cindex block comments
@cindex #!
@cindex !#

@c FIXME::martin: Review me!

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
@subsubsection 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
@subsubsection Keyword Syntax


@node Reader Extensions
@subsubsection 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
@subsection 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
@subsection Procedures for On the Fly Evaluation

@xref{Environments}.

@rnindex eval
@c ARGFIXME environment/environment specifier
@deffn {Scheme Procedure} eval exp module_or_state
@deffnx {C Function} scm_eval (exp, module_or_state)
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.
XXX - dynamic states.
Example: (eval '(+ 1 2) (interaction-environment))
@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 [module]
@deffnx {C Function} scm_eval_string (string)
@deffnx {C Function} scm_eval_string_in_module (string, module)
Evaluate @var{string} as the text representation of a Scheme form or
forms, and return whatever value they produce.  Evaluation takes place
in the given module, or in the current module when no module is given.
While the code is evaluated, the given module is made the current one.
The current module is restored when this procedure returns.
@end deffn

@deftypefn {C Function} SCM scm_c_eval_string (const char *string)
@code{scm_eval_string}, but taking a C string instead of an
@code{SCM}.
@end deftypefn

@deffn {Scheme Procedure} apply proc arg1 @dots{} argN arglst
@deffnx {C Function} scm_apply_0 (proc, arglst)
@deffnx {C Function} scm_apply_1 (proc, arg1, arglst)
@deffnx {C Function} scm_apply_2 (proc, arg1, arg2, arglst)
@deffnx {C Function} scm_apply_3 (proc, arg1, arg2, arg3, arglst)
@deffnx {C Function} scm_apply (proc, arg, rest)
@rnindex apply
Call @var{proc} with arguments @var{arg1} @dots{} @var{argN} plus the
elements of the @var{arglst} list.

@code{scm_apply} takes parameters corresponding to a Scheme level
@code{(lambda (proc arg . rest) ...)}.  So @var{arg} and all but the
last element of the @var{rest} list make up
@var{arg1}@dots{}@var{argN} and the last element of @var{rest} is the
@var{arglst} list.  Or if @var{rest} is the empty list @code{SCM_EOL}
then there's no @var{arg1}@dots{}@var{argN} and @var{arg} is the
@var{arglst}.

@var{arglst} is not modified, but the @var{rest} list passed to
@code{scm_apply} is modified.
@end deffn

@deffn {C Function} scm_call_0 (proc)
@deffnx {C Function} scm_call_1 (proc, arg1)
@deffnx {C Function} scm_call_2 (proc, arg1, arg2)
@deffnx {C Function} scm_call_3 (proc, arg1, arg2, arg3)
@deffnx {C Function} scm_call_4 (proc, arg1, arg2, arg3, arg4)
Call @var{proc} with the given arguments.
@end deffn

@deffn {Scheme Procedure} apply:nconc2last lst
@deffnx {C Function} scm_nconc2last (lst)
@var{lst} should be a list (@var{arg1} @dots{} @var{argN}
@var{arglst}), with @var{arglst} being a list.  This function returns
a list comprising @var{arg1} to @var{argN} plus the elements of
@var{arglst}.  @var{lst} is modified to form the return.  @var{arglst}
is not modified, though the return does share structure with it.

This operation collects up the arguments from a list which is
@code{apply} style parameters.
@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
@subsection Loading Scheme Code from File

@rnindex load
@deffn {Scheme Procedure} load filename [reader]
Load @var{filename} and evaluate its contents in the top-level
environment.  The load paths are not searched.

@var{reader} if provided should be either @code{#f}, or a procedure with
the signature @code{(lambda (port) @dots{})} which reads the next
expression from @var{port}.  If @var{reader} is @code{#f} or absent,
Guile's built-in @code{read} procedure is used (@pxref{Scheme Read}).

The @var{reader} argument takes effect by setting the value of the
@code{current-reader} fluid (see below) before loading the file, and
restoring its previous value when loading is complete.  The Scheme code
inside @var{filename} can itself change the current reader procedure on
the fly by setting @code{current-reader} fluid.

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

@deftypefn {C Function} SCM scm_c_primitive_load (const char *filename)
@code{scm_primitive_load}, but taking a C string instead of an
@code{SCM}.
@end deftypefn

@deffn {Scheme Procedure} primitive-load-path filename
@deffnx {C Function} scm_primitive_load_path (filename)
Search @code{%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 @code{%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 current-reader
@code{current-reader} holds the read procedure that is currently being
used by the above loading procedures to read expressions (from the file
that they are loading).  @code{current-reader} is a fluid, so it has an
independent value in each dynamic root and should be read and set using
@code{fluid-ref} and @code{fluid-set!} (@pxref{Fluids and Dynamic
States}).
@end defvar

@defvar %load-hook
A procedure to be called @code{(%load-hook @var{filename})} whenever a
file is loaded, or @code{#f} for no such call.  @code{%load-hook} is
used by all of the above loading functions (@code{load},
@code{load-path}, @code{primitive-load} and
@code{primitive-load-path}).

For example an application can set this to show what's loaded,

@example
(set! %load-hook (lambda (filename)
                   (format #t "Loading ~a ...\n" filename)))
(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
@subsection Delayed Evaluation
@cindex delayed evaluation
@cindex promises

Promises are a convenient way to defer a calculation until its result
is actually needed, and to run such a calculation only once.

@deffn syntax delay expr
@rnindex delay
Return a promise object which holds the given @var{expr} expression,
ready to be evaluated by a later @code{force}.
@end deffn

@deffn {Scheme Procedure} promise? obj
@deffnx {C Function} scm_promise_p (obj)
Return true if @var{obj} is a promise.
@end deffn

@rnindex force
@deffn {Scheme Procedure} force p
@deffnx {C Function} scm_force (p)
Return the value obtained from evaluating the @var{expr} in the given
promise @var{p}.  If @var{p} has previously been forced then its
@var{expr} is not evaluated again, instead the value obtained at that
time is simply returned.

During a @code{force}, an @var{expr} can call @code{force} again on
its own promise, resulting in a recursive evaluation of that
@var{expr}.  The first evaluation to return gives the value for the
promise.  Higher evaluations run to completion in the normal way, but
their results are ignored, @code{force} always returns the first
value.
@end deffn


@node Local Evaluation
@subsection 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
@subsection 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.

See @ref{Evaluator trap options} for more information on the available
trap handlers.
@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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