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[bpt/guile.git] / 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, 2009
@c   Free Software Foundation, Inc.
@c See the file guile.texi for copying conditions.

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

This chapter describes Guile functions that are concerned with reading,
loading, evaluating, and compiling 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.
* Compilation::                 How to compile Scheme files and procedures.
* Loading::                     Loading Scheme code from file.
* Character Encoding of Source Files:: Loading non-ASCII 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.
* VM Behaviour::                Modifying Guile's virtual machine.
@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}).

@cindex R6RS block comments
@cindex SRFI-30 block comments
Similarly, Guile (starting from version 2.0) supports nested block
comments as specified by R6RS and
@url{http://srfi.schemers.org/srfi-30/srfi-30.html, SRFI-30}:

@lisp
(+  #| this is a #| nested |# block comment |# 2)
@result{} 3
@end lisp

For backward compatibility, this syntax can be overridden with
@code{read-hash-extend} (@pxref{Reader Extensions,
@code{read-hash-extend}}).

There is one special case where the contents of a comment can actually
affect the interpretation of code.  When a character encoding
declaration, such as @code{coding: utf-8} appears in one of the first
few lines of a source file, it indicates to Guile's default reader
that this source code file is not ASCII.  For details see @ref{Character
Encoding of Source Files}.

@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}. 
Passing @code{#f} for @var{proc} will remove a previous setting. 

@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 Compilation
@subsection Compiling Scheme Code

The @code{eval} procedure directly interprets the S-expression
representation of Scheme. An alternate strategy for evaluation is to
determine ahead of time what computations will be necessary to
evaluate the expression, and then use that recipe to produce the
desired results. This is known as @dfn{compilation}.

While it is possible to compile simple Scheme expressions such as
@code{(+ 2 2)} or even @code{"Hello world!"}, compilation is most
interesting in the context of procedures. Compiling a lambda expression
produces a compiled procedure, which is just like a normal procedure
except typically much faster, because it can bypass the generic
interpreter.

Functions from system modules in a Guile installation are normally
compiled already, so they load and run quickly.

Note that well-written Scheme programs will not typically call the
procedures in this section, for the same reason that it is often bad
taste to use @code{eval}. The normal interface to the compiler is the
command-line file compiler, which can be invoked from the shell as
@code{guile-tools compile @var{foo.scm}}. This interface needs more
documentation.

(Why are calls to @code{eval} and @code{compile} usually in bad taste?
Because they are limited, in that they can only really make sense for
top-level expressions. Also, most needs for ``compile-time''
computation are fulfilled by macros and closures. Of course one good
counterexample is the REPL itself, or any code that reads expressions
from a port.)

For more information on the compiler itself, see @ref{Compiling to the
Virtual Machine}. For information on the virtual machine, see @ref{A
Virtual Machine for Guile}.

@deffn {Scheme Procedure} compile exp [env=#f] [from=(current-language)] [to=value] [opts=()]
Compile the expression @var{exp} in the environment @var{env}. If
@var{exp} is a procedure, the result will be a compiled procedure;
otherwise @code{compile} is mostly equivalent to @code{eval}.

For a discussion of languages and compiler options, @xref{Compiling to
the Virtual Machine}.
@end deffn

@deffn {Scheme Procedure} compile-file file [to=objcode] [opts='()]
Compile the file named @var{file}.

Output will be written to a file in the current directory whose name
is computed as @code{(compiled-file-name @var{file})}.
@end deffn

@deffn {Scheme Procedure} compiled-file-name file
Compute an appropriate name for a compiled version of a Scheme file
named @var{file}.

Usually, the result will be the original file name with the
@code{.scm} suffix replaced with @code{.go}, but the exact behavior
depends on the contents of the @code{%load-extensions} and
@code{%load-compiled-extensions} lists.
@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-compiled filename
Load the compiled file named @var{filename}. The load paths are not
searched.

Compiling a source file (@pxref{Read/Load/Eval/Compile}) and then
calling @code{load-compiled} on the resulting file is equivalent to
calling @code{load} on the source file.
@end deffn

@deffn {Scheme Procedure} load-from-path filename
Similar to @code{load}, but searches for @var{filename} in the load
paths. Preferentially loads a compiled version of the file, if it is
available and up-to-date.
@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 [exception-on-not-found]
@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. Preferentially loads a compiled version of the
file, if it is available and up-to-date.

By default or if @var{exception-on-not-found} is true, an exception is
raised if @var{filename} is not found.  If @var{exception-on-not-found}
is @code{#f} and @var{filename} is not found, no exception is raised and
@code{#f} is returned.  For compatibility with Guile 1.8 and earlier,
the C function takes only one argument, which can be either a string
(the file name) or an argument list.
@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}).

Changing @code{current-reader} is typically useful to introduce local
syntactic changes, such that code following the @code{fluid-set!} call
is read using the newly installed reader.  The @code{current-reader}
change should take place at evaluation time when the code is evaluated,
or at compilation time when the code is compiled:

@findex eval-when
@example
(eval-when (compile eval)
  (fluid-set! current-reader my-own-reader))
@end example

The @code{eval-when} form above ensures that the @code{current-reader}
change occurs at the right time.
@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 Character Encoding of Source Files
@subsection Character Encoding of Source Files

@cindex source file encoding
@cindex primitive-load
@cindex load
Scheme source code files are usually encoded in ASCII, but, the
built-in reader can interpret other character encodings.  The
procedure @code{primitive-load}, and by extension the functions that
call it, such as @code{load}, first scan the top 500 characters of the
file for a coding declaration.

A coding declaration has the form @code{coding: XXXXXX}, where
@code{XXXXXX} is the name of a character encoding in which the source
code file has been encoded.  The coding declaration must appear in a
scheme comment.  It can either be a semicolon-initiated comment or a block
@code{#!} comment.

The name of the character encoding in the coding declaration is
typically lower case and containing only letters, numbers, and hyphens,
as recognized by @code{set-port-encoding!} (@pxref{Ports,
@code{set-port-encoding!}}).  Common examples of character encoding
names are @code{utf-8} and @code{iso-8859-1},
@url{http://www.iana.org/assignments/character-sets, as defined by
IANA}.  Thus, the coding declaration is mostly compatible with Emacs.

However, there are some differences in encoding names recognized by
Emacs and encoding names defined by IANA, the latter being essentially a
subset of the former.  For instance, @code{latin-1} is a valid encoding
name for Emacs, but it's not according to the IANA standard, which Guile
follows; instead, you should use @code{iso-8859-1}, which is both
understood by Emacs and dubbed by IANA (IANA writes it uppercase but
Emacs wants it lowercase and Guile is case insensitive.)

For source code, only a subset of all possible character encodings can
be interpreted by the built-in source code reader.  Only those
character encodings in which ASCII text appears unmodified can be
used.  This includes @code{UTF-8} and @code{ISO-8859-1} through
@code{ISO-8859-15}.  The multi-byte character encodings @code{UTF-16}
and @code{UTF-32} may not be used because they are not compatible with
ASCII.

@cindex read
@cindex encoding
@cindex port encoding
@findex set-port-encoding!
There might be a scenario in which one would want to read non-ASCII
code from a port, such as with the function @code{read}, instead of
with @code{load}.  If the port's character encoding is the same as the
encoding of the code to be read by the port, not other special
handling is necessary.  The port will automatically do the character
encoding conversion.  The functions @code{setlocale} or by
@code{set-port-encoding!} are used to set port encodings
(@pxref{Ports}).

If a port is used to read code of unknown character encoding, it can
accomplish this in three steps.  First, the character encoding of the
port should be set to ISO-8859-1 using @code{set-port-encoding!}.
Then, the procedure @code{file-encoding}, described below, is used to
scan for a coding declaration when reading from the port.  As a side
effect, it rewinds the port after its scan is complete. After that,
the port's character encoding should be set to the encoding returned
by @code{file-encoding}, if any, again by using
@code{set-port-encoding!}.  Then the code can be read as normal.

@deffn {Scheme Procedure} file-encoding port
@deffnx {C Function} scm_file_encoding port
Scan the port for an Emacs-like character coding declaration near the
top of the contents of a port with random-accessible contents
(@pxref{Recognize Coding, how Emacs recognizes file encoding,, emacs,
The GNU Emacs Reference Manual}).  The coding declaration is of the form
@code{coding: XXXXX} and must appear in a Scheme comment.  Return a
string containing the character encoding of the file if a declaration
was found, or @code{#f} otherwise.  The port is rewound.
@end deffn


@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

@node VM Behaviour
@subsection VM Behaviour

Like the procedures from the previous section that operate on the
evaluator, there are also procedures to modify the behavior of a
virtual machine.

The most useful thing that a user can do is to add to one of the
virtual machine's predefined hooks:

@deffn {Scheme Procedure} vm-next-hook vm
@deffnx {Scheme Procedure} vm-apply-hook vm
@deffnx {Scheme Procedure} vm-boot-hook vm
@deffnx {Scheme Procedure} vm-return-hook vm
@deffnx {Scheme Procedure} vm-break-hook vm
@deffnx {Scheme Procedure} vm-exit-hook vm
@deffnx {Scheme Procedure} vm-halt-hook vm
@deffnx {Scheme Procedure} vm-enter-hook vm
Accessors to a virtual machine's hooks. Usually you pass
@code{(the-vm)} as the @var{vm}.
@end deffn

@xref{A Virtual Machine for Guile}, for more information on Guile's
virtual machine.

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