[bpt/guile.git] / doc / ref / scheme-procedures.texi

@page
@node Procedures and Macros
@chapter Procedures and Macros

@menu
* Lambda::                      Basic procedure creation using lambda.
* Optional Arguments::          Handling keyword, optional and rest arguments.
* Procedure Properties::        Procedure properties and metainformation.
* Procedures with Setters::     Procedures with setters.
* Macros::                      Lisp style macro definitions.
* Syntax Rules::                Support for R5RS @code{syntax-rules}.
* Syntax Case::                 Support for the @code{syntax-case} system.
* Internal Macros::             Guile's internal representation.
@end menu


@node Lambda
@section Lambda: Basic Procedure Creation

@c FIXME::martin: Review me!

A @code{lambda} expression evaluates to a procedure.  The environment
which is in effect when a @code{lambda} expression is evaluated is
enclosed in the newly created procedure, this is referred to as a
@dfn{closure} (@pxref{About Closure}).

When a procedure created by @code{lambda} is called with some actual
arguments, the environment enclosed in the procedure is extended by
binding the variables named in the formal argument list to new locations
and storing the actual arguments into these locations.  Then the body of
the @code{lambda} expression is evaluation sequentially.  The result of
the last expression in the procedure body is then the result of the
procedure invocation.

The following examples will show how procedures can be created using
@code{lambda}, and what you can do with these procedures.

@lisp
(lambda (x) (+ x x))       @result{} @r{a procedure}
((lambda (x) (+ x x)) 4)   @result{} 8
@end lisp

The fact that the environment in effect when creating a procedure is
enclosed in the procedure is shown with this example:

@lisp
(define add4
  (let ((x 4))
    (lambda (y) (+ x y))))
(add4 6)                   @result{} 10
@end lisp


@deffn syntax lambda formals body
@var{formals} should be a formal argument list as described in the
following table.

@table @code
@item (@var{variable1} @dots{})
The procedure takes a fixed number of arguments; when the procedure is
called, the arguments will be stored into the newly created location for
the formal variables.
@item @var{variable}
The procedure takes any number of arguments; when the procedure is
called, the sequence of actual arguments will converted into a list and
stored into the newly created location for the formal variable.
@item (@var{variable1} @dots{} @var{variablen} . @var{variablen+1})
If a space-delimited period precedes the last variable, then the
procedure takes @var{n} or more variablesm where @var{n} is the number
of formal arguments before the period.  There must be at least one
argument before the period.  The first @var{n} actual arguments will be
stored into the newly allocated locations for the first @var{n} formal
arguments and the sequence of the remaining actual arguments is
converted into a list and the stored into the location for the last
formal argument.  If there are exactly @var{n} actual arguments, the
empty list is stored into the location of the last formal argument.
@end table

@var{body} is a sequence of Scheme expressions which are evaluated in
order when the procedure is invoked.
@end deffn


@node Optional Arguments
@section Optional Arguments

@c FIXME::martin: Review me!

Scheme procedures, as defined in R5RS, can wither handle a fixed number
of actual arguments, or a fixed number of actual arguments followed by
arbitrarily many additional arguments.  Writing procedures of variable
arity can be useful, but unfortunately, the syntactic means for handling
argument lists of varying length is a bit inconvenient.  It is possible
to give names to the fixed number of argument, but the remaining
(optional) arguments can be only referenced as a list of values
(@pxref{Lambda}).

Guile comes with the module @code{(ice-9 optargs)}, which makes using
optional arguments much more convenient.  In addition, this module
provides syntax for handling keywords in argument lists
(@pxref{Keywords}).

Before using any of the procedures or macros defined in this section,
you have to load the module @code{(ice-9 optargs)} with the statement:

@lisp
(use-modules (ice-9 optargs))
@end lisp

@menu
* let-optional Reference::      Locally binding optional arguments.
* let-keywords Reference::      Locally binding keywords arguments.
* lambda* Reference::           Creating advanced argument handling procedures.
* define* Reference::           Defining procedures and macros.
@end menu


@node let-optional Reference
@subsection let-optional Reference

@c FIXME::martin: Review me!

The syntax @code{let-optional} and @code{let-optional*} are for
destructuring rest argument lists and giving names to the various list
elements.  @code{let-optional} binds all variables simultaneously, while
@code{let-optional*} binds them sequentially, consistent with @code{let}
and @code{let*} (@pxref{Local Bindings}).

@deffn {libary syntax} let-optional rest-arg (binding @dots{}) expr @dots{}
@deffnx {library syntax} let-optional* rest-arg (binding @dots{}) expr @dots{}
These two macros give you an optional argument interface that is very
@dfn{Schemey} and introduces no fancy syntax. They are compatible with
the scsh macros of the same name, but are slightly extended. Each of
@var{binding} may be of one of the forms @var{var} or @code{(@var{var}
@var{default-value})}. @var{rest-arg} should be the rest-argument of the
procedures these are used from.  The items in @var{rest-arg} are
sequentially bound to the variable names are given. When @var{rest-arg}
runs out, the remaining vars are bound either to the default values or
left unbound if no default value was specified. @var{rest-arg} remains
bound to whatever may have been left of @var{rest-arg}.

After binding the variables, the expressions @var{expr} @dots{} are
evaluated in order.
@end deffn


@node let-keywords Reference
@subsection let-keywords Reference

@c FIXME::martin: Review me!

@code{let-keywords} and @code{let-keywords*} are used for extracting
values from argument lists which use keywords instead of argument
position for binding local variables to argument values.

@code{let-keywords} binds all variables simultaneously, while
@code{let-keywords*} binds them sequentially, consistent with @code{let}
and @code{let*} (@pxref{Local Bindings}).

@deffn {library syntax} let-keywords rest-arg allow-other-keys? (binding @dots{})  expr @dots{}
@deffnx {library syntax} let-keywords rest-arg allow-other-keys? (binding @dots{})  expr @dots{}
These macros pick out keyword arguments from @var{rest-arg}, but do not
modify it.  This is consistent at least with Common Lisp, which
duplicates keyword arguments in the rest argument. More explanation of what
keyword arguments in a lambda list look like can be found below in
the documentation for @code{lambda*}
 (@pxref{lambda* Reference}).  @var{binding}s can have the same form as
for @code{let-optional}. If @var{allow-other-keys?} is false, an error
will be thrown if anything that looks like a keyword argument but does
not match a known keyword parameter will result in an error.

After binding the variables, the expressions @var{expr} @dots{} are
evaluated in order.
@end deffn


@node lambda* Reference
@subsection lambda* Reference

@c FIXME::martin: Review me!

When using optional and keyword argument lists, using @code{lambda} for
creating procedures and using @code{let-optional} or @code{let-keywords}
is a bit lengthy.  Therefore, @code{lambda*} is provided, which combines
the features of those macros into a single convenient syntax.

For quick reference, here is the syntax of the formal argument list for
@code{lambda*} (brackets are used to indicate grouping only):

@example
ext-param-list ::= [identifier]* [#:optional [ext-var-decl]+]?
  [#:key [ext-var-decl]+ [#:allow-other-keys]?]?
  [[#:rest identifier]|[. identifier]]?

ext-var-decl ::= identifier | ( identifier expression )
@end example

The characters `*', `+' and `?' are not to be taken literally; they mean
respectively, zero or more occurences, one or more occurences, and one
or zero occurences.

@deffn {library syntax} lambda* formals body
@code{lambda*} creates a procedure that takes optional arguments. These
are specified by putting them inside brackets at the end of the
paramater list, but before any dotted rest argument. For example,

@lisp
(lambda* (a b #:optional c d . e) '())
@end lisp

creates a procedure with fixed arguments @var{a} and @var{b}, optional
arguments @var{c} and @var{d}, and rest argument @var{e}. If the
optional arguments are omitted in a call, the variables for them are
unbound in the procedure. This can be checked with the @code{bound?}
macro (documented below).

@code{lambda*} can also take keyword arguments. For example, a procedure
defined like this:

@lisp
(lambda* (#:key xyzzy larch) '())
@end lisp

can be called with any of the argument lists @code{(#:xyzzy 11)}
@code{(#:larch 13)} @code{(#:larch 42 #:xyzzy 19)} @code{()}. Whichever
arguments are given as keywords are bound to values.

Optional and keyword arguments can also be given default values
which they take on when they are not present in a call, by giving a
two-item list in place of an optional argument, for example in:

@lisp
(lambda* (foo #:optional (bar 42) #:key (baz 73))
     (list foo bar baz))
@end lisp

@var{foo} is a fixed argument, @var{bar} is an optional argument with
default value 42, and baz is a keyword argument with default value 73.
Default value expressions are not evaluated unless they are needed and
until the procedure is called.

@code{lambda*} also supports two more special parameter list keywords.

@code{lambda*}-defined procedures now throw an error by default if a
keyword other than one of those specified is found in the actual
passed arguments. However, specifying @code{#:allow-other-keys}
immediately after the keyword argument declarations restores the
previous behavior of ignoring unknown keywords.  @code{lambda*} also now
guarantees that if the same keyword is passed more than once, the
last one passed is the one that takes effect. For example,

@lisp
((lambda* (#:key (heads 0) (tails 0)) (display (list heads tails)))
    #:heads 37 #:tails 42 #:heads 99)
@end lisp

would result in (99 47) being displayed.

@code{#:rest} is also now provided as a synonym for the dotted syntax
rest argument. The argument lists @code{(a . b)} and @code{(a #:rest b)}
are equivalent in all respects to @code{lambda*}. This is provided for
more similarity to DSSSL, MIT-Scheme and Kawa among others, as well as
for refugees from other Lisp dialects.
@end deffn

@deffn {library syntax} bound? variable
Check if a variable is bound in the current environment.

The procedure @code{defined?} doesn't quite cut it as it stands, since
it only checks bindings in the top-level environment, not those in local
scope only.
@end deffn


@node define* Reference
@subsection define* Reference

@c FIXME::martin: Review me!

Just like @code{define} has a shorthand notation for defining procedures
(@pxref{Lambda Alternatives}), @code{define*} is provided as an
abbreviation of the combination of @code{define} and @code{lambda*}.

@code{define*-public} is the @code{lambda*} version of
@code{define-public}; @code{defmacro*} and @code{defmacro*-public} exist
for defining macros with the improved argument list handling
possibilities.  The @code{-public} versions not only define the
procedures/macros, but also export them from the current module.

@deffn {library syntax} define* formals body
@deffnx {library syntax} define*-public formals body
@code{define*} and @code{define*-public} support optional arguments with
a similar syntax to @code{lambda*}. They also support arbitrary-depth
currying, just like Guile's define. Some examples:

@lisp
(define* (x y #:optional a (z 3) #:key w . u)
   (display (list y z u)))
@end lisp
defines a procedure @code{x} with a fixed argument @var{y}, an optional
agument @var{a}, another optional argument @var{z} with default value 3,
a keyword argument @var{w}, and a rest argument @var{u}.

@lisp
(define-public* ((foo #:optional bar) #:optional baz) '())
@end lisp

This illustrates currying. A procedure @code{foo} is defined, which,
when called with an optional argument @var{bar}, returns a procedure
that takes an optional argument @var{baz}.

Of course, @code{define*[-public]} also supports @code{#:rest} and
@code{#:allow-other-keys} in the same way as @code{lambda*}.
@end deffn

@deffn {library syntax} defmacro* name formals body
@deffnx {library syntax} defmacro*-public name formals body
These are just like @code{defmacro} and @code{defmacro-public} except that they
take @code{lambda*}-style extended paramter lists, where @code{#:optional},
@code{#:key}, @code{#:allow-other-keys} and @code{#:rest} are allowed with the usual
semantics. Here is an example of a macro with an optional argument:

@lisp
(defmacro* transmorgify (a #:optional b)
    (a 1))
@end lisp
@end deffn


@node Procedure Properties
@section Procedure Properties and Metainformation

@c FIXME::martin: Review me!

Procedures always have attached the environment in which they were
created and information about how to apply them to actual arguments.  In
addition to that, properties and metainformation can be stored with
procedures.  The procedures in this section can be used to test whether
a given procedure satisfies a condition; and to access and set a
procedure's property.

The first group of procedures are predicates to test whether a Scheme
object is a procedure, or a special procedure, respectively.
@code{procedure?} is the most general predicates, it returns @code{#t}
for any kind of procedure.  @code{closure?} does not return @code{#t}
for primitive procedures, and @code{thunk?} only returns @code{#t} for
procedures which do not accept any arguments.

@rnindex procedure?
@deffn primitive procedure? obj
Return @code{#t} if @var{obj} is a procedure.
@end deffn

@deffn primitive closure? obj
Return @code{#t} if @var{obj} is a closure.
@end deffn

@deffn primitive thunk? obj
Return @code{#t} if @var{obj} is a thunk.
@end deffn

@c FIXME::martin: Is that true?
@cindex procedure properties
Procedure properties are general properties to be attached to
procedures.  These can be the name of a procedure or other relevant
information, such as debug hints.

@deffn primitive procedure-properties proc
Return @var{obj}'s property list.
@end deffn

@deffn primitive procedure-property p k
Return the property of @var{obj} with name @var{key}.
@end deffn

@deffn primitive set-procedure-properties! proc new_val
Set @var{obj}'s property list to @var{alist}.
@end deffn

@deffn primitive set-procedure-property! p k v
In @var{obj}'s property list, set the property named @var{key} to
@var{value}.
@end deffn

@cindex procedure documentation
Documentation for a procedure can be accessed with the procedure
@code{procedure-documentation}.

@deffn primitive procedure-documentation proc
Return the documentation string associated with @code{proc}.  By
convention, if a procedure contains more than one expression and the
first expression is a string constant, that string is assumed to contain
documentation for that procedure.
@end deffn

@cindex source properties
@c FIXME::martin: Is the following true?
Source properties are properties which are related to the source code of
a procedure, such as the line and column numbers, the file name etc.

@deffn primitive set-source-properties! obj plist
Install the association list @var{plist} as the source property
list for @var{obj}.
@end deffn

@deffn primitive set-source-property! obj key datum
Set the source property of object @var{obj}, which is specified by
@var{key} to @var{datum}.  Normally, the key will be a symbol.
@end deffn

@deffn primitive source-properties obj
Return the source property association list of @var{obj}.
@end deffn


@deffn primitive source-property obj key
Return the source property specified by @var{key} from
@var{obj}'s source property list.
@end deffn


@node Procedures with Setters
@section Procedures with Setters

@c FIXME::martin: Review me!

@c FIXME::martin: Document `operator struct'.

@cindex procedure with setter
@cindex setter
A @dfn{procedure with setter} is a special kind of procedure which
normally behaves like any accesor procedure, that is a procedure which
accesses a data structure.  The difference is that this kind of
procedure has a so-called @dfn{setter} attached, which is a procedure
for storing something into a data structure.

Procedures with setters are treated specially when the procedure appears
in the special form @code{set!} (REFFIXME).  How it works is best shown
by example.

Suppose we have a procedure called @code{foo-ref}, which accepts two
arguments, a value of type @code{foo} and an integer.  The procedure
returns the value stored at the given index in the @code{foo} object.
Let @code{f} be a variable containing such a @code{foo} data
structure.@footnote{Working definitions would be:
@lisp
(define foo-ref vector-ref)
(define foo-set! vector-set!)
(define f (make-vector 2 #f))
@end lisp
}

@lisp
(foo-ref f 0)       @result{} bar
(foo-ref f 1)       @result{} braz
@end lisp

Also suppose that a corresponding setter procedure called
@code{foo-set!} does exist.

@lisp
(foo-set! f 0 'bla)
(foo-ref f 0)       @result{} bla
@end lisp

Now we could create a new procedure called @code{foo}, which is a
procedure with setter, by calling @code{make-procedure-with-setter} with
the accessor and setter procedures @code{foo-ref} and @code{foo-set!}.
Let us call this new procedure @code{foo}.

@lisp
(define foo (make-procedure-with-setter foo-ref foo-set!))
@end lisp

@code{foo} can from now an be used to either read from the data
structure stored in @code{f}, or to write into the structure.

@lisp
(set! (foo f 0) 'dum)
(foo f 0)          @result{} dum
@end lisp

@deffn primitive make-procedure-with-setter procedure setter
Create a new procedure which behaves like @var{procedure}, but
with the associated setter @var{setter}.
@end deffn

@deffn primitive procedure-with-setter? obj
Return @code{#t} if @var{obj} is a procedure with an
associated setter procedure.
@end deffn

@deffn primitive procedure proc
Return the procedure of @var{proc}, which must be either a
procedure with setter, or an operator struct.
@end deffn

@deffn primitive setter proc
Return the setter of @var{proc}, which must be either a procedure with
setter or an operator struct.
@end deffn


@node Macros
@section Lisp Style Macro Definitions

@cindex macros
@cindex transformation
Macros are objects which cause the expression that they appear in to be
transformed in some way @emph{before} being evaluated.  In expressions
that are intended for macro transformation, the identifier that names
the relevant macro must appear as the first element, like this:

@lisp
(@var{macro-name} @var{macro-args} @dots{})
@end lisp

In Lisp-like languages, the traditional way to define macros is very
similar to procedure definitions.  The key differences are that the
macro definition body should return a list that describes the
transformed expression, and that the definition is marked as a macro
definition (rather than a procedure definition) by the use of a
different definition keyword: in Lisp, @code{defmacro} rather than
@code{defun}, and in Scheme, @code{define-macro} rather than
@code{define}.

@fnindex defmacro
@fnindex define-macro
Guile supports this style of macro definition using both @code{defmacro}
and @code{define-macro}.  The only difference between them is how the
macro name and arguments are grouped together in the definition:

@lisp
(defmacro @var{name} (@var{args} @dots{}) @var{body} @dots{})
@end lisp

@noindent
is the same as

@lisp
(define-macro (@var{name} @var{args} @dots{}) @var{body} @dots{})
@end lisp

@noindent
The difference is analogous to the corresponding difference between
Lisp's @code{defun} and Scheme's @code{define}.

@code{false-if-exception}, from the @file{boot-9.scm} file in the Guile
distribution, is a good example of macro definition using
@code{defmacro}:

@lisp
(defmacro false-if-exception (expr)
  `(catch #t
          (lambda () ,expr)
          (lambda args #f)))
@end lisp

@noindent
The effect of this definition is that expressions beginning with the
identifier @code{false-if-exception} are automatically transformed into
a @code{catch} expression following the macro definition specification.
For example:

@lisp
(false-if-exception (open-input-file "may-not-exist"))
@equiv{}
(catch #t
       (lambda () (open-input-file "may-not-exist"))
       (lambda args #f))
@end lisp


@node Syntax Rules
@section The R5RS @code{syntax-rules} System
@cindex R5RS syntax-rules system

R5RS defines an alternative system for macro and syntax transformations
using the keywords @code{define-syntax}, @code{let-syntax},
@code{letrec-syntax} and @code{syntax-rules}.

The main difference between the R5RS system and the traditional macros
of the previous section is how the transformation is specified.  In
R5RS, rather than permitting a macro definition to return an arbitrary
expression, the transformation is specified in a pattern language that

@itemize @bullet
@item
does not require complicated quoting and extraction of components of the
source expression using @code{caddr} etc.

@item
is designed such that the bindings associated with identifiers in the
transformed expression are well defined, and such that it is impossible
for the transformed expression to construct new identifiers.
@end itemize

@noindent
The last point is commonly referred to as being @dfn{hygienic}: the R5RS
@code{syntax-case} system provides @dfn{hygienic macros}.

For example, the R5RS pattern language for the @code{false-if-exception}
example of the previous section looks like this:

@lisp
(syntax-rules ()
  ((_ expr)
   (catch #t
          (lambda () expr)
          (lambda args #f))))
@end lisp

In Guile, the @code{syntax-rules} system is provided by the @code{(ice-9
syncase)} module.  To make these facilities available in your code,
include the expression @code{(use-modules (ice-9 syncase))} or
@code{(use-syntax (ice-9 syncase))} (@pxref{Using Guile Modules})
before the first usage of @code{define-syntax} etc.  If you are writing
a Scheme module, you can alternatively use one of the keywords
@code{#:use-module} and @code{#:use-syntax} in your @code{define-module}
declaration (@pxref{Creating Guile Modules}).

@menu
* Pattern Language::            The @code{syntax-rules} pattern language.
* Define-Syntax::               Top level syntax definitions.
* Let-Syntax::                  Local syntax definitions.
@end menu


@node Pattern Language
@subsection The @code{syntax-rules} Pattern Language


@node Define-Syntax
@subsection Top Level Syntax Definitions

define-syntax:  The gist is

  (define-syntax <keyword> <transformer-spec>)

makes the <keyword> into a macro so that

  (<keyword> ...)

expands at _compile_ or _read_ time (i.e. before any
evaluation begins) into some expression that is
given by the <transformer-spec>.


@node Let-Syntax
@subsection Local Syntax Definitions


@node Syntax Case
@section Support for the @code{syntax-case} System



@node Internal Macros
@section Internal Representation of Macros and Syntax

Internally, Guile uses three different flavours of macros.  The three
flavours are called @dfn{acro} (or @dfn{syntax}), @dfn{macro} and
@dfn{mmacro}.

Given the expression

@lisp
(foo @dots{})
@end lisp

@noindent
with @code{foo} being some flavour of macro, one of the following things
will happen when the expression is evaluated.

@itemize @bullet
@item
When @code{foo} has been defined to be an @dfn{acro}, the procedure used
in the acro definition of @code{foo} is passed the whole expression and
the current lexical environment, and whatever that procedure returns is
the value of evaluating the expression.  You can think of this a
procedure that receives its argument as an unevaluated expression.

@item
When @code{foo} has been defined to be a @dfn{macro}, the procedure used
in the macro definition of @code{foo} is passed the whole expression and
the current lexical environment, and whatever that procedure returns is
evaluated again.  That is, the procedure should return a valid Scheme
expression.

@item
When @code{foo} has been defined to be a @dfn{mmacro}, the procedure
used in the mmacro definition of `foo' is passed the whole expression
and the current lexical environment, and whatever that procedure returns
replaces the original expression.  Evaluation then starts over from the
new expression that has just been returned.
@end itemize

The key difference between a @dfn{macro} and a @dfn{mmacro} is that the
expression returned by a @dfn{mmacro} procedure is remembered (or
@dfn{memoized}) so that the expansion does not need to be done again
next time the containing code is evaluated.

The primitives @code{procedure->syntax}, @code{procedure->macro} and
@code{procedure->memoizing-macro} are used to construct acros, macros
and mmacros respectively.  However, if you do not have a very special
reason to use one of these primitives, you should avoid them: they are
very specific to Guile's current implementation and therefore likely to
change.  Use @code{defmacro}, @code{define-macro} (@pxref{Macros}) or
@code{define-syntax} (@pxref{Syntax Rules}) instead.  (In low level
terms, @code{defmacro}, @code{define-macro} and @code{define-syntax} are
all implemented as mmacros.)

@deffn primitive procedure->syntax code
Return a macro which, when a symbol defined to this value appears as the
first symbol in an expression, returns the result of applying @var{code}
to the expression and the environment.
@end deffn

@deffn primitive procedure->macro code
Return a macro which, when a symbol defined to this value appears as the
first symbol in an expression, evaluates the result of applying
@var{code} to the expression and the environment.  For example:

@lisp
(define trace
  (procedure->macro
    (lambda (x env)
      `(set! ,(cadr x) (tracef ,(cadr x) ',(cadr x))))))

(trace @i{foo})
@equiv{}
(set! @i{foo} (tracef @i{foo} '@i{foo})).
@end lisp
@end deffn

@deffn primitive procedure->memoizing-macro code
Return a macro which, when a symbol defined to this value appears as the
first symbol in an expression, evaluates the result of applying
@var{code} to the expression and the environment.
@code{procedure->memoizing-macro} is the same as
@code{procedure->macro}, except that the expression returned by
@var{code} replaces the original macro expression in the memoized form
of the containing code.
@end deffn

In the following primitives, @dfn{acro} flavour macros are referred to
as @dfn{syntax transformers}.

@deffn primitive macro? obj
Return @code{#t} if @var{obj} is a regular macro, a memoizing macro or a
syntax transformer.
@end deffn

@deffn primitive macro-type m
Return one of the symbols @code{syntax}, @code{macro} or
@code{macro!}, depending on whether @var{m} is a syntax
transformer, a regular macro, or a memoizing macro,
respectively.  If @var{m} is not a macro, @code{#f} is
returned.
@end deffn

@deffn primitive macro-name m
Return the name of the macro @var{m}.
@end deffn

@deffn primitive macro-transformer m
Return the transformer of the macro @var{m}.
@end deffn

@deffn primitive cons-source xorig x y
Create and return a new pair whose car and cdr are @var{x} and @var{y}.
Any source properties associated with @var{xorig} are also associated
with the new pair.
@end deffn


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