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[bpt/guile.git] / doc / ref / srfi-modules.texi

@c -*-texinfo-*-
@c This is part of the GNU Guile Reference Manual.
@c Copyright (C)  1996, 1997, 2000, 2001, 2002, 2003, 2004
@c   Free Software Foundation, Inc.
@c See the file guile.texi for copying conditions.

@page
@node SRFI Support
@section SRFI Support Modules
@cindex SRFI

SRFI is an acronym for Scheme Request For Implementation.  The SRFI
documents define a lot of syntactic and procedure extensions to standard
Scheme as defined in R5RS.

Guile has support for a number of SRFIs.  This chapter gives an overview
over the available SRFIs and some usage hints.  For complete
documentation, design rationales and further examples, we advise you to
get the relevant SRFI documents from the SRFI home page
@url{http://srfi.schemers.org}.

@menu
* About SRFI Usage::            What to know about Guile's SRFI support.
* SRFI-0::                      cond-expand
* SRFI-1::                      List library.
* SRFI-2::                      and-let*.
* SRFI-4::                      Homogeneous numeric vector datatypes.
* SRFI-6::                      Basic String Ports.
* SRFI-8::                      receive.
* SRFI-9::                      define-record-type.
* SRFI-10::                     Hash-Comma Reader Extension.
* SRFI-11::                     let-values and let-values*.
* SRFI-13::                     String library.
* SRFI-14::                     Character-set library.
* SRFI-16::                     case-lambda
* SRFI-17::                     Generalized set!
* SRFI-19::                     Time/Date library.
* SRFI-26::                     Specializing parameters
* SRFI-31::                     A special form `rec' for recursive evaluation
* SRFI-39::                     Parameter objects
@end menu


@node About SRFI Usage
@subsection About SRFI Usage

@c FIXME::martin: Review me!

SRFI support in Guile is currently implemented partly in the core
library, and partly as add-on modules.  That means that some SRFIs are
automatically available when the interpreter is started, whereas the
other SRFIs require you to use the appropriate support module
explicitly.

There are several reasons for this inconsistency.  First, the feature
checking syntactic form @code{cond-expand} (@pxref{SRFI-0}) must be
available immediately, because it must be there when the user wants to
check for the Scheme implementation, that is, before she can know that
it is safe to use @code{use-modules} to load SRFI support modules.  The
second reason is that some features defined in SRFIs had been
implemented in Guile before the developers started to add SRFI
implementations as modules (for example SRFI-6 (@pxref{SRFI-6})).  In
the future, it is possible that SRFIs in the core library might be
factored out into separate modules, requiring explicit module loading
when they are needed.  So you should be prepared to have to use
@code{use-modules} someday in the future to access SRFI-6 bindings.  If
you want, you can do that already.  We have included the module
@code{(srfi srfi-6)} in the distribution, which currently does nothing,
but ensures that you can write future-safe code.

Generally, support for a specific SRFI is made available by using
modules named @code{(srfi srfi-@var{number})}, where @var{number} is the
number of the SRFI needed.  Another possibility is to use the command
line option @code{--use-srfi}, which will load the necessary modules
automatically (@pxref{Invoking Guile}).


@node SRFI-0
@subsection SRFI-0 - cond-expand
@cindex SRFI-0

This SRFI lets a portable Scheme program test for the presence of
certain features, and adapt itself by using different blocks of code,
or fail if the necessary features are not available.  There's no
module to load, this is in the Guile core.

A program designed only for Guile will generally not need this
mechanism, such a program can of course directly use the various
documented parts of Guile.

@deffn syntax cond-expand (feature body@dots{}) @dots{}
Expand to the @var{body} of the first clause whose @var{feature}
specification is satisfied.  It is an error if no @var{feature} is
satisfied.

Features are symbols such as @code{srfi-1}, and a feature
specification can use @code{and}, @code{or} and @code{not} forms to
test combinations.  The last clause can be an @code{else}, to be used
if no other passes.

For example, define a private version of @code{alist-cons} if SRFI-1
is not available.

@example
(cond-expand (srfi-1
              )
             (else
              (define (alist-cons key val alist)
                (cons (cons key val) alist))))
@end example

Or demand a certain set of SRFIs (list operations, string ports,
@code{receive} and string operations), failing if they're not
available.

@example
(cond-expand ((and srfi-1 srfi-6 srfi-8 srfi-13)
              ))
@end example
@end deffn

@noindent
The Guile core has the following features,

@example
guile
r5rs
srfi-0
srfi-4
srfi-6
srfi-13
srfi-14
@end example

Other SRFI feature symbols are defined once their code has been loaded
with @code{use-modules}, since only then are their bindings available.

The @samp{--use-srfi} command line option (@pxref{Invoking Guile}) is
a good way to load SRFIs to satisfy @code{cond-expand} when running a
portable program.

Testing the @code{guile} feature allows a program to adapt itself to
the Guile module system, but still run on other Scheme systems.  For
example the following demands SRFI-8 (@code{receive}), but also knows
how to load it with the Guile mechanism.

@example
(cond-expand (srfi-8
              )
             (guile
              (use-modules (srfi srfi-8))))
@end example

It should be noted that @code{cond-expand} is separate from the
@code{*features*} mechanism (@pxref{Feature Tracking}), feature
symbols in one are unrelated to those in the other.


@node SRFI-1
@subsection SRFI-1 - List library
@cindex SRFI-1
@cindex list

@c FIXME::martin: Review me!

The list library defined in SRFI-1 contains a lot of useful list
processing procedures for construction, examining, destructuring and
manipulating lists and pairs.

Since SRFI-1 also defines some procedures which are already contained
in R5RS and thus are supported by the Guile core library, some list
and pair procedures which appear in the SRFI-1 document may not appear
in this section.  So when looking for a particular list/pair
processing procedure, you should also have a look at the sections
@ref{Lists} and @ref{Pairs}.

@menu
* SRFI-1 Constructors::         Constructing new lists.
* SRFI-1 Predicates::           Testing list for specific properties.
* SRFI-1 Selectors::            Selecting elements from lists.
* SRFI-1 Length Append etc::    Length calculation and list appending.
* SRFI-1 Fold and Map::         Higher-order list processing.
* SRFI-1 Filtering and Partitioning::  Filter lists based on predicates.
* SRFI-1 Searching::            Search for elements.
* SRFI-1 Deleting::             Delete elements from lists.
* SRFI-1 Association Lists::    Handle association lists.
* SRFI-1 Set Operations::       Use lists for representing sets.
@end menu

@node SRFI-1 Constructors
@subsubsection Constructors
@cindex list constructor

@c FIXME::martin: Review me!

New lists can be constructed by calling one of the following
procedures.

@deffn {Scheme Procedure} xcons d a
Like @code{cons}, but with interchanged arguments.  Useful mostly when
passed to higher-order procedures.
@end deffn

@deffn {Scheme Procedure} list-tabulate n init-proc
Return an @var{n}-element list, where each list element is produced by
applying the procedure @var{init-proc} to the corresponding list
index.  The order in which @var{init-proc} is applied to the indices
is not specified.
@end deffn

@deffn {Scheme Procedure} list-copy lst
Return a new list containing the elements of the list @var{lst}.

This function differs from the core @code{list-copy} (@pxref{List
Constructors}) in accepting improper lists too.  And if @var{lst} is
not a pair at all then it's treated as the final tail of an improper
list and simply returned.
@end deffn

@deffn {Scheme Procedure} circular-list elt1 elt2 @dots{}
Return a circular list containing the given arguments @var{elt1}
@var{elt2} @dots{}.
@end deffn

@deffn {Scheme Procedure} iota count [start step]
Return a list containing @var{count} numbers, starting from
@var{start} and adding @var{step} each time.  The default @var{start}
is 0, the default @var{step} is 1.  For example,

@example
(iota 6)        @result{} (0 1 2 3 4 5)
(iota 4 2.5 -2) @result{} (2.5 0.5 -1.5 -3.5)
@end example

This function takes its name from the corresponding primitive in the
APL language.
@end deffn


@node SRFI-1 Predicates
@subsubsection Predicates
@cindex list predicate

@c FIXME::martin: Review me!

The procedures in this section test specific properties of lists.

@deffn {Scheme Procedure} proper-list? obj
Return @code{#t} if @var{obj} is a proper list, or @code{#f}
otherwise.  This is the same as the core @code{list?} (@pxref{List
Predicates}).

A proper list is a list which ends with the empty list @code{()} in
the usual way.  The empty list @code{()} itself is a proper list too.

@example
(proper-list? '(1 2 3))  @result{} #t
(proper-list? '())       @result{} #t
@end example
@end deffn

@deffn {Scheme Procedure} circular-list? obj
Return @code{#t} if @var{obj} is a circular list, or @code{#f}
otherwise.

A circular list is a list where at some point the @code{cdr} refers
back to a previous pair in the list (either the start or some later
point), so that following the @code{cdr}s takes you around in a
circle, with no end.

@example
(define x (list 1 2 3 4))
(set-cdr! (last-pair x) (cddr x))
x @result{} (1 2 3 4 3 4 3 4 ...)
(circular-list? x)  @result{} #t
@end example
@end deffn

@deffn {Scheme Procedure} dotted-list? obj
Return @code{#t} if @var{obj} is a dotted list, or @code{#f}
otherwise.

A dotted list is a list where the @code{cdr} of the last pair is not
the empty list @code{()}.  Any non-pair @var{obj} is also considered a
dotted list, with length zero.

@example
(dotted-list? '(1 2 . 3))  @result{} #t
(dotted-list? 99)          @result{} #t
@end example
@end deffn

It will be noted that any Scheme object passes exactly one of the
above three tests @code{proper-list?}, @code{circular-list?} and
@code{dotted-list?}.  Non-lists are @code{dotted-list?}, finite lists
are either @code{proper-list?} or @code{dotted-list?}, and infinite
lists are @code{circular-list?}.

@sp 1
@deffn {Scheme Procedure} null-list? lst
Return @code{#t} if @var{lst} is the empty list @code{()}, @code{#f}
otherwise.  If something else than a proper or circular list is passed
as @var{lst}, an error is signalled.  This procedure is recommended
for checking for the end of a list in contexts where dotted lists are
not allowed.
@end deffn

@deffn {Scheme Procedure} not-pair? obj
Return @code{#t} is @var{obj} is not a pair, @code{#f} otherwise.
This is shorthand notation @code{(not (pair? @var{obj}))} and is
supposed to be used for end-of-list checking in contexts where dotted
lists are allowed.
@end deffn

@deffn {Scheme Procedure} list= elt= list1 @dots{}
Return @code{#t} if all argument lists are equal, @code{#f} otherwise.
List equality is determined by testing whether all lists have the same
length and the corresponding elements are equal in the sense of the
equality predicate @var{elt=}.  If no or only one list is given,
@code{#t} is returned.
@end deffn


@node SRFI-1 Selectors
@subsubsection Selectors
@cindex list selector

@c FIXME::martin: Review me!

@deffn {Scheme Procedure} first pair
@deffnx {Scheme Procedure} second pair
@deffnx {Scheme Procedure} third pair
@deffnx {Scheme Procedure} fourth pair
@deffnx {Scheme Procedure} fifth pair
@deffnx {Scheme Procedure} sixth pair
@deffnx {Scheme Procedure} seventh pair
@deffnx {Scheme Procedure} eighth pair
@deffnx {Scheme Procedure} ninth pair
@deffnx {Scheme Procedure} tenth pair
These are synonyms for @code{car}, @code{cadr}, @code{caddr}, @dots{}.
@end deffn

@deffn {Scheme Procedure} car+cdr pair
Return two values, the @sc{car} and the @sc{cdr} of @var{pair}.
@end deffn

@deffn {Scheme Procedure} take lst i
@deffnx {Scheme Procedure} take! lst i
Return a list containing the first @var{i} elements of @var{lst}.

@code{take!} may modify the structure of the argument list @var{lst}
in order to produce the result.
@end deffn

@deffn {Scheme Procedure} drop lst i
Return a list containing all but the first @var{i} elements of
@var{lst}.
@end deffn

@deffn {Scheme Procedure} take-right lst i
Return the a list containing the @var{i} last elements of @var{lst}.
@end deffn

@deffn {Scheme Procedure} drop-right lst i
@deffnx {Scheme Procedure} drop-right! lst i
Return the a list containing all but the @var{i} last elements of
@var{lst}.

@code{drop-right!} may modify the structure of the argument list
@var{lst} in order to produce the result.
@end deffn

@deffn {Scheme Procedure} split-at lst i
@deffnx {Scheme Procedure} split-at! lst i
Return two values, a list containing the first @var{i} elements of the
list @var{lst} and a list containing the remaining elements.

@code{split-at!} may modify the structure of the argument list
@var{lst} in order to produce the result.
@end deffn

@deffn {Scheme Procedure} last lst
Return the last element of the non-empty, finite list @var{lst}.
@end deffn


@node SRFI-1 Length Append etc
@subsubsection Length, Append, Concatenate, etc.

@c FIXME::martin: Review me!

@deffn {Scheme Procedure} length+ lst
Return the length of the argument list @var{lst}.  When @var{lst} is a
circular list, @code{#f} is returned.
@end deffn

@deffn {Scheme Procedure} concatenate list-of-lists
@deffnx {Scheme Procedure} concatenate! list-of-lists
Construct a list by appending all lists in @var{list-of-lists}.

@code{concatenate!} may modify the structure of the given lists in
order to produce the result.

@code{concatenate} is the same as @code{(apply append
@var{list-of-lists})}.  It exists because some Scheme implementations
have a limit on the number of arguments a function takes, which the
@code{apply} might exceed.  In Guile there is no such limit.
@end deffn

@deffn {Scheme Procedure} append-reverse rev-head tail
@deffnx {Scheme Procedure} append-reverse! rev-head tail
Reverse @var{rev-head}, append @var{tail} and return the result.  This
is equivalent to @code{(append (reverse @var{rev-head}) @var{tail})},
but more efficient.

@code{append-reverse!} may modify @var{rev-head} in order to produce
the result.
@end deffn

@deffn {Scheme Procedure} zip lst1 lst2 @dots{}
Return a list as long as the shortest of the argument lists, where
each element is a list.  The first list contains the first elements of
the argument lists, the second list contains the second elements, and
so on.
@end deffn

@deffn {Scheme Procedure} unzip1 lst
@deffnx {Scheme Procedure} unzip2 lst
@deffnx {Scheme Procedure} unzip3 lst
@deffnx {Scheme Procedure} unzip4 lst
@deffnx {Scheme Procedure} unzip5 lst
@code{unzip1} takes a list of lists, and returns a list containing the
first elements of each list, @code{unzip2} returns two lists, the
first containing the first elements of each lists and the second
containing the second elements of each lists, and so on.
@end deffn

@deffn {Scheme Procedure} count pred lst1 @dots{} lstN
Return a count of the number of times @var{pred} returns true when
called on elements from the given lists.

@var{pred} is called with @var{N} parameters @code{(@var{pred}
@var{elem1} @dots{} @var{elemN})}, each element being from the
corresponding @var{lst1} @dots{} @var{lstN}.  The first call is with
the first element of each list, the second with the second element
from each, and so on.

Counting stops when the end of the shortest list is reached.  At least
one list must be non-circular.
@end deffn


@node SRFI-1 Fold and Map
@subsubsection Fold, Unfold & Map
@cindex list fold
@cindex list map

@c FIXME::martin: Review me!

@deffn {Scheme Procedure} fold proc init lst1 @dots{} lstN
@deffnx {Scheme Procedure} fold-right proc init lst1 @dots{} lstN
Apply @var{proc} to the elements of @var{lst1} @dots{} @var{lstN} to
build a result, and return that result.

Each @var{proc} call is @code{(@var{proc} @var{elem1} @dots{}
@var{elemN} @var{previous})}, where @var{elem1} is from @var{lst1},
through @var{elemN} from @var{lstN}.  @var{previous} is the return
from the previous call to @var{proc}, or the given @var{init} for the
first call.  If any list is empty, just @var{init} is returned.

@code{fold} works through the list elements from first to last.  The
following shows a list reversal and the calls it makes,

@example
(fold cons '() '(1 2 3))

(cons 1 '())
(cons 2 '(1))
(cons 3 '(2 1)
@result{} (3 2 1)
@end example

@code{fold-right} works through the list elements from last to first,
ie.@: from the right.  So for example the following finds the longest
string, and the last among equal longest,

@example
(fold-right (lambda (str prev)
              (if (> (string-length str) (string-length prev))
                  str
                  prev))
            ""
            '("x" "abc" "xyz" "jk"))
@result{} "xyz"
@end example

If @var{lst1} through @var{lstN} have different lengths, @code{fold}
stops when the end of the shortest is reached; @code{fold-right}
commences at the last element of the shortest.  Ie.@: elements past
the length of the shortest are ignored in the other @var{lst}s.  At
least one @var{lst} must be non-circular.

@code{fold} should be preferred over @code{fold-right} if the order of
processing doesn't matter, or can be arranged either way, since
@code{fold} is a little more efficient.

The way @code{fold} builds a result from iterating is quite general,
it can do more than other iterations like say @code{map} or
@code{filter}.  The following for example removes adjacent duplicate
elements from a list,

@example
(define (delete-adjacent-duplicates lst)
  (fold-right (lambda (elem ret)
                (if (equal? elem (first ret))
                    ret
                    (cons elem ret)))
              (list (last lst))
              lst))
(delete-adjacent-duplicates '(1 2 3 3 4 4 4 5))
@result{} (1 2 3 4 5)
@end example

Clearly the same sort of thing can be done with a @code{for-each} and
a variable in which build the result, but a self-contained @var{proc}
can be re-used in multiple contexts, where a @code{for-each} would
have to be written out each time.
@end deffn

@deffn {Scheme Procedure} pair-fold proc init lst1 @dots{} lstN
@deffnx {Scheme Procedure} pair-fold-right proc init lst1 @dots{} lstN
The same as @code{fold} and @code{fold-right}, but apply @var{proc} to
the pairs of the lists instead of the list elements.
@end deffn

@deffn {Scheme Procedure} reduce proc identity lst
@deffnx {Scheme Procedure} reduce-right proc identity lst
@code{reduce} is a variant of @code{fold} (see above), designed for
cases where the initial value is an ``identity'', so that @var{proc}
can start directly from the first element of @var{lst}.

Consider folding with @code{+} and initial value 0, calls would be for
instance

@example
(fold + 0 '(4 5 6))
@result{}
(+ 4 0)
(+ 5 4)
(+ 6 9)
@end example

The first call @code{(+ 4 0)} is unnecessary, adding 0 to 4 doesn't
change that value.  The first element @code{4} may as well be the
initial ``previous'' value argument to @var{proc}.  This is what
@code{reduce} does.

@example
(reduce + 0 '(4 5 6))
@result{}
(+ 5 4)
(+ 6 9)
@end example

The @var{identity} parameter is the return when @var{lst} is empty,
that's the only time @var{identity} is used.  If @var{lst} has just
one element, that's the return with no calls to @var{proc}.

@code{reduce-right} is a similar variation on @code{fold-right},
working from the end (ie.@: the right) of @var{lst}.  Calls in this
case become

@example
(reduce-right + 0 '(4 5 6))
@result{}
(+ 5 6)
(+ 4 11)
@end example

@code{reduce} should be preferred over @code{reduce-right} if the
order of processing doesn't matter, or can be arranged either way,
since @code{reduce} is a little more efficient.

In the above examples of course @code{+} takes any number of
arguments, so it can be used on a list just with @code{apply}.  An
example where that's not the case would be SRFI-19 @code{add-duration}
(@pxref{SRFI-19 Time}) on a list of durations,

@example
(use-modules (srfi srfi-19))
(reduce add-duration
        (make-time time-duration 0 0)
        (list (make-time time-duration 0 4)
              (make-time time-duration 0 5)
              (make-time time-duration 0 6)))
@result{} #<time duration 15 seconds>
@end example
@end deffn

@deffn {Scheme Procedure} unfold p f g seed [tail-gen]
@code{unfold} is defined as follows:

@lisp
(unfold p f g seed) =
   (if (p seed) (tail-gen seed)
       (cons (f seed)
             (unfold p f g (g seed))))
@end lisp

@table @var
@item p
Determines when to stop unfolding.

@item f
Maps each seed value to the corresponding list element.

@item g
Maps each seed value to next seed valu.

@item seed
The state value for the unfold.

@item tail-gen
Creates the tail of the list; defaults to @code{(lambda (x) '())}.
@end table

@var{g} produces a series of seed values, which are mapped to list
elements by @var{f}.  These elements are put into a list in
left-to-right order, and @var{p} tells when to stop unfolding.
@end deffn

@deffn {Scheme Procedure} unfold-right p f g seed [tail]
Construct a list with the following loop.

@lisp
(let lp ((seed seed) (lis tail))
   (if (p seed) lis
       (lp (g seed)
           (cons (f seed) lis))))
@end lisp

@table @var
@item p
Determines when to stop unfolding.

@item f
Maps each seed value to the corresponding list element.

@item g
Maps each seed value to next seed valu.

@item seed
The state value for the unfold.

@item tail-gen
Creates the tail of the list; defaults to @code{(lambda (x) '())}.
@end table

@end deffn

@deffn {Scheme Procedure} map f lst1 lst2 @dots{}
Map the procedure over the list(s) @var{lst1}, @var{lst2}, @dots{} and
return a list containing the results of the procedure applications.
This procedure is extended with respect to R5RS, because the argument
lists may have different lengths.  The result list will have the same
length as the shortest argument lists.  The order in which @var{f}
will be applied to the list element(s) is not specified.
@end deffn

@deffn {Scheme Procedure} for-each f lst1 lst2 @dots{}
Apply the procedure @var{f} to each pair of corresponding elements of
the list(s) @var{lst1}, @var{lst2}, @dots{}.  The return value is not
specified.  This procedure is extended with respect to R5RS, because
the argument lists may have different lengths.  The shortest argument
list determines the number of times @var{f} is called.  @var{f} will
be applied to the list elements in left-to-right order.

@end deffn

@deffn {Scheme Procedure} append-map f lst1 lst2 @dots{}
@deffnx {Scheme Procedure} append-map! f lst1 lst2 @dots{}
Equivalent to

@lisp
(apply append (map f clist1 clist2 ...))
@end lisp

and

@lisp
(apply append! (map f clist1 clist2 ...))
@end lisp

Map @var{f} over the elements of the lists, just as in the @code{map}
function. However, the results of the applications are appended
together to make the final result. @code{append-map} uses
@code{append} to append the results together; @code{append-map!} uses
@code{append!}.

The dynamic order in which the various applications of @var{f} are
made is not specified.
@end deffn

@deffn {Scheme Procedure} map! f lst1 lst2 @dots{}
Linear-update variant of @code{map} -- @code{map!} is allowed, but not
required, to alter the cons cells of @var{lst1} to construct the
result list.

The dynamic order in which the various applications of @var{f} are
made is not specified. In the n-ary case, @var{lst2}, @var{lst3},
@dots{} must have at least as many elements as @var{lst1}.
@end deffn

@deffn {Scheme Procedure} pair-for-each f lst1 lst2 @dots{}
Like @code{for-each}, but applies the procedure @var{f} to the pairs
from which the argument lists are constructed, instead of the list
elements.  The return value is not specified.
@end deffn

@deffn {Scheme Procedure} filter-map f lst1 lst2 @dots{}
Like @code{map}, but only results from the applications of @var{f}
which are true are saved in the result list.
@end deffn


@node SRFI-1 Filtering and Partitioning
@subsubsection Filtering and Partitioning
@cindex list filter
@cindex list partition

@c FIXME::martin: Review me!

Filtering means to collect all elements from a list which satisfy a
specific condition.  Partitioning a list means to make two groups of
list elements, one which contains the elements satisfying a condition,
and the other for the elements which don't.

The @code{filter} and @code{filter!} functions are implemented in the
Guile core, @xref{List Modification}.

@deffn {Scheme Procedure} partition pred lst
@deffnx {Scheme Procedure} partition! pred lst
Split @var{lst} into those elements which do and don't satisfy the
predicate @var{pred}.

The return is two values (@pxref{Multiple Values}), the first being a
list of all elements from @var{lst} which satisfy @var{pred}, the
second a list of those which do not.

The elements in the result lists are in the same order as in @var{lst}
but the order in which the calls @code{(@var{pred} elem)} are made on
the list elements is unspecified.

@code{partition} does not change @var{lst}, but one of the returned
lists may share a tail with it.  @code{partition!} may modify
@var{lst} to construct its return.
@end deffn

@deffn {Scheme Procedure} remove pred lst
@deffnx {Scheme Procedure} remove! pred lst
Return a list containing all elements from @var{lst} which do not
satisfy the predicate @var{pred}.  The elements in the result list
have the same order as in @var{lst}.  The order in which @var{pred} is
applied to the list elements is not specified.

@code{remove!} is allowed, but not required to modify the structure of
the input list.
@end deffn


@node SRFI-1 Searching
@subsubsection Searching
@cindex list search

@c FIXME::martin: Review me!

The procedures for searching elements in lists either accept a
predicate or a comparison object for determining which elements are to
be searched.

@deffn {Scheme Procedure} find pred lst
Return the first element of @var{lst} which satisfies the predicate
@var{pred} and @code{#f} if no such element is found.
@end deffn

@deffn {Scheme Procedure} find-tail pred lst
Return the first pair of @var{lst} whose @sc{car} satisfies the
predicate @var{pred} and @code{#f} if no such element is found.
@end deffn

@deffn {Scheme Procedure} take-while pred lst
@deffnx {Scheme Procedure} take-while! pred lst
Return the longest initial prefix of @var{lst} whose elements all
satisfy the predicate @var{pred}.

@code{take-while!} is allowed, but not required to modify the input
list while producing the result.
@end deffn

@deffn {Scheme Procedure} drop-while pred lst
Drop the longest initial prefix of @var{lst} whose elements all
satisfy the predicate @var{pred}.
@end deffn

@deffn {Scheme Procedure} span pred lst
@deffnx {Scheme Procedure} span! pred lst
@deffnx {Scheme Procedure} break pred lst
@deffnx {Scheme Procedure} break! pred lst
@code{span} splits the list @var{lst} into the longest initial prefix
whose elements all satisfy the predicate @var{pred}, and the remaining
tail.  @code{break} inverts the sense of the predicate.

@code{span!} and @code{break!} are allowed, but not required to modify
the structure of the input list @var{lst} in order to produce the
result.

Note that the name @code{break} conflicts with the @code{break}
binding established by @code{while} (@pxref{while do}).  Applications
wanting to use @code{break} from within a @code{while} loop will need
to make a new define under a different name.
@end deffn

@deffn {Scheme Procedure} any pred lst1 lst2 @dots{} lstN
Test whether any set of elements from @var{lst1} @dots{} lstN
satisfies @var{pred}.  If so the return value is the return from the
successful @var{pred} call, or if not the return is @code{#f}.

Each @var{pred} call is @code{(@var{pred} @var{elem1} @dots{}
@var{elemN})} taking an element from each @var{lst}.  The calls are
made successively for the first, second, etc elements of the lists,
stopping when @var{pred} returns non-@code{#f}, or when the end of the
shortest list is reached.

The @var{pred} call on the last set of elements (ie.@: when the end of
the shortest list has been reached), if that point is reached, is a
tail call.
@end deffn

@deffn {Scheme Procedure} every pred lst1 lst2 @dots{} lstN
Test whether every set of elements from @var{lst1} @dots{} lstN
satisfies @var{pred}.  If so the return value is the return from the
final @var{pred} call, or if not the return is @code{#f}.

Each @var{pred} call is @code{(@var{pred} @var{elem1} @dots{}
@var{elemN})} taking an element from each @var{lst}.  The calls are
made successively for the first, second, etc elements of the lists,
stopping if @var{pred} returns @code{#f}, or when the end of any of
the lists is reached.

The @var{pred} call on the last set of elements (ie.@: when the end of
the shortest list has been reached) is a tail call.

If one of @var{lst1} @dots{} @var{lstN} is empty then no calls to
@var{pred} are made, and the return is @code{#t}.
@end deffn

@deffn {Scheme Procedure} list-index pred lst1 @dots{} lstN
Return the index of the first set of elements, one from each of
@var{lst1}@dots{}@var{lstN}, which satisfies @var{pred}.

@var{pred} is called as @code{(@var{pred} elem1 @dots{} elemN)}.
Searching stops when the end of the shortest @var{lst} is reached.
The return index starts from 0 for the first set of elements.  If no
set of elements pass then the return is @code{#f}.

@example
(list-index odd? '(2 4 6 9))      @result{} 3
(list-index = '(1 2 3) '(3 1 2))  @result{} #f
@end example
@end deffn

@deffn {Scheme Procedure} member x lst [=]
Return the first sublist of @var{lst} whose @sc{car} is equal to
@var{x}.  If @var{x} does not appear in @var{lst}, return @code{#f}.

Equality is determined by @code{equal?}, or by the equality predicate
@var{=} if given.  @var{=} is called @code{(= @var{x} elem)},
ie.@: with the given @var{x} first, so for example to find the first
element greater than 5,

@example
(member 5 '(3 5 1 7 2 9) <) @result{} (7 2 9)
@end example

This version of @code{member} extends the core @code{member}
(@pxref{List Searching}) by accepting an equality predicate.
@end deffn


@node SRFI-1 Deleting
@subsubsection Deleting
@cindex list delete

@c FIXME::martin: Review me!

@deffn {Scheme Procedure} delete x lst [=]
@deffnx {Scheme Procedure} delete! x lst [=]
Return a list containing the elements of @var{lst} but with those
equal to @var{x} deleted.  The returned elements will be in the same
order as they were in @var{lst}.

Equality is determined by the @var{=} predicate, or @code{equal?} if
not given.  An equality call is made just once for each element, but
the order in which the calls are made on the elements is unspecified.

The equality calls are always @code{(= x elem)}, ie.@: the given @var{x}
is first.  This means for instance elements greater than 5 can be
deleted with @code{(delete 5 lst <)}.

@code{delete} does not modify @var{lst}, but the return might share a
common tail with @var{lst}.  @code{delete!} may modify the structure
of @var{lst} to construct its return.

These functions extend the core @code{delete} and @code{delete!} in
accepting an equality predicate.  (@pxref{List Modification})
@end deffn

@deffn {Scheme Procedure} delete-duplicates lst [=]
@deffnx {Scheme Procedure} delete-duplicates! lst [=]
Return a list containing the elements of @var{lst} but without
duplicates.

When elements are equal, only the first in @var{lst} is retained.
Equal elements can be anywhere in @var{lst}, they don't have to be
adjacent.  The returned list will have the retained elements in the
same order as they were in @var{lst}.

Equality is determined by the @var{=} predicate, or @code{equal?} if
not given.  Calls @code{(= x y)} are made with element @var{x} being
before @var{y} in @var{lst}.  A call is made at most once for each
combination, but the sequence of the calls across the elements is
unspecified.

@code{delete-duplicates} does not modify @var{lst}, but the return
might share a common tail with @var{lst}.  @code{delete-duplicates!}
may modify the structure of @var{lst} to construct its return.

In the worst case, this is an @math{O(N^2)} algorithm because it must
check each element against all those preceding it.  For long lists it
is more efficient to sort and then compare only adjacent elements.
@end deffn


@node SRFI-1 Association Lists
@subsubsection Association Lists
@cindex association list
@cindex alist

@c FIXME::martin: Review me!

Association lists are described in detail in section @ref{Association
Lists}.  The present section only documents the additional procedures
for dealing with association lists defined by SRFI-1.

@deffn {Scheme Procedure} assoc key alist [=]
Return the pair from @var{alist} which matches @var{key}.  Equality is
determined by @var{=}, which defaults to @code{equal?} if not given.
@var{alist} must be an association lists---a list of pairs.

This function extends the core @code{assoc} by accepting an equality
predicate.  (@pxref{Association Lists})
@end deffn

@deffn {Scheme Procedure} alist-cons key datum alist
Equivalent to

@lisp
(cons (cons @var{key} @var{datum}) @var{alist})
@end lisp

This procedure is used to coons a new pair onto an existing
association list.
@end deffn

@deffn {Scheme Procedure} alist-copy alist
Return a newly allocated copy of @var{alist}, that means that the
spine of the list as well as the pairs are copied.
@end deffn

@deffn {Scheme Procedure} alist-delete key alist [=]
@deffnx {Scheme Procedure} alist-delete! key alist [=]
Return a list containing the elements of @var{alist} but with those
elements whose keys are equal to @var{key} deleted.  The returned
elements will be in the same order as they were in @var{alist}.

Equality is determined by the @var{=} predicate, or @code{equal?} if
not given.  The order in which elements are tested is unspecified, but
each equality call is made @code{(= key alistkey)}, ie. the given
@var{key} parameter is first and the key from @var{alist} second.
This means for instance all associations with a key greater than 5 can
be removed with @code{(alist-delete 5 alist <)}.

@code{alist-delete} does not modify @var{alist}, but the return might
share a common tail with @var{alist}.  @code{alist-delete!} may modify
the list structure of @var{alist} to construct its return.
@end deffn


@node SRFI-1 Set Operations
@subsubsection Set Operations on Lists
@cindex list set operation

@c FIXME::martin: Review me!

Lists can be used for representing sets of objects.  The procedures
documented in this section can be used for such set representations.
Man combining several sets or adding elements, they make sure that no
object is contained more than once in a given list.  Please note that
lists are not a too efficient implementation method for sets, so if
you need high performance, you should think about implementing a
custom data structure for representing sets, such as trees, bitsets,
hash tables or something similar.

All these procedures accept an equality predicate as the first
argument.  This predicate is used for testing the objects in the list
sets for sameness.

@deffn {Scheme Procedure} lset<= = list1 @dots{}
Return @code{#t} if every @var{listi} is a subset of @var{listi+1},
otherwise return @code{#f}.  Returns @code{#t} if called with less
than two arguments. @var{=} is used for testing element equality.
@end deffn

@deffn {Scheme Procedure} lset= = list1 list2 @dots{}
Return @code{#t} if all argument lists are equal. @var{=} is used for
testing element equality.
@end deffn

@deffn {Scheme Procedure} lset-adjoin = list elt1 @dots{}
Add all @var{elts} to the list @var{list}, suppressing duplicates and
return the resulting list.  @var{=} is used for testing
element equality.
@end deffn

@deffn {Scheme Procedure} lset-union = list1 @dots{}
@deffnx {Scheme Procedure} lset-union! = list1 @dots{}
Return the union of all argument list sets.  The union is the set of
all elements which appear in any of the argument sets.
@code{lset-union!} is allowed, but not required to modify its first
argument. @var{=} is used for testing element equality.
@end deffn

@deffn {Scheme Procedure} lset-intersection = list1 list2 @dots{}
@deffnx {Scheme Procedure} lset-intersection! = list1 list2 @dots{}
Return the intersection of all argument list sets.  The intersection
is the set containing all elements which appear in all argument sets.
@code{lset-intersection!} is allowed, but not required to modify its
first argument. @var{=} is used for testing element equality.
@end deffn

@deffn {Scheme Procedure} lset-difference = list1 list2 @dots{}
@deffnx {Scheme Procedure} lset-difference! = list1 list2 @dots{}
Return the difference of all argument list sets.  The difference is
the the set containing all elements of the first list which do not
appear in the other lists.  @code{lset-difference!}  is allowed, but
not required to modify its first argument. @var{=} is used for testing
element equality.
@end deffn

@deffn {Scheme Procedure} lset-xor = list1 @dots{}
@deffnx {Scheme Procedure} lset-xor! = list1 @dots{}
Return the set containing all elements which appear in the first
argument list set, but not in the second; or, more generally: which
appear in an odd number of sets.  @code{lset-xor!}  is allowed, but
not required to modify its first argument. @var{=} is used for testing
element equality.
@end deffn

@deffn {Scheme Procedure} lset-diff+intersection = list1 list2 @dots{}
@deffnx {Scheme Procedure} lset-diff+intersection! = list1 list2 @dots{}
Return two values, the difference and the intersection of the argument
list sets. This works like a combination of @code{lset-difference} and
@code{lset-intersection}, but is more efficient.
@code{lset-diff+intersection!}  is allowed, but not required to modify
its first argument. @var{=} is used for testing element equality.  You
have to use some means to deal with the multiple values these
procedures return (@pxref{Multiple Values}).
@end deffn


@node SRFI-2
@subsection SRFI-2 - and-let*
@cindex SRFI-2

@noindent
The following syntax can be obtained with

@lisp
(use-modules (srfi srfi-2))
@end lisp

@deffn {library syntax} and-let* (clause @dots{}) body @dots{}
A combination of @code{and} and @code{let*}.

Each @var{clause} is evaluated in turn, and if @code{#f} is obtained
then evaluation stops and @code{#f} is returned.  If all are
non-@code{#f} then @var{body} is evaluated and the last form gives the
return value, or if @var{body} is empty then the result is @code{#t}.
Each @var{clause} should be one of the following,

@table @code
@item (symbol expr)
Evaluate @var{expr}, check for @code{#f}, and bind it to @var{symbol}.
Like @code{let*}, that binding is available to subsequent clauses.
@item (expr)
Evaluate @var{expr} and check for @code{#f}.
@item symbol
Get the value bound to @var{symbol} and check for @code{#f}.
@end table

Notice that @code{(expr)} has an ``extra'' pair of parentheses, for
instance @code{((eq? x y))}.  One way to remember this is to imagine
the @code{symbol} in @code{(symbol expr)} is omitted.

@code{and-let*} is good for calculations where a @code{#f} value means
termination, but where a non-@code{#f} value is going to be needed in
subsequent expressions.

The following illustrates this, it returns text between brackets
@samp{[...]} in a string, or @code{#f} if there are no such brackets
(ie.@: either @code{string-index} gives @code{#f}).

@example
(define (extract-brackets str)
  (and-let* ((start (string-index str #\[))
             (end   (string-index str #\] start)))
    (substring str (1+ start) end)))
@end example

The following shows plain variables and expressions tested too.
@code{diagnostic-levels} is taken to be an alist associating a
diagnostic type with a level.  @code{str} is printed only if the type
is known and its level is high enough.

@example
(define (show-diagnostic type str)
  (and-let* (want-diagnostics
             (level (assq-ref diagnostic-levels type))
             ((>= level current-diagnostic-level)))
    (display str)))
@end example

The advantage of @code{and-let*} is that an extended sequence of
expressions and tests doesn't require lots of nesting as would arise
from separate @code{and} and @code{let*}, or from @code{cond} with
@code{=>}.

@end deffn


@node SRFI-4
@subsection SRFI-4 - Homogeneous numeric vector datatypes
@cindex SRFI-4

The SRFI-4 procedures and data types are always available, @xref{Uniform
Numeric Vectors}.

@node SRFI-6
@subsection SRFI-6 - Basic String Ports
@cindex SRFI-6

SRFI-6 defines the procedures @code{open-input-string},
@code{open-output-string} and @code{get-output-string}.  These
procedures are included in the Guile core, so using this module does not
make any difference at the moment.  But it is possible that support for
SRFI-6 will be factored out of the core library in the future, so using
this module does not hurt, after all.

@node SRFI-8
@subsection SRFI-8 - receive
@cindex SRFI-8

@code{receive} is a syntax for making the handling of multiple-value
procedures easier.  It is documented in @xref{Multiple Values}.


@node SRFI-9
@subsection SRFI-9 - define-record-type
@cindex SRFI-9
@cindex record

This SRFI is a syntax for defining new record types and creating
predicate, constructor, and field getter and setter functions.  In
Guile this is simply an alternate interface to the core record
functionality (@pxref{Records}).  It can be used with,

@example
(use-modules (srfi srfi-9))
@end example

@deffn {library syntax} define-record-type type @* (constructor fieldname @dots{}) @* predicate @* (fieldname accessor [modifier]) @dots{}
@sp 1
Create a new record type, and make various @code{define}s for using
it.  This syntax can only occur at the top-level, not nested within
some other form.

@var{type} is bound to the record type, which is as per the return
from the core @code{make-record-type}.  @var{type} also provides the
name for the record, as per @code{record-type-name}.

@var{constructor} is bound to a function to be called as
@code{(@var{constructor} fieldval @dots{})} to create a new record of
this type.  The arguments are initial values for the fields, one
argument for each field, in the order they appear in the
@code{define-record-type} form.

The @var{fieldname}s provide the names for the record fields, as per
the core @code{record-type-fields} etc, and are referred to in the
subsequent accessor/modifier forms.

@var{predictate} is bound to a function to be called as
@code{(@var{predicate} obj)}.  It returns @code{#t} or @code{#f}
according to whether @var{obj} is a record of this type.

Each @var{accessor} is bound to a function to be called
@code{(@var{accessor} record)} to retrieve the respective field from a
@var{record}.  Similarly each @var{modifier} is bound to a function to
be called @code{(@var{modifier} record val)} to set the respective
field in a @var{record}.
@end deffn

@noindent
An example will illustrate typical usage,

@example
(define-record-type employee-type
  (make-employee name age salary)
  employee?
  (name    get-employee-name)
  (age     get-employee-age    set-employee-age)
  (salary  get-employee-salary set-employee-salary))
@end example

This creates a new employee data type, with name, age and salary
fields.  Accessor functions are created for each field, but no
modifier function for the name (the intention in this example being
that it's established only when an employee object is created).  These
can all then be used as for example,

@example
employee-type @result{} #<record-type employee-type>

(define fred (make-employee "Fred" 45 20000.00))

(employee? fred)        @result{} #t
(get-employee-age fred) @result{} 45
(set-employee-salary fred 25000.00)  ;; pay rise
@end example

The functions created by @code{define-record-type} are ordinary
top-level @code{define}s.  They can be redefined or @code{set!} as
desired, exported from a module, etc.


@node SRFI-10
@subsection SRFI-10 - Hash-Comma Reader Extension
@cindex SRFI-10

@cindex hash-comma
@cindex #,()
This SRFI implements a reader extension @code{#,()} called hash-comma.
It allows the reader to give new kinds of objects, for use both in
data and as constants or literals in source code.  This feature is
available with

@example
(use-modules (srfi srfi-10))
@end example

@noindent
The new read syntax is of the form

@example
#,(@var{tag} @var{arg}@dots{})
@end example

@noindent
where @var{tag} is a symbol and the @var{arg}s are objects taken as
parameters.  @var{tag}s are registered with the following procedure.

@deffn {Scheme Procedure} define-reader-ctor tag proc
Register @var{proc} as the constructor for a hash-comma read syntax
starting with symbol @var{tag}, ie. @nicode{#,(@var{tag} arg@dots{})}.
@var{proc} is called with the given arguments @code{(@var{proc}
arg@dots{})} and the object it returns is the result of the read.
@end deffn

@noindent
For example, a syntax giving a list of @var{N} copies of an object.

@example
(define-reader-ctor 'repeat
  (lambda (obj reps)
    (make-list reps obj)))

(display '#,(repeat 99 3))
@print{} (99 99 99)
@end example

Notice the quote @nicode{'} when the @nicode{#,( )} is used.  The
@code{repeat} handler returns a list and the program must quote to use
it literally, the same as any other list.  Ie.

@example
(display '#,(repeat 99 3))
@result{}
(display '(99 99 99))
@end example

When a handler returns an object which is self-evaluating, like a
number or a string, then there's no need for quoting, just as there's
no need when giving those directly as literals.  For example an
addition,

@example
(define-reader-ctor 'sum
  (lambda (x y)
    (+ x y)))
(display #,(sum 123 456)) @print{} 579
@end example

A typical use for @nicode{#,()} is to get a read syntax for objects
which don't otherwise have one.  For example, the following allows a
hash table to be given literally, with tags and values, ready for fast
lookup.

@example
(define-reader-ctor 'hash
  (lambda elems
    (let ((table (make-hash-table)))
      (for-each (lambda (elem)
		  (apply hash-set! table elem))
		elems)
      table)))

(define (animal->family animal)
  (hash-ref '#,(hash ("tiger" "cat")
                     ("lion"  "cat")
                     ("wolf"  "dog"))
            animal))

(animal->family "lion") @result{} "cat"
@end example

Or for example the following is a syntax for a compiled regular
expression (@pxref{Regular Expressions}).

@example
(use-modules (ice-9 regex))

(define-reader-ctor 'regexp make-regexp)

(define (extract-angs str)
  (let ((match (regexp-exec '#,(regexp "<([A-Z0-9]+)>") str)))
    (and match
         (match:substring match 1))))

(extract-angs "foo <BAR> quux") @result{} "BAR"
@end example

@sp 1
@nicode{#,()} is somewhat similar to @code{define-macro}
(@pxref{Macros}) in that handler code is run to produce a result, but
@nicode{#,()} operates at the read stage, so it can appear in data for
@code{read} (@pxref{Scheme Read}), not just in code to be executed.

Because @nicode{#,()} is handled at read-time it has no direct access
to variables etc.  A symbol in the arguments is just a symbol, not a
variable reference.  The arguments are essentially constants, though
the handler procedure can use them in any complicated way it might
want.

Once @code{(srfi srfi-10)} has loaded, @nicode{#,()} is available
globally, there's no need to use @code{(srfi srfi-10)} in later
modules.  Similarly the tags registered are global and can be used
anywhere once registered.

There's no attempt to record what previous @nicode{#,()} forms have
been seen, if two identical forms occur then two calls are made to the
handler procedure.  The handler might like to maintain a cache or
similar to avoid making copies of large objects, depending on expected
usage.

In code the best uses of @nicode{#,()} are generally when there's a
lot of objects of a particular kind as literals or constants.  If
there's just a few then some local variables and initializers are
fine, but that becomes tedious and error prone when there's a lot, and
the anonymous and compact syntax of @nicode{#,()} is much better.


@node SRFI-11
@subsection SRFI-11 - let-values
@cindex SRFI-11

@findex let-values
@findex let-values*
This module implements the binding forms for multiple values
@code{let-values} and @code{let-values*}.  These forms are similar to
@code{let} and @code{let*} (@pxref{Local Bindings}), but they support
binding of the values returned by multiple-valued expressions.

Write @code{(use-modules (srfi srfi-11))} to make the bindings
available.

@lisp
(let-values (((x y) (values 1 2))
             ((z f) (values 3 4)))
   (+ x y z f))
@result{}
10
@end lisp

@code{let-values} performs all bindings simultaneously, which means that
no expression in the binding clauses may refer to variables bound in the
same clause list.  @code{let-values*}, on the other hand, performs the
bindings sequentially, just like @code{let*} does for single-valued
expressions.


@node SRFI-13
@subsection SRFI-13 - String Library
@cindex SRFI-13

The SRFI-13 procedures are always available, @xref{Strings}.

@node SRFI-14
@subsection SRFI-14 - Character-set Library
@cindex SRFI-14

The SRFI-14 data type and procedures are always available,
@xref{Character Sets}.

@node SRFI-16
@subsection SRFI-16 - case-lambda
@cindex SRFI-16
@cindex variable arity
@cindex arity, variable

@c FIXME::martin: Review me!

@findex case-lambda
The syntactic form @code{case-lambda} creates procedures, just like
@code{lambda}, but has syntactic extensions for writing procedures of
varying arity easier.

The syntax of the @code{case-lambda} form is defined in the following
EBNF grammar.

@example
@group
<case-lambda>
   --> (case-lambda <case-lambda-clause>)
<case-lambda-clause>
   --> (<formals> <definition-or-command>*)
<formals>
   --> (<identifier>*)
     | (<identifier>* . <identifier>)
     | <identifier>
@end group
@end example

The value returned by a @code{case-lambda} form is a procedure which
matches the number of actual arguments against the formals in the
various clauses, in order.  @dfn{Formals} means a formal argument list
just like with @code{lambda} (@pxref{Lambda}). The first matching clause
is selected, the corresponding values from the actual parameter list are
bound to the variable names in the clauses and the body of the clause is
evaluated.  If no clause matches, an error is signalled.

The following (silly) definition creates a procedure @var{foo} which
acts differently, depending on the number of actual arguments.  If one
argument is given, the constant @code{#t} is returned, two arguments are
added and if more arguments are passed, their product is calculated.

@lisp
(define foo (case-lambda
              ((x) #t)
              ((x y) (+ x y))
              (z
                (apply * z))))
(foo 'bar)
@result{}
#t
(foo 2 4)
@result{}
6
(foo 3 3 3)
@result{}
27
(foo)
@result{}
1
@end lisp

The last expression evaluates to 1 because the last clause is matched,
@var{z} is bound to the empty list and the following multiplication,
applied to zero arguments, yields 1.


@node SRFI-17
@subsection SRFI-17 - Generalized set!
@cindex SRFI-17

This is an implementation of SRFI-17: Generalized set!

@findex getter-with-setter
It exports the Guile procedure @code{make-procedure-with-setter} under
the SRFI name @code{getter-with-setter} and exports the standard
procedures @code{car}, @code{cdr}, @dots{}, @code{cdddr},
@code{string-ref} and @code{vector-ref} as procedures with setters, as
required by the SRFI.

SRFI-17 was heavily criticized during its discussion period but it was
finalized anyway.  One issue was its concept of globally associating
setter @dfn{properties} with (procedure) values, which is non-Schemy.
For this reason, this implementation chooses not to provide a way to set
the setter of a procedure.  In fact, @code{(set!  (setter @var{proc})
@var{setter})} signals an error.  The only way to attach a setter to a
procedure is to create a new object (a @dfn{procedure with setter}) via
the @code{getter-with-setter} procedure. This procedure is also
specified in the SRFI.  Using it avoids the described problems.


@node SRFI-19
@subsection SRFI-19 - Time/Date Library
@cindex SRFI-19
@cindex time
@cindex date

This is an implementation of the SRFI-19 time/date library.  The
functions and variables described here are provided by

@example
(use-modules (srfi srfi-19))
@end example

@menu
* SRFI-19 Introduction::        
* SRFI-19 Time::                
* SRFI-19 Date::                
* SRFI-19 Time/Date conversions::  
* SRFI-19 Date to string::      
* SRFI-19 String to date::      
@end menu

@node SRFI-19 Introduction
@subsubsection SRFI-19 Introduction

@cindex universal time
@cindex atomic time
@cindex UTC
@cindex TAI
This module implements time and date representations and calculations,
in various time systems, including universal time (UTC) and atomic
time (TAI).

For those not familiar with these time systems, TAI is based on a
fixed length second derived from oscillations of certain atoms.  UTC
differs from TAI by an integral number of seconds, which is increased
or decreased at announced times to keep UTC aligned to a mean solar
day (the orbit and rotation of the earth are not quite constant).

@cindex leap second
So far, only increases in the TAI
@tex
$\leftrightarrow$
@end tex
@ifnottex
<->
@end ifnottex
UTC difference have been needed.  Such an increase is a ``leap
second'', an extra second of TAI introduced at the end of a UTC day.
When working entirely within UTC this is never seen, every day simply
has 86400 seconds.  But when converting from TAI to a UTC date, an
extra 23:59:60 is present, where normally a day would end at 23:59:59.
Effectively the UTC second from 23:59:59 to 00:00:00 has taken two TAI
seconds.

@cindex system clock
In the current implementation, the system clock is assumed to be UTC,
and a table of leap seconds in the code converts to TAI.  See comments
in @file{srfi-19.scm} for how to update this table.

@cindex julian day
@cindex modified julian day
Also, for those not familiar with the terminology, a @dfn{Julian Day}
is a real number which is a count of days and fraction of a day, in
UTC, starting from -4713-01-01T12:00:00Z, ie.@: midday Monday 1 Jan
4713 B.C.  A @dfn{Modified Julian Day} is the same, but starting from
1858-11-17T00:00:00Z, ie.@: midnight 17 November 1858 UTC.  That time
is julian day 2400000.5.

@c  The SRFI-1 spec says -4714-11-24T12:00:00Z (November 24, -4714 at
@c  noon, UTC), but this is incorrect.  It looks like it might have
@c  arisen from the code incorrectly treating years a multiple of 100
@c  but not 400 prior to 1582 as non-leap years, where instead the Julian
@c  calendar should be used so all multiples of 4 before 1582 are leap
@c  years.


@node SRFI-19 Time
@subsubsection SRFI-19 Time
@cindex time

A @dfn{time} object has type, seconds and nanoseconds fields
representing a point in time starting from some epoch.  This is an
arbitrary point in time, not just a time of day.  Although times are
represented in nanoseconds, the actual resolution may be lower.

The following variables hold the possible time types.  For instance
@code{(current-time time-process)} would give the current CPU process
time.

@defvar time-utc
Universal Coordinated Time (UTC).
@cindex UTC
@end defvar

@defvar time-tai
International Atomic Time (TAI).
@cindex TAI
@end defvar

@defvar time-monotonic
Monotonic time, meaning a monotonically increasing time starting from
an unspecified epoch.

Note that in the current implementation @code{time-monotonic} is the
same as @code{time-tai}, and unfortunately is therefore affected by
adjustments to the system clock.  Perhaps this will change in the
future.
@end defvar

@defvar time-duration
A duration, meaning simply a difference between two times.
@end defvar

@defvar time-process
CPU time spent in the current process, starting from when the process
began.
@cindex process time
@end defvar

@defvar time-thread
CPU time spent in the current thread.  Not currently implemented.
@cindex thread time
@end defvar

@sp 1
@defun time? obj
Return @code{#t} if @var{obj} is a time object, or @code{#f} if not.
@end defun

@defun make-time type nanoseconds seconds
Create a time object with the given @var{type}, @var{seconds} and
@var{nanoseconds}.
@end defun

@defun time-type time
@defunx time-nanosecond time
@defunx time-second time
@defunx set-time-type! time type
@defunx set-time-nanosecond! time nsec
@defunx set-time-second! time sec
Get or set the type, seconds or nanoseconds fields of a time object.

@code{set-time-type!} merely changes the field, it doesn't convert the
time value.  For conversions, see @ref{SRFI-19 Time/Date conversions}.
@end defun

@defun copy-time time
Return a new time object, which is a copy of the given @var{time}.
@end defun

@defun current-time [type]
Return the current time of the given @var{type}.  The default
@var{type} is @code{time-utc}.

Note that the name @code{current-time} conflicts with the Guile core
@code{current-time} function (@pxref{Time}).  Applications wanting to
use both will need to use a different name for one of them.
@end defun

@defun time-resolution [type]
Return the resolution, in nanoseconds, of the given time @var{type}.
The default @var{type} is @code{time-utc}.
@end defun

@defun time<=? t1 t2
@defunx time<? t1 t2
@defunx time=? t1 t2
@defunx time>=? t1 t2
@defunx time>? t1 t2
Return @code{#t} or @code{#f} according to the respective relation
between time objects @var{t1} and @var{t2}.  @var{t1} and @var{t2}
must be the same time type.
@end defun

@defun time-difference t1 t2
@defunx time-difference! t1 t2
Return a time object of type @code{time-duration} representing the
period between @var{t1} and @var{t2}.  @var{t1} and @var{t2} must be
the same time type.

@code{time-difference} returns a new time object,
@code{time-difference!} may modify @var{t1} to form its return.
@end defun

@defun add-duration time duration
@defunx add-duration! time duration
@defunx subtract-duration time duration
@defunx subtract-duration! time duration
Return a time object which is @var{time} with the given @var{duration}
added or subtracted.  @var{duration} must be a time object of type
@code{time-duration}.

@code{add-duration} and @code{subtract-duration} return a new time
object.  @code{add-duration!} and @code{subtract-duration!} may modify
the given @var{time} to form their return.
@end defun


@node SRFI-19 Date
@subsubsection SRFI-19 Date
@cindex date

A @dfn{date} object represents a date in the Gregorian calendar and a
time of day on that date in some timezone.

The fields are year, month, day, hour, minute, second, nanoseconds and
timezone.  A date object is immutable, its fields can be read but they
cannot be modified once the object is created.

@defun date? obj
Return @code{#t} if @var{obj} is a date object, or @code{#f} if not.
@end defun

@defun make-date nsecs seconds minutes hours date month year zone-offset
Create a new date object.
@c
@c  FIXME: What can we say about the ranges of the values.  The
@c  current code looks it doesn't normalize, but expects then in their
@c  usual range already.
@c
@end defun

@defun date-nanosecond date
Nanoseconds, 0 to 999999999.
@end defun

@defun date-second date
Seconds, 0 to 59, or 60 for a leap second.  60 is never seen when working
entirely within UTC, it's only when converting to or from TAI.
@end defun

@defun date-minute date
Minutes, 0 to 59.
@end defun

@defun date-hour date
Hour, 0 to 23.
@end defun

@defun date-day date
Day of the month, 1 to 31 (or less, according to the month).
@end defun

@defun date-month date
Month, 1 to 12.
@end defun

@defun date-year date
Year, eg.@: 2003.  Dates B.C.@: are negative, eg.@: @math{-46} is 46
B.C.  There is no year 0, year @math{-1} is followed by year 1.
@end defun

@defun date-zone-offset date
Time zone, an integer number of seconds east of Greenwich.
@end defun

@defun date-year-day date
Day of the year, starting from 1 for 1st January.
@end defun

@defun date-week-day date
Day of the week, starting from 0 for Sunday.
@end defun

@defun date-week-number date dstartw
Week of the year, ignoring a first partial week.  @var{dstartw} is the
day of the week which is taken to start a week, 0 for Sunday, 1 for
Monday, etc.
@c
@c  FIXME: The spec doesn't say whether numbering starts at 0 or 1.
@c  The code looks like it's 0, if that's the correct intention.
@c
@end defun

@c  The SRFI text doesn't actually give the default for tz-offset, but
@c  the reference implementation has the local timezone and the
@c  conversions functions all specify that, so it should be ok to
@c  document it here.
@c
@defun current-date [tz-offset]
Return a date object representing the current date/time, in UTC offset
by @var{tz-offset}.  @var{tz-offset} is seconds east of Greenwich and
defaults to the local timezone.
@end defun

@defun current-julian-day
@cindex julian day
Return the current Julian Day.
@end defun

@defun current-modified-julian-day
@cindex modified julian day
Return the current Modified Julian Day.
@end defun


@node SRFI-19 Time/Date conversions
@subsubsection SRFI-19 Time/Date conversions
@cindex time conversion
@cindex date conversion

@defun date->julian-day date
@defunx date->modified-julian-day date
@defunx date->time-monotonic date
@defunx date->time-tai date
@defunx date->time-utc date
@end defun
@defun julian-day->date jdn [tz-offset]
@defunx julian-day->time-monotonic jdn
@defunx julian-day->time-tai jdn
@defunx julian-day->time-utc jdn
@end defun
@defun modified-julian-day->date jdn [tz-offset]
@defunx modified-julian-day->time-monotonic jdn
@defunx modified-julian-day->time-tai jdn
@defunx modified-julian-day->time-utc jdn
@end defun
@defun time-monotonic->date time [tz-offset]
@defunx time-monotonic->time-tai time
@defunx time-monotonic->time-tai! time
@defunx time-monotonic->time-utc time
@defunx time-monotonic->time-utc! time
@end defun
@defun time-tai->date time [tz-offset]
@defunx time-tai->julian-day time
@defunx time-tai->modified-julian-day time
@defunx time-tai->time-monotonic time
@defunx time-tai->time-monotonic! time
@defunx time-tai->time-utc time
@defunx time-tai->time-utc! time
@end defun
@defun time-utc->date time [tz-offset]
@defunx time-utc->julian-day time
@defunx time-utc->modified-julian-day time
@defunx time-utc->time-monotonic time
@defunx time-utc->time-monotonic! time
@defunx time-utc->time-tai time
@defunx time-utc->time-tai! time
@sp 1
Convert between dates, times and days of the respective types.  For
instance @code{time-tai->time-utc} accepts a @var{time} object of type
@code{time-tai} and returns an object of type @code{time-utc}.

The @code{!} variants may modify their @var{time} argument to form
their return.  The plain functions create a new object.

For conversions to dates, @var{tz-offset} is seconds east of
Greenwich.  The default is the local timezone, at the given time, as
provided by the system, using @code{localtime} (@pxref{Time}).

On 32-bit systems, @code{localtime} is limited to a 32-bit
@code{time_t}, so a default @var{tz-offset} is only available for
times between Dec 1901 and Jan 2038.  For prior dates an application
might like to use the value in 1902, though some locations have zone
changes prior to that.  For future dates an application might like to
assume today's rules extend indefinitely.  But for correct daylight
savings transitions it will be necessary to take an offset for the
same day and time but a year in range and which has the same starting
weekday and same leap/non-leap (to support rules like last Sunday in
October).
@end defun

@node SRFI-19 Date to string
@subsubsection SRFI-19 Date to string
@cindex date to string
@cindex string, from date

@defun date->string date [format]
Convert a date to a string under the control of a format.
@var{format} should be a string containing @samp{~} escapes, which
will be expanded as per the following conversion table.  The default
@var{format} is @samp{~c}, a locale-dependent date and time.

Many of these conversion characters are the same as POSIX
@code{strftime} (@pxref{Time}), but there are some extras and some
variations.

@multitable {MMMM} {MMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMM}
@item @nicode{~~} @tab literal ~
@item @nicode{~a} @tab locale abbreviated weekday, eg.@: @samp{Sun}
@item @nicode{~A} @tab locale full weekday, eg.@: @samp{Sunday}
@item @nicode{~b} @tab locale abbreviated month, eg.@: @samp{Jan}
@item @nicode{~B} @tab locale full month, eg.@: @samp{January}
@item @nicode{~c} @tab locale date and time, eg.@: @*
@samp{Fri Jul 14 20:28:42-0400 2000}
@item @nicode{~d} @tab day of month, zero padded, @samp{01} to @samp{31}

@c  Spec says d/m/y, reference implementation says m/d/y.
@c  Apparently the reference code was the intention, but would like to
@c  see an errata published for the spec before contradicting it here.
@c
@c  @item @nicode{~D} @tab date @nicode{~d/~m/~y}

@item @nicode{~e} @tab day of month, blank padded, @samp{ 1} to @samp{31}
@item @nicode{~f} @tab seconds and fractional seconds,
with locale decimal point, eg.@: @samp{5.2}
@item @nicode{~h} @tab same as @nicode{~b}
@item @nicode{~H} @tab hour, 24-hour clock, zero padded, @samp{00} to @samp{23}
@item @nicode{~I} @tab hour, 12-hour clock, zero padded, @samp{01} to @samp{12}
@item @nicode{~j} @tab day of year, zero padded, @samp{001} to @samp{366}
@item @nicode{~k} @tab hour, 24-hour clock, blank padded, @samp{ 0} to @samp{23}
@item @nicode{~l} @tab hour, 12-hour clock, blank padded, @samp{ 1} to @samp{12}
@item @nicode{~m} @tab month, zero padded, @samp{01} to @samp{12}
@item @nicode{~M} @tab minute, zero padded, @samp{00} to @samp{59}
@item @nicode{~n} @tab newline
@item @nicode{~N} @tab nanosecond, zero padded, @samp{000000000} to @samp{999999999}
@item @nicode{~p} @tab locale AM or PM
@item @nicode{~r} @tab time, 12 hour clock, @samp{~I:~M:~S ~p}
@item @nicode{~s} @tab number of full seconds since ``the epoch'' in UTC
@item @nicode{~S} @tab second, zero padded @samp{00} to @samp{60} @*
(usual limit is 59, 60 is a leap second)
@item @nicode{~t} @tab horizontal tab character
@item @nicode{~T} @tab time, 24 hour clock, @samp{~H:~M:~S}
@item @nicode{~U} @tab week of year, Sunday first day of week,
@samp{00} to @samp{52}
@item @nicode{~V} @tab week of year, Monday first day of week,
@samp{01} to @samp{53}
@item @nicode{~w} @tab day of week, 0 for Sunday, @samp{0} to @samp{6}
@item @nicode{~W} @tab week of year, Monday first day of week,
@samp{00} to @samp{52}

@c  The spec has ~x as an apparent duplicate of ~W, and ~X as a locale
@c  date.  The reference code has ~x as the locale date and ~X as a
@c  locale time.  The rule is apparently that the code should be
@c  believed, but would like to see an errata for the spec before
@c  contradicting it here.
@c
@c  @item @nicode{~x} @tab week of year, Monday as first day of week,
@c  @samp{00} to @samp{53}
@c  @item @nicode{~X} @tab locale date, eg.@: @samp{07/31/00}

@item @nicode{~y} @tab year, two digits, @samp{00} to @samp{99}
@item @nicode{~Y} @tab year, full, eg.@: @samp{2003}
@item @nicode{~z} @tab time zone, RFC-822 style
@item @nicode{~Z} @tab time zone symbol (not currently implemented)
@item @nicode{~1} @tab ISO-8601 date, @samp{~Y-~m-~d}
@item @nicode{~2} @tab ISO-8601 time+zone, @samp{~k:~M:~S~z}
@item @nicode{~3} @tab ISO-8601 time, @samp{~k:~M:~S}
@item @nicode{~4} @tab ISO-8601 date/time+zone, @samp{~Y-~m-~dT~k:~M:~S~z}
@item @nicode{~5} @tab ISO-8601 date/time, @samp{~Y-~m-~dT~k:~M:~S}
@end multitable
@end defun

Conversions @samp{~D}, @samp{~x} and @samp{~X} are not currently
described here, since the specification and reference implementation
differ.

Currently Guile doesn't implement any localizations for the above, all
outputs are in English, and the @samp{~c} conversion is POSIX
@code{ctime} style @samp{~a ~b ~d ~H:~M:~S~z ~Y}.  This may change in
the future.


@node SRFI-19 String to date
@subsubsection SRFI-19 String to date
@cindex string to date
@cindex date, from string

@c  FIXME: Can we say what happens when an incomplete date is
@c  converted?  Ie. fields left as 0, or what?  The spec seems to be
@c  silent on this.

@defun string->date input template
Convert an @var{input} string to a date under the control of a
@var{template} string.  Return a newly created date object.

Literal characters in @var{template} must match characters in
@var{input} and @samp{~} escapes must match the input forms described
in the table below.  ``Skip to'' means characters up to one of the
given type are ignored, or ``no skip'' for no skipping.  ``Read'' is
what's then read, and ``Set'' is the field affected in the date
object.

For example @samp{~Y} skips input characters until a digit is reached,
at which point it expects a year and stores that to the year field of
the date.

@multitable {MMMM} {@nicode{char-alphabetic?}} {MMMMMMMMMMMMMMMMMMMMMMMMM} {@nicode{date-zone-offset}}
@item
@tab Skip to
@tab Read
@tab Set

@item @nicode{~~}
@tab no skip
@tab literal ~
@tab nothing

@item @nicode{~a}
@tab @nicode{char-alphabetic?}
@tab locale abbreviated weekday name
@tab nothing

@item @nicode{~A}
@tab @nicode{char-alphabetic?}
@tab locale full weekday name
@tab nothing

@c  Note that the SRFI spec says that ~b and ~B don't set anything,
@c  but that looks like a mistake.  The reference implementation sets
@c  the month field, which seems sensible and is what we describe
@c  here.

@item @nicode{~b}
@tab @nicode{char-alphabetic?}
@tab locale abbreviated month name
@tab @nicode{date-month}

@item @nicode{~B}
@tab @nicode{char-alphabetic?}
@tab locale full month name
@tab @nicode{date-month}

@item @nicode{~d}
@tab @nicode{char-numeric?}
@tab day of month
@tab @nicode{date-day}

@item @nicode{~e}
@tab no skip
@tab day of month, blank padded
@tab @nicode{date-day}

@item @nicode{~h}
@tab same as @samp{~b}

@item @nicode{~H}
@tab @nicode{char-numeric?}
@tab hour
@tab @nicode{date-hour}

@item @nicode{~k}
@tab no skip
@tab hour, blank padded
@tab @nicode{date-hour}

@item @nicode{~m}
@tab @nicode{char-numeric?}
@tab month
@tab @nicode{date-month}

@item @nicode{~M}
@tab @nicode{char-numeric?}
@tab minute
@tab @nicode{date-minute}

@item @nicode{~S}
@tab @nicode{char-numeric?}
@tab second
@tab @nicode{date-second}

@item @nicode{~y}
@tab no skip
@tab 2-digit year
@tab @nicode{date-year} within 50 years

@item @nicode{~Y}
@tab @nicode{char-numeric?}
@tab year
@tab @nicode{date-year}

@item @nicode{~z}
@tab no skip
@tab time zone
@tab date-zone-offset
@end multitable

Notice that the weekday matching forms don't affect the date object
returned, instead the weekday will be derived from the day, month and
year.

Currently Guile doesn't implement any localizations for the above,
month and weekday names are always expected in English.  This may
change in the future.
@end defun


@node SRFI-26
@subsection SRFI-26 - specializing parameters
@cindex SRFI-26
@cindex parameter specialize
@cindex argument specialize
@cindex specialize parameter

This SRFI provides a syntax for conveniently specializing selected
parameters of a function.  It can be used with,

@example
(use-modules (srfi srfi-26))
@end example

@deffn {library syntax} cut slot @dots{}
@deffnx {library syntax} cute slot @dots{}
Return a new procedure which will make a call (@var{slot} @dots{}) but
with selected parameters specialized to given expressions.

An example will illustrate the idea.  The following is a
specialization of @code{write}, sending output to
@code{my-output-port},

@example
(cut write <> my-output-port)
@result{}
(lambda (obj) (write obj my-output-port))
@end example

The special symbol @code{<>} indicates a slot to be filled by an
argument to the new procedure.  @code{my-output-port} on the other
hand is an expression to be evaluated and passed, ie.@: it specializes
the behaviour of @code{write}.

@table @nicode
@item <>
A slot to be filled by an argument from the created procedure.
Arguments are assigned to @code{<>} slots in the order they appear in
the @code{cut} form, there's no way to re-arrange arguments.

The first argument to @code{cut} is usually a procedure (or expression
giving a procedure), but @code{<>} is allowed there too.  For example,

@example
(cut <> 1 2 3)
@result{}
(lambda (proc) (proc 1 2 3))
@end example

@item <...>
A slot to be filled by all remaining arguments from the new procedure.
This can only occur at the end of a @code{cut} form.

For example, a procedure taking a variable number of arguments like
@code{max} but in addition enforcing a lower bound,

@example
(define my-lower-bound 123)

(cut max my-lower-bound <...>)
@result{}
(lambda arglist (apply max my-lower-bound arglist))
@end example
@end table

For @code{cut} the specializing expressions are evaluated each time
the new procedure is called.  For @code{cute} they're evaluated just
once, when the new procedure is created.  The name @code{cute} stands
for ``@code{cut} with evaluated arguments''.  In all cases the
evaluations take place in an unspecified order.

The following illustrates the difference between @code{cut} and
@code{cute},

@example
(cut format <> "the time is ~s" (current-time))
@result{}
(lambda (port) (format port "the time is ~s" (current-time)))

(cute format <> "the time is ~s" (current-time))
@result{}
(let ((val (current-time)))
  (lambda (port) (format port "the time is ~s" val))
@end example

(There's no provision for a mixture of @code{cut} and @code{cute}
where some expressions would be evaluated every time but others
evaluated only once.)

@code{cut} is really just a shorthand for the sort of @code{lambda}
forms shown in the above examples.  But notice @code{cut} avoids the
need to name unspecialized parameters, and is more compact.  Use in
functional programming style or just with @code{map}, @code{for-each}
or similar is typical.

@example
(map (cut * 2 <>) '(1 2 3 4))         

(for-each (cut write <> my-port) my-list)  
@end example
@end deffn

@node SRFI-31
@subsection SRFI-31 - A special form `rec' for recursive evaluation
@cindex SRFI-31
@cindex recursive expression
@findex rec

SRFI-31 defines a special form that can be used to create
self-referential expressions more conveniently.  The syntax is as
follows:

@example
@group
<rec expression> --> (rec <variable> <expression>)
<rec expression> --> (rec (<variable>+) <body>)
@end group
@end example

The first syntax can be used to create self-referential expressions,
for example:

@lisp
  guile> (define tmp (rec ones (cons 1 (delay ones))))
@end lisp

The second syntax can be used to create anonymous recursive functions:

@lisp
  guile> (define tmp (rec (display-n item n)
                       (if (positive? n)
                           (begin (display n) (display-n (- n 1))))))
  guile> (tmp 42 3)
  424242
  guile>
@end lisp


@node SRFI-39
@subsection SRFI-39 - Parameters
@cindex SRFI-39
@cindex parameter object
@tindex Parameter

This SRFI provides parameter objects, which implement dynamically
bound locations for values.  The functions below are available from

@example
(use-modules (srfi srfi-39))
@end example

A parameter object is a procedure.  Called with no arguments it
returns its value, called with one argument it sets the value.

@example
(define my-param (make-parameter 123))
(my-param) @result{} 123
(my-param 456)
(my-param) @result{} 456
@end example

The @code{parameterize} special form establishes new locations for
parameters, those new locations having effect within the dynamic scope
of the @code{parameterize} body.  Leaving restores the previous
locations, or re-entering through a saved continuation will again use
the new locations.

@example
(parameterize ((my-param 789))
  (my-param) @result{} 789
  )
(my-param) @result{} 456
@end example

Parameters are like dynamically bound variables in other Lisp dialets.
They allow an application to establish parameter settings (as the name
suggests) just for the execution of a particular bit of code,
restoring when done.  Examples of such parameters might be
case-sensitivity for a search, or a prompt for user input.

Global variables are not as good as parameter objects for this sort of
thing.  Changes to them are visible to all threads, but in Guile
parameter object locations are per-thread, thereby truely limiting the
effect of @code{parameterize} to just its dynamic execution.

Passing arguments to functions is thread-safe, but that soon becomes
tedious when there's more than a few or when they need to pass down
through several layers of calls before reaching the point they should
affect.  And introducing a new setting to existing code is often
easier with a parameter object than adding arguments.


@sp 1
@defun make-parameter init [converter]
Return a new parameter object, with initial value @var{init}.

A parameter object is a procedure.  When called @code{(param)} it
returns its value, or a call @code{(param val)} sets its value.  For
example,

@example
(define my-param (make-parameter 123))
(my-param) @result{} 123

(my-param 456)
(my-param) @result{} 456
@end example

If a @var{converter} is given, then a call @code{(@var{converter}
val)} is made for each value set, its return is the value stored.
Such a call is made for the @var{init} initial value too.

A @var{converter} allows values to be validated, or put into a
canonical form.  For example,

@example
(define my-param (make-parameter 123
                   (lambda (val)
                     (if (not (number? val))
                         (error "must be a number"))
                     (inexact->exact val))))
(my-param 0.75)
(my-param) @result{} 3/4
@end example
@end defun

@deffn {library syntax} parameterize ((param value) @dots{}) body @dots{}
Establish a new dynamic scope with the given @var{param}s bound to new
locations and set to the given @var{value}s.  @var{body} is evaluated
in that environment, the result is the return from the last form in
@var{body}.

Each @var{param} is an expression which is evaluated to get the
parameter object.  Often this will just be the name of a variable
holding the object, but it can be anything that evaluates to a
parameter.

The @var{param} expressions and @var{value} expressions are all
evaluated before establishing the new dynamic bindings, and they're
evaluated in an unspecified order.

For example,

@example
(define prompt (make-parameter "Type something: "))
(define (get-input)
  (display (prompt))
  ...)

(parameterize ((prompt "Type a number: "))
  (get-input)
  ...)
@end example
@end deffn

@deffn {Parameter object} current-input-port [new-port]
@deffnx {Parameter object} current-output-port [new-port]
@deffnx {Parameter object} current-error-port [new-port]
This SRFI extends the core @code{current-input-port} and
@code{current-output-port}, making them parameter objects.  The
Guile-specific @code{current-error-port} is extended too, for
consistency.  (@pxref{Default Ports}.)

This is an upwardly compatible extension, a plain call like
@code{(current-input-port)} still returns the current input port, and
@code{set-current-input-port} can still be used.  But the port can now
also be set with @code{(current-input-port my-port)} and bound
dynamically with @code{parameterize}.
@end deffn

@defun with-parameters* param-list value-list thunk
Establish a new dynamic scope, as per @code{parameterize} above,
taking parameters from @var{param-list} and corresponding values from
@var{values-list}.  A call @code{(@var{thunk})} is made in the new
scope and the result from that @var{thunk} is the return from
@code{with-parameters*}.

This function is a Guile-specific addition to the SRFI, it's similar
to the core @code{with-fluids*} (@pxref{Fluids}).
@end defun


@sp 1
Parameter objects are implemented using fluids (@pxref{Fluids}), so
each dynamic root has it's own parameter locations.  That includes the
separate locations when outside any @code{parameterize} form.  When a
parameter is created it gets a separate initial location in each
dynamic root, all initialized to the given @var{init} value.

As alluded to above, because each thread is a separate dynamic root,
each thread has it's own locations behind parameter objects, and
changes in one thread are not visible to any other.  When a new
dynamic root or thread is created, the values of parameters in the
originating context are copied, into new locations.

SRFI-39 doesn't specify the interaction between parameter objects and
threads, so the threading behaviour described here should be regarded
as Guile-specific.


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