@c This file is part of the MIT/GNU Scheme Reference Manual.
-@c $Id: special-forms.texi,v 1.2 2003/04/25 20:40:22 cph Exp $
+@c $Id: special-forms.texi,v 1.3 2005/12/09 20:25:59 riastradh Exp $
@c Copyright 1991,1992,1993,1994,1995 Massachusetts Institute of Technology
@c Copyright 1996,1997,1999,2000,2001 Massachusetts Institute of Technology
-@c Copyright 2002,2003 Massachusetts Institute of Technology
+@c Copyright 2002,2003,2005 Massachusetts Institute of Technology
@c See file scheme.texinfo for copying conditions.
@node Special Forms, Equivalence Predicates, Overview, Top
@menu
* cond-expand (SRFI 0)::
* receive (SRFI 8)::
+* and-let* (SRFI 2)::
* define-record-type (SRFI 9)::
@end menu
by @acronym{SRFI 0}, which only allows @code{cond-expand} at top level.
@end deffn
-@node receive (SRFI 8), define-record-type (SRFI 9), cond-expand (SRFI 0), SRFI syntax
+@node receive (SRFI 8), and-let* (SRFI 2), cond-expand (SRFI 0), SRFI syntax
@subsection receive (SRFI 8)
@cindex SRFI 8
body are the values of the @samp{receive} expression.
@end deffn
-@node define-record-type (SRFI 9), , receive (SRFI 8), SRFI syntax
+@node and-let* (SRFI 2), define-record-type (SRFI 9), receive (SRFI 8), SRFI syntax
+@subsection and-let* (SRFI 2)
+
+@cindex SRFI 2
+@uref{http://srfi.schemers.org/srfi-2/srfi-2.html,@acronym{SRFI} 2}
+provides a form that combines @samp{and} and @samp{let*} for a
+logically short-circuiting sequential binding operator.
+
+@deffn {special form} and-let* (clause @dots{}) body
+Runs through each of the clauses left-to-right, short-circuiting like
+@samp{and} in that the first false clause will result in the whole
+@samp{and-let*} form returning false. If a body is supplied, and all
+of the clauses evaluate true, then the body is evaluated sequentially
+as if in a @samp{begin} form, and the value of the @samp{and-let*}
+expression is the value of the last body form, evaluated in a tail
+position with respect to the @samp{and-let*} expression. If no body
+is supplied, the value of the last clause, also evaluated in a tail
+position with respect to the @samp{and-let*} expression, is used
+instead.
+
+Each @var{clause} should have one of the following forms:
+
+@table @samp
+@item @var{identifier}
+in which case @var{identifier}'s value is tested.
+
+@item (@var{expression})
+in which case the value of @var{expression} is tested.
+
+@item (@var{identifier} @var{expression})
+in which case @var{expression} is evaluated, and, if its value is not
+false, @var{identifier} is bound to that value for the remainder of
+the clauses and the optional body.
+@end table
+@end deffn
+
+@comment More examples, please!
+
+Example:
+
+@example
+@group
+(and-let* ((list (compute-list))
+ ((pair? list))
+ (item (car list))
+ ((integer? item)))
+ (sqrt item))
+@end group
+@end example
+
+@node define-record-type (SRFI 9), , and-let* (SRFI 2), SRFI syntax
@subsection define-record-type (SRFI 9)
@cindex SRFI 9
#| -*-Scheme-*-
-$Id: mit-syntax.scm,v 14.23 2005/11/20 04:12:59 riastradh Exp $
+$Id: mit-syntax.scm,v 14.24 2005/12/09 20:25:59 riastradh Exp $
Copyright 1989,1990,1991,2001,2002,2003 Massachusetts Institute of Technology
Copyright 2004 Massachusetts Institute of Technology
(define supported-srfi-features
'(SRFI-0
+ SRFI-2
SRFI-6
SRFI-8
SRFI-9
(syntax-check '(KEYWORD EXPRESSION) form history)
(descend-quasiquote (cadr form) 0 finalize-quasiquote)))))))
\f
+;;;; SRFI 2: AND-LET*
+
+;;; The SRFI document is a little unclear about the semantics, imposes
+;;; the weird restriction that variables may be duplicated (citing
+;;; LET*'s similar restriction, which doesn't actually exist), and the
+;;; reference implementation is highly non-standard and hard to
+;;; follow. This passes all of the tests except for the one that
+;;; detects duplicate bound variables, though.
+
+(define-er-macro-transformer 'AND-LET* system-global-environment
+ (lambda (form rename compare)
+ compare
+ (let ((%and (rename 'AND))
+ (%let (rename 'LET))
+ (%begin (rename 'BEGIN)))
+ (cond ((syntax-match? '(() * FORM) (cdr form))
+ `(,%begin #T ,@(cddr form)))
+ ((syntax-match? '((* DATUM) * FORM) (cdr form))
+ (let ((clauses (cadr form))
+ (body (cddr form)))
+ (define (expand clause recur)
+ (cond ((syntax-match? 'IDENTIFIER clause)
+ (recur clause))
+ ((syntax-match? '(EXPRESSION) clause)
+ (recur (car clause)))
+ ((syntax-match? '(IDENTIFIER EXPRESSION) clause)
+ (let ((tail (recur (car clause))))
+ (and tail `(,%let (,clause) ,tail))))
+ (else #f)))
+ (define (recur clauses make-body)
+ (expand (car clauses)
+ (let ((clauses (cdr clauses)))
+ (if (null? clauses)
+ make-body
+ (lambda (conjunct)
+ `(,%and ,conjunct
+ ,(recur clauses make-body)))))))
+ (or (recur clauses
+ (if (null? body)
+ (lambda (conjunct) conjunct)
+ (lambda (conjunct)
+ `(,%and ,conjunct (,%begin ,@body)))))
+ (ill-formed-syntax form))))
+ (else
+ (ill-formed-syntax form))))))
+\f
;;;; MIT-specific syntax
(define-er-macro-transformer 'ACCESS system-global-environment
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