Apologies for the rapid-fire posts, but having looked at Racket's
SRFI-45 module, I see that they added 'promise?' to the list of exports,
and I think we should too. As things stand now, (promise? (delay 1))
returns #f if you have imported SRFI-45, and that seems wrong. If we
replace 'delay' and 'force', we should also replace 'promise?'.
Here's an updated patch. Comments and suggestions solicited.
Mark
>From 1225cc23b9088e8d52a99f1bac2814b0905826ee Mon Sep 17 00:00:00 2001
From: Mark H Weaver <m...@netris.org>
Date: Mon, 18 Mar 2013 20:01:12 -0400
Subject: [PATCH] SRFI-45: Support multiple values; add promise? predicate.
* module/srfi/srfi-45.scm (eager): Accept any number of arguments.
Store the list of arguments in the value record. Previously, only one
argument was accepted, and that value was stored in the value record.
(delay): Support expressions that return any number of arguments.
(force): Return the list of values stored in the value record.
(promise?): Export.
* doc/ref/srfi-modules.texi (SRFI-45): Update docs. Remove typing
for simplicity in discussing multiple values.
* test-suite/tests/srfi-45.test: Add tests.
---
doc/ref/srfi-modules.texi | 57 +++++++++++++++++++++++++----------------
module/srfi/srfi-45.scm | 21 ++++++++-------
test-suite/tests/srfi-45.test | 40 +++++++++++++++++++++++++++++
3 files changed, 87 insertions(+), 31 deletions(-)
diff --git a/doc/ref/srfi-modules.texi b/doc/ref/srfi-modules.texi
index af1afc0..c0d077b 100644
--- a/doc/ref/srfi-modules.texi
+++ b/doc/ref/srfi-modules.texi
@@ -3833,45 +3833,58 @@ words, no program that uses the R5RS definitions of delay and force will
break if those definition are replaced by the SRFI-45 definitions of
delay and force.
+Guile compatibly extends SRFI-45 to support multiple values. It also
+adds @code{promise?} to the list of exports.
+
@deffn {Scheme Syntax} delay expression
-Takes an expression of arbitrary type @var{a} and returns a promise of
-type @code{(Promise @var{a})} which at some point in the future may be
-asked (by the @code{force} procedure) to evaluate the expression and
-deliver the resulting value.
+Takes an expression and returns a promise which at some point in the
+future may be asked (by the @code{force} procedure) to evaluate the
+expression and deliver the resulting value(s).
@end deffn
@deffn {Scheme Syntax} lazy expression
-Takes an expression of type @code{(Promise @var{a})} and returns a
-promise of type @code{(Promise @var{a})} which at some point in the
-future may be asked (by the @code{force} procedure) to evaluate the
-expression and deliver the resulting promise.
+Takes an expression (which must evaluate to a promise) and returns a
+promise which at some point in the future may be asked (by the
+@code{force} procedure) to evaluate the expression and deliver the
+resulting promise.
@end deffn
-@deffn {Scheme Procedure} force expression
-Takes an argument of type @code{(Promise @var{a})} and returns a value
-of type @var{a} as follows: If a value of type @var{a} has been computed
-for the promise, this value is returned. Otherwise, the promise is
-first evaluated, then overwritten by the obtained promise or value, and
-then force is again applied (iteratively) to the promise.
+@deffn {Scheme Procedure} force promise
+Takes a promise and returns the associated value(s) as follows: If
+value(s) have been computed for the promise, these value(s) are
+returned. Otherwise, the promise is first evaluated, then overwritten
+by the obtained promise or value(s), and then force is again applied
+(iteratively) to the promise.
@end deffn
-@deffn {Scheme Procedure} eager expression
-Takes an argument of type @var{a} and returns a value of type
-@code{(Promise @var{a})}. As opposed to @code{delay}, the argument is
-evaluated eagerly. Semantically, writing @code{(eager expression)} is
-equivalent to writing
+@deffn {Scheme Procedure} eager obj ...
+Takes any number of argument(s) and returns a promise. As opposed to
+@code{delay}, the argument(s) are evaluated eagerly. Semantically,
+writing @code{(eager expression)} is equivalent to writing
@lisp
(let ((value expression)) (delay value)).
@end lisp
However, the former is more efficient since it does not require
-unnecessary creation and evaluation of thunks. We also have the
-equivalence
+unnecessary creation and evaluation of thunks. For expressions that
+return a single value, we also have the equivalence
@lisp
(delay expression) = (lazy (eager expression))
@end lisp
+
+More generally, the following equivalence holds:
+
+@lisp
+(delay expression) = (lazy (call-with-values
+ (lambda () expression)
+ eager))
+@end lisp
+@end deffn
+
+@deffn {Scheme Procedure} promise? obj
+Return true if @var{obj} is an SRFI-45 promise.
@end deffn
The following reduction rules may be helpful for reasoning about these
@@ -3881,7 +3894,7 @@ usage semantics specified above:
@lisp
(force (delay expression)) -> expression
(force (lazy expression)) -> (force expression)
-(force (eager value)) -> value
+(force (eager obj ...)) -> (values obj ...)
@end lisp
@subsubheading Correct usage
diff --git a/module/srfi/srfi-45.scm b/module/srfi/srfi-45.scm
index 29b0393..47e3ba6 100644
--- a/module/srfi/srfi-45.scm
+++ b/module/srfi/srfi-45.scm
@@ -1,6 +1,6 @@
;;; srfi-45.scm -- Primitives for Expressing Iterative Lazy Algorithms
-;; Copyright (C) 2010, 2011 Free Software Foundation, Inc.
+;; Copyright (C) 2010, 2011, 2013 Free Software Foundation, Inc.
;; Copyright (C) 2003 André van Tonder. All Rights Reserved.
;; Permission is hereby granted, free of charge, to any person
@@ -25,8 +25,8 @@
;;; Commentary:
-;; This is the code of the reference implementation of SRFI-45, slightly
-;; modified to use SRFI-9.
+;; This is the code of the reference implementation of SRFI-45,
+;; modified to use SRFI-9 and to support multiple values.
;; This module is documented in the Guile Reference Manual.
@@ -36,8 +36,9 @@
#:export (delay
lazy
force
- eager)
- #:replace (delay force)
+ eager
+ promise?)
+ #:replace (delay force promise?)
#:use-module (srfi srfi-9))
(define-record-type promise (make-promise val) promise?
@@ -50,16 +51,18 @@
(define-syntax-rule (lazy exp)
(make-promise (make-value 'lazy (lambda () exp))))
-(define (eager x)
- (make-promise (make-value 'eager x)))
+(define (eager . xs)
+ (make-promise (make-value 'eager xs)))
(define-syntax-rule (delay exp)
- (lazy (eager exp)))
+ (lazy (call-with-values
+ (lambda () exp)
+ eager)))
(define (force promise)
(let ((content (promise-val promise)))
(case (value-tag content)
- ((eager) (value-proc content))
+ ((eager) (apply values (value-proc content)))
((lazy) (let* ((promise* ((value-proc content)))
(content (promise-val promise))) ; *
(if (not (eqv? (value-tag content) 'eager)) ; *
diff --git a/test-suite/tests/srfi-45.test b/test-suite/tests/srfi-45.test
index 573eea0..7771d09 100644
--- a/test-suite/tests/srfi-45.test
+++ b/test-suite/tests/srfi-45.test
@@ -258,3 +258,43 @@
;; Commented out since it takes too long
#;
(test-equal 300000000 (force (times3 100000000))) ;==> bounded space
+
+
+;======================================================================
+; Test promise? predicate (non-standard Guile extension)
+
+(pass-if "promise? predicate"
+ (promise? (delay 1)))
+
+;======================================================================
+; Test memoization of multiple values (non-standard Guile extension)
+
+(with-test-prefix "Multiple values (non-standard)"
+
+ (let ((promise (delay (values 1 2 3))))
+ (pass-if-equal "Multiple values delay"
+ '(1 2 3)
+ (call-with-values
+ (lambda () (force promise))
+ list)))
+
+ (let ((promise (eager 1 2 3)))
+ (pass-if-equal "Multiple values eager"
+ '(1 2 3)
+ (call-with-values
+ (lambda () (force promise))
+ list)))
+
+ (let ((promise (delay (values))))
+ (pass-if-equal "Zero values delay"
+ '()
+ (call-with-values
+ (lambda () (force promise))
+ list)))
+
+ (let ((promise (eager)))
+ (pass-if-equal "Zero values eager"
+ '()
+ (call-with-values
+ (lambda () (force promise))
+ list))))
--
1.7.10.4
