[PATCH] SRFI-45: Support multiple values; add promise? predicate

[Mark H Weaver]

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 <address@hidden>
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
address@hidden procedure) to evaluate the expression and deliver the
+resulting promise.
 @end deffn
 
address@hidden {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.
address@hidden {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
 
address@hidden {Scheme Procedure} eager expression
-Takes an argument of type @var{a} and returns a value of type
address@hidden(Promise @var{a})}.  As opposed to @code{delay}, the argument is
-evaluated eagerly. Semantically, writing @code{(eager expression)} is
-equivalent to writing
address@hidden {Scheme Procedure} eager obj ...
+Takes any number of argument(s) and returns a promise.  As opposed to
address@hidden, 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:
+
address@hidden
+(delay expression) = (lazy (call-with-values
+                               (lambda () expression)
+                             eager))
address@hidden lisp
address@hidden deffn
+
address@hidden {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