Implement `(ice-9 threads)' high-level constructs in terms of futures.

* module/ice-9/threads.scm (parallel, par-mapper): Rewrite in terms of
  `future' and `touch'.

* test-suite/tests/threads.test ("par-map", "par-for-each"): New test
  prefixes.

* doc/ref/api-scheduling.texi (Parallel Forms): Add cross-ref to
  futures.  Recommend against the `n-' variants.

doc/ref/api-scheduling.texi

@@ -904,6 +904,11 @@ The functions described in this section are available from
 (use-modules (ice-9 threads))
 @end example
 
+They provide high-level parallel constructs.  The following functions
+are implemented in terms of futures (@pxref{Futures}).  Thus they are
+relatively cheap as they re-use existing threads, and portable, since
+they automatically use one thread per available CPU core.
+
 @deffn syntax parallel expr1 @dots{} exprN
 Evaluate each @var{expr} expression in parallel, each in its own thread.
 Return the results as a set of @var{N} multiple values
@@ -935,6 +940,16 @@ These functions are like @code{map} and @code{for-each} (@pxref{List
 Mapping}), but make their @var{proc} calls in parallel.
 @end deffn
 
+Unlike those above, the functions described below take a number of
+threads as an argument.  This makes them inherently non-portable since
+the specified number of threads may differ from the number of available
+CPU cores as returned by @code{current-processor-count}
+(@pxref{Processes}).  In addition, these functions create the specified
+number of threads when they are called and terminate them upon
+completion, which makes them quite expensive.
+
+Therefore, they should be avoided.
+
 @deffn {Scheme Procedure} n-par-map n proc lst1 @dots{} lstN
 @deffnx {Scheme Procedure} n-par-for-each n proc lst1 @dots{} lstN
 Call @var{proc} on the elements of the given lists, in the same way as

module/ice-9/threads.scm

@@ -32,19 +32,20 @@
 ;;; Code:
 
 (define-module (ice-9 threads)
-  :export (begin-thread
-           parallel
-           letpar
-           make-thread
-           with-mutex
-           monitor
+  #:use-module (ice-9 futures)
+  #:export (begin-thread
+            parallel
+            letpar
+            make-thread
+            with-mutex
+            monitor
 
-           par-map
-	   par-for-each
-	   n-par-map
-	   n-par-for-each
-	   n-for-each-par-map
-	   %thread-handler))
+            par-map
+            par-for-each
+            n-par-map
+            n-par-for-each
+            n-for-each-par-map
+            %thread-handler))
 
 
 
@@ -62,10 +63,9 @@
     (syntax-case x ()
       ((_ e0 ...)
        (with-syntax (((tmp0 ...) (generate-temporaries (syntax (e0 ...)))))
-         (syntax
-          (let ((tmp0 (begin-thread e0))
-                ...)
-            (values (join-thread tmp0) ...))))))))
+         #'(let ((tmp0 (future e0))
+                 ...)
+             (values (touch tmp0) ...)))))))
 
 (define-syntax letpar
   (syntax-rules ()
@@ -99,10 +99,10 @@
 
 (define (par-mapper mapper)
   (lambda (proc . arglists)
-    (mapper join-thread
+    (mapper touch
             (apply map
                    (lambda args
-                     (begin-thread (apply proc args)))
+                     (future (apply proc args)))
                    arglists))))
 
 (define par-map (par-mapper map))

test-suite/tests/threads.test

@@ -1,6 +1,6 @@
 ;;;; threads.test --- Tests for Guile threading.    -*- scheme -*-
 ;;;;
-;;;; Copyright 2003, 2006, 2007, 2009 Free Software Foundation, Inc.
+;;;; Copyright 2003, 2006, 2007, 2009, 2010 Free Software Foundation, Inc.
 ;;;;
 ;;;; This library is free software; you can redistribute it and/or
 ;;;; modify it under the terms of the GNU Lesser General Public
@@ -17,8 +17,9 @@
 ;;;; Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
 
 (define-module (test-threads)
-  :use-module (ice-9 threads)
-  :use-module (test-suite lib))
+  #:use-module (ice-9 threads)
+  #:use-module (system base compile)
+  #:use-module (test-suite lib))
 
 (define (asyncs-still-working?)
   (let ((a #f))
@@ -70,6 +71,38 @@
 		   (equal? y 2)
 		   (equal? z 3))))))
 
+      ;;
+      ;; par-map
+      ;;
+
+      (with-test-prefix "par-map"
+
+        (pass-if "simple"
+          (compile '(letrec ((fibo (lambda (n)
+                                     (if (<= n 1)
+                                         n
+                                         (+ (fibo (- n 1))
+                                            (fibo (- n 2)))))))
+                      (equal? (par-map fibo (iota 13))
+                              (map fibo (iota 13))))
+                   #:to 'value
+                   #:env (current-module))))
+
+      ;;
+      ;; par-for-each
+      ;;
+
+      (with-test-prefix "par-for-each"
+
+        (pass-if "simple"
+          (compile '(let ((v (make-vector 6 #f)))
+                      (par-for-each (lambda (n)
+                                      (vector-set! v n n))
+                                    (iota 6))
+                      (equal? v (list->vector (iota 6))))
+                   #:to 'value
+                   #:env (current-module))))
+
       ;;
       ;; n-par-for-each
       ;;