From a1d943146bfce6c737b9c3a4477e6eab9ba5015f Mon Sep 17 00:00:00 2001
From: Eli Barzilay <eli@racket-lang.org>
Date: Sun, 3 May 2009 20:41:37 +0000
Subject: [PATCH] added a nullary case to compose

svn: r14697
---
 collects/scheme/private/list.ss                 | 3 ++-
 collects/scribblings/reference/procedures.scrbl | 3 ++-
 collects/tests/mzscheme/function.ss             | 1 +
 3 files changed, 5 insertions(+), 2 deletions(-)

diff --git a/collects/scheme/private/list.ss b/collects/scheme/private/list.ss
index bd3dc7565c..07305f74b5 100644
--- a/collects/scheme/private/list.ss
+++ b/collects/scheme/private/list.ss
@@ -232,7 +232,7 @@
 
   (define compose
     (case-lambda
-      [(f) (if (procedure? f) 
+      [(f) (if (procedure? f)
                f
                (raise-type-error 'compose "procedure" f))]
       [(f g)
@@ -247,6 +247,7 @@
                    (call-with-values (lambda () (g a)) f))
                  (lambda args
                    (call-with-values (lambda () (apply g args)) f)))))]
+      [() values]
       [(f . more)
        (if (procedure? f)
            (let ([m (apply compose more)])
diff --git a/collects/scribblings/reference/procedures.scrbl b/collects/scribblings/reference/procedures.scrbl
index 329ca50ed4..c3fa23d127 100644
--- a/collects/scribblings/reference/procedures.scrbl
+++ b/collects/scribblings/reference/procedures.scrbl
@@ -38,7 +38,8 @@ Returns a procedure that composes the given functions, applying the
 last @scheme[proc] first and the first @scheme[proc] last. The
 composed functions can consume and produce any number of values, as
 long as each function produces as many values as the preceding
-function consumes.
+function consumes.  When no @scheme[proc] arguments are given, the
+result is @scheme[values].
 
 @mz-examples[
 ((compose - sqrt) 10)
diff --git a/collects/tests/mzscheme/function.ss b/collects/tests/mzscheme/function.ss
index 9dfa210674..13d5a524d3 100644
--- a/collects/tests/mzscheme/function.ss
+++ b/collects/tests/mzscheme/function.ss
@@ -14,6 +14,7 @@
 (test 'hi (compose (case-lambda [(x) 'bye][(y z) 'hi]) (lambda () (values 1 2))))
 (test 'ok (compose (lambda () 'ok) (lambda () (values))))
 (test 'ok (compose (lambda () 'ok) (lambda (w) (values))) 5)
+(test 0 (compose) 0)
 (test-values '(1 2 3) (lambda () ((compose (lambda (x) (values x (add1 x) (+ x 2))) (lambda (y) y)) 1)))
 
 (err/rt-test (compose 5))