From 5571463e79247d6cb338a015e5bbf94d4bafde44 Mon Sep 17 00:00:00 2001
From: David Thompson <dthompson2@worcester.edu>
Date: Wed, 10 Feb 2016 21:59:10 -0500
Subject: signal: Add signal-map-maybe.

* sly/signal.scm (signal-map-maybe): New procedure.
---
 sly/signal.scm | 13 ++++++++++++-
 1 file changed, 12 insertions(+), 1 deletion(-)

diff --git a/sly/signal.scm b/sly/signal.scm
index 42cce40..41365a9 100644
--- a/sly/signal.scm
+++ b/sly/signal.scm
@@ -41,7 +41,9 @@
             signal-set!
             signal-merge
             signal-zip
-            signal-map signal-sample-on
+            signal-map
+            signal-map-maybe
+            signal-sample-on
             signal-negate
             signal-fold
             signal-filter
@@ -273,6 +275,15 @@ or more SIGNALs."
                          (%signal-set! self (current-value)))
                        inputs)))
 
+(define (signal-map-maybe proc signal . rest)
+  "Create a new signal that applies PROC to the values stored in
+SIGNAL ... if all of them are not #f."
+  (apply signal-map
+         (lambda args
+           (and (every identity args)
+                (apply proc args)))
+         signal rest))
+
 (define (signal-sample-on value-signal sample-signal)
   "Create a new signal that takes on the value of VALUE-SIGNAL
 whenever SAMPLE-SIGNAL receives a new value."
-- 
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