From c3c5ee977acea4aa9839b15c1cfb8e110316d168 Mon Sep 17 00:00:00 2001
From: David Thompson <dthompson@member.fsf.org>
Date: Mon, 2 Dec 2013 09:36:46 -0500
Subject: Add signal-fold procedure.

* 2d/signals.scm (signal-fold): New procedure.
---
 2d/signals.scm | 10 ++++++++++
 1 file changed, 10 insertions(+)

(limited to '2d/signals.scm')

diff --git a/2d/signals.scm b/2d/signals.scm
index 400f61e..0d7c75b 100644
--- a/2d/signals.scm
+++ b/2d/signals.scm
@@ -45,6 +45,7 @@
             signal-lift4
             signal-merge
             signal-combine
+            signal-fold
             signal-count
             signal-if
             signal-and
@@ -202,6 +203,15 @@ list."
                    (map signal-ref signals))
    #:connectors signals))
 
+(define (signal-fold proc init signal)
+  "Create a new signal that accumulates the current and previous
+values of SIGNAL using PROC."
+  (make-signal
+   #:init init
+   #:transformer (lambda (value prev from)
+                   (proc value prev))
+   #:connectors (list signal)))
+
 (define (signal-count signal)
   "Create a new signal that increments a counter every time the value
 of SIGNAL changes."
-- 
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