Add signal-fold procedure.

* 2d/signals.scm (signal-fold): New procedure.

1 files changed, 10 insertions, 0 deletions

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."