From a7472fd0500a00bb97bf1fb0475213f49c9d57f6 Mon Sep 17 00:00:00 2001
From: David Thompson <dthompson2@worcester.edu>
Date: Sun, 29 Jun 2014 21:19:04 -0400
Subject: Add signal-drop-repeats procedure.

* sly/signal.scm (signal-drop-repeats): New procedure.
---
 sly/signal.scm | 15 +++++++++++++++
 1 file changed, 15 insertions(+)

diff --git a/sly/signal.scm b/sly/signal.scm
index a77e3bb..f8545eb 100644
--- a/sly/signal.scm
+++ b/sly/signal.scm
@@ -40,6 +40,7 @@
             signal-fold
             signal-filter
             signal-reject
+            signal-drop-repeats
             signal-constant
             signal-count
             signal-tap
@@ -243,6 +244,20 @@ SIGNAL when it satisfies the procedure PREDICATE.  The value of the
 signal is DEFAULT when the predicate is never satisfied."
   (signal-filter (lambda (x) (not (predicate x))) default signal))
 
+(define* (signal-drop-repeats signal #:optional (equal? equal?))
+  "Create a new signal that filters out new values from SIGNAL that
+are equivalent to the current value.  By default, equal? is used for
+testing equivalence."
+  (signal-reject (let ((prev (signal-ref signal)))
+                   (lambda (current)
+                     (if (equal? prev current)
+                         #t
+                         (begin
+                           (set! prev current)
+                           #f))))
+                 (signal-ref signal)
+                 signal))
+
 (define (signal-constant constant signal)
   "Create a new signal whose value is always CONSTANT regardless of
 what the value received from SIGNAL."
-- 
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