Add signal-switch procedure.

* sly/signal.scm (signal-switch): New procedure.

1 files changed, 13 insertions, 0 deletions

diff --git a/sly/signal.scm b/sly/signal.scm
index f8545eb..3e5ce26 100644
--- a/sly/signal.scm
+++ b/sly/signal.scm
@@ -41,6 +41,7 @@
             signal-filter
             signal-reject
             signal-drop-repeats
+            signal-switch
             signal-constant
             signal-count
             signal-tap
@@ -258,6 +259,18 @@ testing equivalence."
                  (signal-ref signal)
                  signal))
 
+(define (signal-switch pred on off)
+  "Create a new signal whose value is that of the signal ON when the
+signal PRED is true, or the value of the signal OFF otherwise."
+  (define (current-value)
+    (if (signal-ref pred)
+        (signal-ref on)
+        (signal-ref off)))
+  (make-boxed-signal (current-value)
+                     (lambda (self value)
+                       (%signal-set! self (current-value)))
+                     (list pred)))
+
 (define (signal-constant constant signal)
   "Create a new signal whose value is always CONSTANT regardless of
 what the value received from SIGNAL."