example-partial-retraction.rkt

racket/syndicate/examples/actor/example-partial-retraction.rkt

@@ -0,0 +1,39 @@
+#lang syndicate
+;; Illustrates the response of asserted / retracted / during to
+;; observation of assertions discarding some of their dimensions.
+
+(require syndicate/actor)
+
+(struct ready (what) #:prefab)
+(struct entry (key val) #:prefab)
+
+(actor (forever
+        (assert (ready 'listener))
+        (on (asserted (entry $key _))
+            (log-info "key ~v asserted" key)
+            (until (retracted (entry key _))
+                   (on (asserted (entry key $value))
+                       (log-info "add binding: ~v -> ~v" key value))
+                   (on (retracted (entry key $value))
+                       (log-info "del binding: ~v -> ~v" key value)))
+            (log-info "key ~v retracted" key))))
+
+(define (pause)
+  (log-info "pause")
+  (until (asserted (ready 'pause))
+         (assert (ready 'pause))))
+
+(actor (until (asserted (ready 'listener)))
+       (assert! (entry 'a 1))
+       (assert! (entry 'a 2))
+       (assert! (entry 'b 3))
+       (assert! (entry 'c 33))
+       (assert! (entry 'a 4))
+       (assert! (entry 'a 5))
+       (pause)
+       (retract! (entry 'a 2))
+       (retract! (entry 'c 33))
+       (assert! (entry 'a 9))
+       (pause)
+       (retract! (entry 'a ?))
+       (pause))