Take actors to ... the upside down [1]

Actors in the upside down may communicate with each other, but not the outside
world. However, the outside world can see what actors in the upside-down are
saying. The intention is to use this facility for testing.

[1] (http://strangerthings.wikia.com/wiki/Upside_Down).

racket/syndicate/upside-down.rkt

@@ -0,0 +1,77 @@
+#lang racket
+
+(provide (struct-out upside-down)
+         upside-down-parenthesis
+         spawn-upside-down)
+
+(require "core.rkt")
+(require "trie.rkt")
+
+#|
+A module that takes actors to ... the upside down [1]. Actors in the upside down
+may communicate with each other, but not the outside world. However, the outside
+world can see what actors in the upside-down are saying. The intention is to use
+this facility for testing.
+
+[1] (http://strangerthings.wikia.com/wiki/Upside_Down).
+|#
+
+(struct upside-down (v) #:transparent)
+(define upside-down-parenthesis (open-parenthesis 1 struct:upside-down))
+
+(define (spawn-upside-down inner-spawn)
+  (make-actor (lambda ()
+                (define-values (proc initial-transition)
+                  (actor->process+transition inner-spawn))
+                (list (upside-down-behavior (process-behavior proc))
+                      (upside-down-transition initial-transition)
+                      (process-name proc)))))
+
+;; Transition -> Transition
+(define (upside-down-transition t)
+  (match t
+    [(<quit> exn actions)
+     (<quit> exn (upside-down-actions actions))]
+    [(transition st actions)
+     (transition st (upside-down-actions actions))]
+    [(or #f (? void?))
+     t]))
+
+(define ((upside-down-behavior b) e s)
+  (define rightside-up-e
+    (turn-event-rightside-up e))
+  (upside-down-transition (b rightside-up-e s)))
+
+(define (upside-down-actions acs)
+  (filter-map turn-action-upside-down (clean-actions acs)))
+
+;; Action -> Action
+(define (turn-action-upside-down a)
+  (match a
+    [#f #f]
+    [(message v)
+     (message (upside-down v))]
+    [(patch added removed)
+     (patch (turn-trie-upside-down added)
+            (turn-trie-upside-down removed))]
+    [(? actor? a) a]))
+
+;; Event -> Event
+(define (turn-event-rightside-up e)
+  (match e
+    [#f #f]
+    [(patch added removed)
+     (patch (trie-project added (upside-down (?!)))
+            (trie-project removed (upside-down (?!))))]
+    [(message (upside-down v))
+     (message v)]))
+
+;; Trie -> Trie
+;; x ∈ T => (upside-down x) ∈ T'
+;; ?x ∈ T => ?(upside-down x) ∈ T'
+(define (turn-trie-upside-down t)
+  (define subscriptions (trie-project t (observe (?!))))
+  (trie-union (trie-prepend upside-down-parenthesis t)
+              (trie-prepend observe-parenthesis
+                            (trie-prepend upside-down-parenthesis
+                                          subscriptions))))