Make send-message topic optional, defaulting to a publish of the body itself.

os2.rkt

@@ -27,7 +27,8 @@
 	 (rename-out [make-add-role add-role])
 	 (except-out (struct-out delete-role) delete-role)
 	 (rename-out [make-delete-role delete-role])
-	 (struct-out send-message)
+	 (except-out (struct-out send-message) send-message)
+	 (rename-out [make-send-message send-message])
 	 (except-out (struct-out spawn) spawn)
 	 (rename-out [make-spawn spawn])
 	 (except-out (struct-out kill) kill)
@@ -101,7 +102,7 @@
 ;; Ks are various TrapKs or #f, signifying lack of interest.
 (struct add-role (topics handlers k) #:prefab)
 (struct delete-role (eid reason) #:prefab)
-(struct send-message (topic body) #:prefab)
+(struct send-message (body topic) #:prefab)
 (struct spawn (main k) #:prefab)
 (struct kill (pid reason) #:prefab)
 
@@ -152,6 +153,7 @@
 (define (make-transition state . actions) (transition state actions))
 (define (make-add-role topics handlers [k #f]) (add-role topics handlers k))
 (define (make-delete-role eid [reason #f]) (delete-role eid reason))
+(define (make-send-message body [topic (topic-publisher body)]) (send-message body topic))
 (define (make-spawn main [k #f]) (spawn main k))
 (define (make-kill [pid #f] [reason #f]) (kill pid reason))
 
@@ -250,7 +252,7 @@
   (match preaction
     [(add-role topics hs k) (do-subscribe pid (ensure-topic-union topics) hs k state)]
     [(delete-role eid reason) (do-unsubscribe pid eid reason state)]
-    [(send-message topic body) (route-and-deliver topic body state)]
+    [(send-message body topic) (route-and-deliver topic body state)]
     [(spawn main k) (do-spawn pid main k state)]
     [(kill pid-to-kill reason) (do-kill (or pid-to-kill pid) reason state)]))