on-receive does not have to exist for a world to be a part of a universe, weird but feasible; Closes PR12401

collects/2htdp/tests/small-client-server.rkt

@@ -0,0 +1,32 @@
+#lang racket/load
+
+(module client racket
+  (require 2htdp/universe 2htdp/image)
+  
+  ;; Color -> Boolean 
+  (define (client c)
+    (big-bang #true
+              (to-draw (lambda (w) (text (if w "hello world" "good bye") 22 c)))
+              (register LOCALHOST)
+              (name c)
+              (on-receive (lambda (w msg) (not w)))))
+  
+  (launch-many-worlds (client "blue") (client "red")))
+
+(module server racket 
+  (require 2htdp/universe)
+  
+  (define (server tick-rate)
+    (universe '()
+              (on-new (lambda (w n) (make-bundle (cons n w) '() '())))
+              (on-tick 
+               (lambda (w*)
+                 (make-bundle w* (map (lambda (w) (make-mail w 'go)) w*) '()))
+               tick-rate)
+              (on-msg void)))
+  
+  (thread server))
+
+(require 'server)
+
+(require 'client)

collects/2htdp/tests/universe-receive.rkt

@@ -0,0 +1,15 @@
+#lang racket
+
+(require 2htdp/universe)
+(require 2htdp/image)
+
+(launch-many-worlds
+ (big-bang '*
+           (on-tick (lambda (w) w) 1/3 2)
+           (to-draw (λ (w) (empty-scene 200 200)))
+           (register LOCALHOST))
+ 
+ (universe '*
+           (on-tick (lambda (w) (make-bundle '* '() '())) 1/2 2)
+           (on-new (λ (u iw) (make-bundle '* (list (make-mail iw 'boo!)) '())))
+           (on-msg (λ (u iw msg) (make-bundle '* empty empty)))))

collects/2htdp/universe.rkt

@@ -96,7 +96,7 @@
   [on-release DEFAULT #f (function-with-arity 2)]
   ;; (U #f (World S-expression -> World))
   ;; -- on-receive must specify a receive handler 
-  [on-receive DEFAULT #'#f (function-with-arity 2)]
+  [on-receive DEFAULT #'(lambda (w m) w) (function-with-arity 2)]
   ;; World -> Boolean 
   ;; -- stop-when must specify a predicate; it may specify a rendering function
   [stop-when DEFAULT #'False

collects/2htdp/xtest

@@ -37,4 +37,5 @@ run on-tick-universe-with-limit.rkt
 run on-tick-with-limit.rkt
 run on-release-no-key.rkt
 run struct-universe.rkt
+run universe-receive.rkt
 

doc/release-notes/teachpack/HISTORY.txt

@@ -1,4 +1,9 @@
 ------------------------------------------------------------------------
+
+* on-receive doesn't have to exist for universe/world interactions 
+  -- fixed
+
+------------------------------------------------------------------------
 Version 5.2				  [Tue Oct 18 12:34:16 EDT 2011]
 
 * bug fixes in 2htdp/universe