Introduce "?" as identifier-syntax for "(wild)"

chat-os2-paper.rkt

@@ -54,10 +54,8 @@
        (cout (term->bytes
 	      `(,who ,did-what))))))
   (list
-   (role (topic-publisher
+   (role (topic-publisher `(,me says ,?)))
-	  `(,me says ,(wild))))
+   (role (topic-subscriber `(,? says ,?))
-   (role (topic-subscriber
-	  `(,(wild) says ,(wild)))
     #:topic t
     #:on-presence (announce t 'arrived)
     #:on-absence  (announce t 'departed)

chat-os2.rkt

@@ -18,8 +18,8 @@
   (match-define (topic _ (tcp-channel connection-id _ _) _) t)
   (define-values (cin cout in-topic out-topic) (tcp-accept t))
   (transition 'no-state
-    (role (topic-publisher `(,connection-id says ,(wild))))
+    (role (topic-publisher `(,connection-id says ,?)))
-    (role (topic-subscriber `(,(wild) says ,(wild)))
+    (role (topic-subscriber `(,? says ,?))
       #:topic t
       #:on-presence (match t [(topic _ (list who _ _) _)
 			      (unless (equal? who connection-id)

os2.rkt

@@ -77,6 +77,9 @@
 	 non-wild?
 	 ground?
 
+	 ;; Identifier-syntax for (wild)
+	 ?
+
 	 ;; Reexports from racket/match for convenience
 	 (all-from-out racket/match)
 
@@ -294,6 +297,9 @@
 (define-syntax-rule (yield-macro #:state state-pattern body ...)
   (yield (match-lambda [state-pattern body ...])))
 
+;; A fresh unification variable, as identifier-syntax.
+(define-syntax ? (syntax-id-rules () (_ (wild))))
+
 ;;---------------------------------------------------------------------------
 ;; Smarter constructors for transitions and preactions.