Add wild? and non-wild? predicates.

presence/conversation.rkt

@@ -29,6 +29,8 @@
 
 	 ;; Reexported from unify.rkt for convenience
 	 wild
+	 wild?
+	 non-wild?
 	 )
 
 (struct arrived (who) #:prefab) ;; someone arrived

presence/unify.rkt

@@ -7,6 +7,8 @@
 (provide (struct-out variable)
 	 (struct-out canonical-variable)
 	 wild
+	 wild?
+	 non-wild?
 	 variables-in
 	 unify
 	 unify/env
@@ -45,6 +47,16 @@
 (define (wild [base-name '_])
   (variable (gensym base-name)))
 
+;; Any -> Boolean
+;; True iff the argument is a variable or canonical-variable.
+(define (wild? x)
+  (or (variable? x) (canonical-variable? x)))
+
+;; Any -> Boolean
+;; True iff the argument is neither a variable nor a canonical-variable.
+(define (non-wild? x)
+  (not (wild? x)))
+
 ;; Any -> Set<Variable>
 (define (variables-in x)
   (let walk ((x x) (acc (set)))
@@ -134,7 +146,7 @@
   (define env (make-hash)) ;; cheeky use of mutation
   (let walk ((t t))
     (cond
-     [(or (upper-case-symbol? t) (variable? t) (canonical-variable? t))
+     [(or (upper-case-symbol? t) (wild? t))
       (cond [(hash-ref env t #f)]
 	    [else (define v (canonical-variable (hash-count env))) (hash-set! env t v) v])]
      [(pair? t) (cons (walk (car t)) (walk (cdr t)))]
@@ -178,7 +190,7 @@
   (define env (make-hash)) ;; cheeky use of mutation
   (let walk ((t t))
     (cond
-     [(or (variable? t) (canonical-variable? t))
+     [(wild? t)
       (cond [(hash-ref env t #f)]
 	    [else (define v ((if (canonical-variable? t) canon-handler var-handler) t env))
 		  (hash-set! env t v)
@@ -246,8 +258,8 @@
 (define (specialization? a b)
   (let walk ((a a) (b b))
     (cond
-     [(or (variable? b) (canonical-variable? b)) #t]
+     [(wild? b) #t]
-     [(or (variable? a) (canonical-variable? a)) #f]
+     [(wild? a) #f]
      [(and (pair? a) (pair? b))
       (and (walk (car a) (car b)) (walk (cdr a) (cdr b)))]
      [(and (vector? a) (vector? b) (= (vector-length a) (vector-length b)))