added metafunction result caching

svn: r11883

collects/redex/private/reduction-semantics.ss

@@ -1046,8 +1046,13 @@
         [codom-compiled-pattern (compile-pattern lang codom-contract-pat #f)])
     (values
      (wrap
-      (letrec ([metafunc
+      (letrec ([cache (make-hash)]
+               [not-in-cache (gensym)]
+               [metafunc
                 (λ (exp)
+                  (let ([cache-ref (hash-ref cache exp not-in-cache)])
+                    (cond
+                      [(eq? cache-ref not-in-cache)
                        (when dom-compiled-pattern
                          (unless (match-pattern dom-compiled-pattern exp)
                            (redex-error name
@@ -1078,7 +1083,10 @@
                                    (let ([ans (rhs metafunc (mtch-bindings (car mtchs)))])
                                      (unless (match-pattern codom-compiled-pattern ans)
                                        (redex-error name "codomain test failed for ~s, call was ~s" ans `(,name ,@exp)))
-                                ans)])))])))])
+                                     (hash-set! cache exp ans)
+                                     ans)])))]))]
+                      [else 
+                       cache-ref])))])
         metafunc)
       compiled-patterns
       rhss)

collects/redex/redex.scrbl

@@ -840,6 +840,11 @@ Raises an exception recognized by @scheme[exn:fail:redex?] if
 no clauses match, if one of the clauses matches multiple ways, or
 if the contract is violated.
 
+Note that metafunctions are assumed to always return the same results
+for the same inputs, and their results are cached. Accordingly, if a
+metafunction is called with the same inputs twice, then its body is
+only evaluated a single time.
+
 As an example, these metafunctions finds the free variables in
 an expression in the lc-lang above: