Functional dict-update.

src/dictionary.scm

@@ -37,6 +37,7 @@
   dict-map
   dict-filter
   dict-reduce
+  dict-update
   dictionary-tests!
   )
 
@@ -145,6 +146,11 @@
 	 (loop (cdr d)
 	       (proc acc (caar d) (cdar d))))))
 
+ ;; Functional update
+ (define (dict-update d k proc . dfls)
+   (dict-set d k
+	     (proc (apply dict-ref d k dfls))))
+
  ;; Performs self-tests of the dictionary module.
  (define (dictionary-tests!)
    (run-tests