added update functions

collections-lib/data/collection/indexable.rkt

@@ -7,15 +7,26 @@
 
 (provide
  gen:indexable indexable? indexable/c
- ref set-ref)
+ ref set-ref update-ref)
 
 (define-generics indexable
   (ref indexable . _)
   (set-ref indexable . _)
+  (update-ref indexable . _)
   #:defaults
   ([hash? (define ref hash-ref)
-          (define set-ref hash-set)]
+          (define set-ref hash-set)
+          (define update-ref hash-update)]
    [dict? (define ref dict-ref)
-          (define set-ref dict-set)]
+          (define set-ref dict-set)
+          (define update-ref dict-update)]
    [sequence? (define ref nth)
-              (define set-ref set-nth)]))
+              (define set-ref set-nth)
+              (define update-ref update-nth)]
+   )
+  #:fallbacks
+  [(define/generic ref* ref)
+   (define/generic set-ref* set-ref)
+   (define (update-ref indexable index proc)
+     (let ([old (ref* indexable index)])
+       (set-ref* indexable index (proc old))))])