add contract for set-filterer-lens

unstable/lens/set-filterer.rkt

@@ -1,9 +1,12 @@
 #lang racket/base
 
-(provide set-filterer-lens)
+(require racket/contract/base)
+(provide (contract-out
+          [set-filterer-lens
+           (-> predicate/c (lens/c functional-set? functional-set?))]
+          ))
 
 (require lens/base/main
-         racket/contract/base
          racket/list
          racket/set
          fancy-app
@@ -27,6 +30,10 @@
   (for/fold ([set set]) ([elem (in-set set)] #:when (pred elem))
     (set-remove set elem)))
 
+(define (functional-set? st)
+  (and (generic-set? st)
+       (set-implements? st 'set-add 'set-remove)))
+
 (module+ test
   (check-equal? (lens-view (set-filterer-lens number?) '(1 a 2 b c 3 d e))
                 '(1 2 3))

unstable/lens/set-filterer.scrbl

@@ -6,7 +6,7 @@
 
 @defmodule[unstable/lens/set-filterer]
 
-@defproc[(set-filterer-lens [pred (-> any/c any/c)]) lens?]{
+@defproc[(set-filterer-lens [pred (-> any/c any/c)]) (lens/c functional-set? functional-set?)]{
 Creates a lens that filters a set by the predicate @racket[pred].
 @lenses-unstable-examples[
   (lens-view (set-filterer-lens number?) (set 1 'a 2 'b 'c 3 'd 'e))