split functional-dict? predicate into separate module

lens/private/dict.rkt

@@ -6,7 +6,7 @@
            (-> any/c (lens/c functional-dict? any/c))]
           ))
 
-(require racket/dict fancy-app "base/main.rkt")
+(require lens/private/util/functional-dict racket/dict fancy-app "base/main.rkt")
 (module+ test
   (require rackunit))
 
@@ -14,9 +14,6 @@
   (make-lens (dict-ref _ key)
              (dict-set _ key _)))
 
-(define (functional-dict? v)
-  (and (dict? v) (dict-can-functional-set? v)))
-
 (module+ test
   (check-equal? (lens-transform/list '((a . 1) (b . 2) (c . 3)) (dict-ref-lens 'a) (* 100 _))
                 '((a . 100) (b . 2) (c . 3))))

lens/private/util/functional-dict.rkt

@@ -0,0 +1,15 @@
+#lang sweet-exp racket/base
+
+provide functional-dict?
+
+require racket/dict
+module+ test
+  require rackunit
+
+(define (functional-dict? v)
+  (and (dict? v) (dict-can-functional-set? v)))
+
+module+ test
+  (check-true (functional-dict? (hash 'a 1 'b 2)))
+  (check-true (functional-dict? '((a . 1) (b . 2))))
+  (check-false (functional-dict? (make-hash '((a . 1) (b . 2)))))