add lens/c

lens/base/base.rkt

@@ -12,7 +12,9 @@
                        [focus-lens (-> lens? any/c
                                        (values any/c (-> any/c any/c)))]
                        [use-applicable-lenses! (-> void?)]
-                       [lens? predicate/c]))
+                       [lens? predicate/c]
+                       [lens/c (contract? contract? . -> . contract?)]
+                       ))
 
 
 (define lenses-applicable? (make-parameter #f))
@@ -27,6 +29,9 @@
         ((lens-struct-get this) target)
         (error "cannot apply a non-applicable lens as a function"))))
 
+(define (lens/c target/c view/c)
+  (struct/c lens-struct (-> target/c view/c) (-> target/c view/c target/c)))
+
 (module+ test
   (require rackunit)
   (check-exn exn:fail? (thunk (first-lens '(a b c)))))

lens/base/contract.scrbl

@@ -0,0 +1,16 @@
+#lang scribble/manual
+
+@(require lens/doc-util/main)
+
+@title{Lens Contracts}
+
+@defproc[(lens/c [target/c contract?] [view/c contract?]) contract?]{
+A contract constructor for lenses. The @racket[target/c] contract is used for
+the second argument in @racket[(lens-view lens target)], the second argument
+and the return value of @racket[(lens-set lens target view)], for example, the
+@racket[view/c] contract is used for the return value of
+@racket[(lens-view lens target)] and the third argument of
+@racket[(lens-set lens target view)], as well as other places where targets or
+views of the lens are used as inputs or outputs.
+}
+

lens/base/main.scrbl

@@ -6,4 +6,5 @@
 @include-section["view-set.scrbl"]
 @include-section["laws.scrbl"]
 @include-section["transform.scrbl"]
+@include-section["contract.scrbl"]
 @include-section["compose.scrbl"]