add dict-ref-lens

lens/dict.rkt

@@ -0,0 +1,16 @@
+#lang racket/base
+
+(provide dict-ref-lens)
+
+(require racket/dict fancy-app "base/main.rkt")
+(module+ test
+  (require rackunit))
+
+(define (dict-ref-lens key)
+  (make-lens (dict-ref _ key)
+             (dict-set _ key _)))
+
+(module+ test
+  (check-equal? (lens-transform* '((a . 1) (b . 2) (c . 3)) (dict-ref-lens 'a) (* 100 _))
+                '((a . 100) (b . 2) (c . 3)))
+  )

lens/dict.scrbl

@@ -0,0 +1,22 @@
+#lang scribble/manual
+
+@(require scribble/eval
+          "lenses-examples.rkt"
+          (for-label lens
+                     lens/dict
+                     racket/base
+                     racket/contract))
+
+
+@title{Dict lenses}
+
+@defmodule[lens/dict]
+
+@defproc[(dict-ref-lens [key any/c]) lens?]{
+  Returns a lens for viewing the value mapped to @racket[key] in a dict.
+  @lenses-examples[
+    (require lens/dict)
+    (define dict '((a . 1) (b . 2) (c . 3)))
+    (lens-view (dict-ref-lens 'a) dict)
+    (lens-set (dict-ref-lens 'a) dict 100)
+]}

lens/main.scrbl

@@ -16,4 +16,5 @@ source code: @url["https://github.com/jackfirth/lens"]
 @include-section["base/main.scrbl"]
 @include-section["list/main.scrbl"]
 @include-section["struct.scrbl"]
+@include-section["dict.scrbl"]
 @include-section["applicable.scrbl"]