From c9fa6fb8ea78142e0f529835f82d2026fb90af4b Mon Sep 17 00:00:00 2001
From: AlexKnauth <alexander@knauth.org>
Date: Mon, 31 Aug 2015 15:48:38 -0400
Subject: [PATCH] provide and document transformer-lens

---
 unstable/lens/main.rkt          |  1 +
 unstable/lens/main.scrbl        |  1 +
 unstable/lens/transformer.scrbl | 26 ++++++++++++++++++++++++++
 3 files changed, 28 insertions(+)
 create mode 100644 unstable/lens/transformer.scrbl

diff --git a/unstable/lens/main.rkt b/unstable/lens/main.rkt
index 4393467..6c69a5e 100644
--- a/unstable/lens/main.rkt
+++ b/unstable/lens/main.rkt
@@ -13,4 +13,5 @@
 "struct-nested.rkt"
 "sublist.rkt"
 "syntax.rkt"
+"transformer.rkt"
 "view-set.rkt"
diff --git a/unstable/lens/main.scrbl b/unstable/lens/main.scrbl
index 10d51a5..2a2d1e9 100644
--- a/unstable/lens/main.scrbl
+++ b/unstable/lens/main.scrbl
@@ -26,6 +26,7 @@ this library being backwards-compatible.
   "struct-nested.scrbl"
   "sublist.scrbl"
   "syntax.scrbl"
+  "transformer.scrbl"
   "view-set.scrbl"
   )
 
diff --git a/unstable/lens/transformer.scrbl b/unstable/lens/transformer.scrbl
new file mode 100644
index 0000000..f6275e5
--- /dev/null
+++ b/unstable/lens/transformer.scrbl
@@ -0,0 +1,26 @@
+#lang scribble/manual
+
+@(require lens/private/doc-util/main)
+
+@title{Lenses that transform subpieces}
+
+@defmodule[unstable/lens/transformer]
+
+@defproc[(transformer-lens [lens lens?] [transform-lens lens?]) lens?]{
+Creates a lens that transforms the subpiece of the target that @racket[lens]
+views with @racket[transform-lens].
+
+@racketblock[(lens-view (transformer-lens lens transform-lens) target)]
+is equivalent to:
+@racketblock[(lens-transform lens target (λ (v) (lens-view transform-lens v)))]
+
+@lens-unstable-examples[
+  (define first-sym->str
+    (transformer-lens first-lens symbol->string-lens))
+  (lens-view first-sym->str '(a b c))
+  (lens-set first-sym->str '(a b c) '("a" b c))
+  (lens-set first-sym->str '(a b c) '("z" b c))
+  (lens-set first-sym->str '(a b c) '("z" bee sea))
+  (lens-view first-sym->str (lens-set first-sym->str '(a b c) '("z" bee sea)))
+]}
+