provide and document string lenses

unstable/lens/main.rkt

@@ -4,6 +4,7 @@
          "join.rkt"
          "list.rkt"
          "hash.rkt"
+         "string.rkt"
          "view-set.rkt"
          "sublist.rkt"
          "struct.rkt"
@@ -15,6 +16,7 @@
                        "join.rkt"
                        "list.rkt"
                        "hash.rkt"
+                       "string.rkt"
                        "view-set.rkt"
                        "sublist.rkt"
                        "struct.rkt"

unstable/lens/main.scrbl

@@ -13,6 +13,7 @@ this library being backwards-compatible.
 @include-section["join.scrbl"]
 @include-section["list.scrbl"]
 @include-section["hash.scrbl"]
+@include-section["string.scrbl"]
 @include-section["syntax.scrbl"]
 @include-section["sublist.scrbl"]
 @include-section["struct.scrbl"]

unstable/lens/string.scrbl

@@ -0,0 +1,23 @@
+#lang scribble/manual
+
+@(require lens/doc-util/main)
+
+@title{String Lenses}
+
+@defproc[(string-ref-lens [i exact-nonnegative-integer?]) lens?]{
+Returns a lens for viewing the @racket[i]th character of a string.
+@lenses-unstable-examples[
+  (lens-view (string-ref-lens 2) "abcdef")
+  (lens-set (string-ref-lens 2) "abcdef" #\C)
+]}
+
+@defproc[(string-pluck-lens [i exact-nonnegative-integer?]) lens?]{
+Like @racket[list-refs-lens], but for strings.
+Equivalent to @racket[(lens-join/string (string-ref-lens i) ...)].
+@lenses-unstable-examples[
+  (define 1-5-6-lens (string-pluck-lens 1 5 6))
+  (check-equal? (lens-view 1-5-6-lens "abcdefg")
+                "bfg")
+  (check-equal? (lens-set 1-5-6-lens "abcdefg" "BFG")
+                "aBcdeFG")
+]}