From 1ab56f079407f3411046c8e6984e0e7747418391 Mon Sep 17 00:00:00 2001
From: Jack Firth <jackhfirth@gmail.com>
Date: Fri, 10 Jul 2015 21:23:19 -0700
Subject: [PATCH] Add list-refs lens

---
 unstable/lens/list.rkt   | 22 ++++++++++++++++++----
 unstable/lens/list.scrbl |  9 +++++++++
 2 files changed, 27 insertions(+), 4 deletions(-)

diff --git a/unstable/lens/list.rkt b/unstable/lens/list.rkt
index 36b2747..bce7392 100644
--- a/unstable/lens/list.rkt
+++ b/unstable/lens/list.rkt
@@ -1,17 +1,31 @@
 #lang racket
 
-(require lens)
+(require lens
+         "compound.rkt")
 
 (module+ test
   (require rackunit))
 
 (provide
   (contract-out
-    [list-ref-nested-lens (->* () #:rest (listof exact-nonnegative-integer?) lens?)]))
+    [list-ref-nested-lens (->* () #:rest (listof exact-nonnegative-integer?) lens?)]
+    [list-refs-lens (->* () #:rest (listof exact-nonnegative-integer?) lens?)]))
 
-(define (list-ref-nested-lens . args)
-  (apply lens-thrush (map list-ref-lens args)))
+
+(define (list-ref-nested-lens . indices)
+  (apply lens-thrush (map list-ref-lens indices)))
 
 (module+ test
   (check-equal? (lens-transform/list '(a (b c) (d e f)) (list-ref-nested-lens 2 1) symbol->string)
                 '(a (b c) (d "e" f))))
+
+
+(define (list-refs-lens . indices)
+  (apply compound-list-lens (map list-ref-lens indices)))
+
+(module+ test
+  (define 1-5-6-lens (list-refs-lens 1 5 6))
+  (check-equal? (lens-view 1-5-6-lens '(a b c d e f g))
+                '(b f g))
+  (check-equal? (lens-set 1-5-6-lens '(a b c d e f g) '(1 2 3))
+                '(a 1 c d e 2 3)))
diff --git a/unstable/lens/list.scrbl b/unstable/lens/list.scrbl
index e8852ee..0a6be1a 100644
--- a/unstable/lens/list.scrbl
+++ b/unstable/lens/list.scrbl
@@ -15,3 +15,12 @@
     (lens-view first-of-second-lens '(1 (a b c) 2 3))
     (lens-set first-of-second-lens '(1 (a b c) 2 3) 'foo)
 ]}
+
+@defproc[(list-refs-lens [index exact-nonnegative-integer?] ...) lens?]{
+  Constructs a lens that views each @racket[index] item in a list.
+  Indexing starts from zero in the same was as @racket[list-ref-lens].
+  @lenses-unstable-examples[
+    (define 1-5-6-lens (list-refs-lens 1 5 6))
+    (lens-view 1-5-6-lens '(a b c d e f g))
+    (lens-set 1-5-6-lens '(a b c d e f g) '(1 2 3))
+]}