Merge pull request #112 from AlexKnauth/hash-slice

add hash-slice-lens, closes #102

unstable/lens/hash-pluck.rkt

@@ -0,0 +1,24 @@
+#lang racket/base
+
+(provide hash-pluck-lens)
+
+(require racket/list
+         lens/base/main
+         "hash.rkt"
+         "join.rkt")
+(module+ test
+  (require rackunit))
+
+(define (hash-pluck-lens . ks)
+  (apply lens-join/hash
+         (append-map
+          (λ (k)
+            (list k (hash-ref-lens k)))
+          ks)))
+
+(module+ test
+  (check-equal? (lens-view (hash-pluck-lens 'a 'c) (hash 'a 1 'b 2 'c 3))
+                (hash 'a 1 'c 3))
+  (check-equal? (lens-set (hash-pluck-lens 'a 'c) (hash 'a 1 'b 2 'c 3) (hash 'a 4 'c 5))
+                (hash 'a 4 'b 2 'c 5))
+  )

unstable/lens/hash-pluck.scrbl

@@ -0,0 +1,16 @@
+#lang scribble/manual
+
+@(require lens/doc-util/main)
+
+@title{Viewing a subset of a hash table by key}
+
+@defmodule[unstable/lens/hash-pluck]
+
+@defproc[(hash-pluck-lens [key any/c] ...) lens?]{
+Creates a lens that views a subset of the target hash-table with the given
+@racket[key]s. The view, is another hash-table with only the given keys and
+their corrosponding values in the target hash-table.
+@lenses-unstable-examples[
+  (lens-view (hash-slice-lens 'a 'c) (hash 'a 1 'b 2 'c 3))
+  (lens-set (hash-slice-lens 'a 'c) (hash 'a 1 'b 2 'c 3) (hash 'a 4 'c 5))
+]}

unstable/lens/main.rkt

@@ -8,6 +8,7 @@
          "sublist.rkt"
          "struct.rkt"
          "arrow.rkt"
+         "hash-pluck.rkt"
          )
 
 (provide (all-from-out "syntax.rkt"
@@ -18,4 +19,5 @@
                        "sublist.rkt"
                        "struct.rkt"
                        "arrow.rkt"
+                       "hash-pluck.rkt"
                        ))

unstable/lens/main.scrbl

@@ -17,3 +17,4 @@ this library being backwards-compatible.
 @include-section["sublist.scrbl"]
 @include-section["struct.scrbl"]
 @include-section["arrow.scrbl"]
+@include-section["hash-pluck.scrbl"]