add dict-ref-nested-lens

unstable/lens/dict-nested.rkt

@@ -0,0 +1,24 @@
+#lang sweet-exp racket/base
+
+require racket/contract/base
+provide
+  contract-out
+    dict-ref-nested-lens (->* [] #:rest (listof any/c) (lens/c functional-dict? any/c))
+
+require lens/private/base/main
+        lens/private/compound/thrush
+        lens/private/dict
+        lens/private/util/functional-dict
+module+ test
+  require rackunit fancy-app
+
+(define (dict-ref-nested-lens . ks)
+  (apply lens-thrush (map dict-ref-lens ks)))
+
+module+ test
+  (define a-x (dict-ref-nested-lens 'a 'x))
+  (let-lens [val ctxt] a-x '([a . ([x . 1] [y . 2])] '[b . ([z . 3])])
+    (check-equal? val 1)
+    (check-equal? (ctxt 100) '([a . ([x . 100] [y . 2])] '[b . ([z . 3])])))
+  (check-equal? (lens-transform/list '([a . ([x . 1] [y . 2])] '[b . ([z . 3])]) a-x (* 10 _))
+                '([a . ([x . 10] [y . 2])] '[b . ([z . 3])]))