add lazy-lens and rec-lens

unstable/lens/lazy.rkt

@@ -0,0 +1,29 @@
+#lang sweet-exp racket/base
+
+provide lazy-lens
+        rec-lens
+
+require fancy-app lens/private/base/main racket/promise
+module+ test
+  require rackunit "if.rkt" "isomorphism/data.rkt" "mapper.rkt"
+
+(define-syntax-rule (lazy-lens expr)
+  (let ([p (delay expr)])
+    (make-lens (lens-view (force p) _) (lens-set (force p) _ _))))
+
+(define-syntax-rule (rec-lens name expr)
+  (letrec ([name (lazy-lens expr)])
+    name))
+
+module+ test
+  (define (tree-mapper-lens item-lens)
+    (rec-lens the-tree-lens
+      (lens-cond [list? (mapper-lens the-tree-lens)]
+                 [else item-lens])))
+  (check-equal? (lens-view (tree-mapper-lens symbol->string-lens) '(a (b (() c)) (d)))
+                '("a" ("b" (() "c")) ("d")))
+  (check-equal? (lens-set (tree-mapper-lens symbol->string-lens)
+                          '(a (b (() c)) (d))
+                          '("hay" ("bee" (() "sea")) ("deep")))
+                '(hay (bee (() sea)) (deep)))
+