add lens-if

unstable/lens/if.rkt

@@ -0,0 +1,26 @@
+#lang racket/base
+
+(provide lens-if)
+
+(require lens/base/main)
+(module+ test
+  (require rackunit lens/list/main lens/vector/main))
+
+(define (lens-if pred lens1 lens2)
+  (make-lens
+   (λ (tgt)
+     (if (pred tgt)
+         (lens-view lens1 tgt)
+         (lens-view lens2 tgt)))
+   (λ (tgt nvw)
+     (if (pred tgt)
+         (lens-set lens1 tgt nvw)
+         (lens-set lens2 tgt nvw)))))
+
+(module+ test
+  (define if-lens (lens-if list? first-lens (vector-ref-lens 0)))
+  (check-equal? (lens-view if-lens '(1 2 3)) 1)
+  (check-equal? (lens-view if-lens '#(1 2 3)) 1)
+  (check-equal? (lens-set if-lens '(1 2 3) 'a) '(a 2 3))
+  (check-equal? (lens-set if-lens '#(1 2 3) 'a) '#(a 2 3))
+  )