Add core lens forms

lenses/core.rkt

@@ -0,0 +1,43 @@
+#lang racket
+
+(provide lens/c
+         let-lens
+         lens-view
+         lens-set
+         lens-transform
+         lens-compose)
+
+(define (lens/c input subcomponent)
+  (-> input
+      (values subcomponent
+              (-> subcomponent
+                  input))))
+
+(define-syntax-rule (let-lens (view setter) lens-call-expr body ...)
+  (let-values ([(view setter) lens-call-expr])
+    body ...))
+
+(define (lens-view lens v)
+  (let-lens (view _) (lens v)
+    view))
+
+(define (lens-set lens v x)
+  (let-lens (_ setter) (lens v)
+    (setter x)))
+
+(define (lens-transform lens v f)
+  (let-lens (view setter) (lens v)
+    (setter (f view))))
+
+(define ((lens-compose2 sub-lens super-lens) v)
+  (let-lens (super-view super-setter) (super-lens v)
+    (let-lens (sub-view sub-setter) (sub-lens super-view)
+      (values sub-view
+              (compose super-setter sub-setter)))))
+
+(define ((generalize-operator op) v . vs)
+  (if (empty? vs)
+      v
+      (foldl (λ (next-v previous) (op previous next-v)) vs)))
+
+(define lens-compose (generalize-operator lens-compose2))