One option is to avoid the struct: approach and go with the following Lens definition.
(define-type Lens (All (A B) (case-> (A -> B) (B A -> A)))) Then along the lines below works. (: mod (All (A B) (Lens A B) (B -> B) A -> A)) (define (mod lens f a) (lens (f (lens a)) a)) On Sat, Jun 2, 2012 at 7:27 PM, Ray Racine <[email protected]> wrote: > I'd like to create a Lens structure which is a procedure structure. The > structures exec procedure would be a case-lambda parameterized by A, B. > > (define-struct/exec: (A B) Lens ([getter : (A -> B)][setter : (B A -> > A)]) [fn : (case-> (A -> B) (B A -> A))]) > > where fn would be the getter and setter detemined by arity at the call > site. > > Looks like currently a define-struct/exec: cannot be parameterized. Is > this something that might be available down the road? > > The goal is to say > > (int-lens 3 (int-lens 2)) > > as opposed to say > > (set int-lens 3 (get int-lens 2)) > > > ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;; > ;; Below is current effort. Want to do a define-struct/exec > > ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;; > > (struct: (A B) Lens ([getter : (A -> B)] > [setter : (B A -> A)])) > > (: get (All (A B) (Lens A B) A -> B)) > (define (get lens a) > ((Lens-getter lens) a)) > > (: set (All (A B) (Lens A B) B A -> A)) > (define (set lens b a) > ((Lens-setter lens) b a)) > > (: mod (All (A B) (Lens A B) (B -> B) A -> A)) > (define (mod lens f a) > (set lens (f (get lens a)) a)) > > (: and-then (All (A B C) (Lens A B) (Lens B C) -> (Lens A C))) > (define (and-then lens-ab lens-bc) > (Lens (λ: ((a : A)) > ((Lens-getter lens-bc) ((Lens-getter lens-ab) a))) > (λ: ((c : C) (a : A)) > (mod lens-ab (λ: ((b : B)) ((Lens-setter lens-bc) c b)) a)))) > > (: compose (All (A B C) (Lens B C) (Lens A B) -> (Lens A C))) > (define (compose lens-ab lens-ca) > (and-then lens-ca lens-ab)) > >
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