unzip.ml

module type app = sig
   type 'a t                                                                          
   val pure : 'a -> 'a t
   val ( <$> ) : ('a -> 'b) t -> 'a t -> 'b t
end

module Unzip(App:app) = struct
  open App
  let rec f = function 
  | [] -> pure [] 
  | a :: q -> pure List.cons <$> a <$> f q
end

module Pair =
  struct
    type 'a t = 'a * 'a
    let pure x = x, x
    let (<$>) (f,g) (x,y) = (f x,g y)
end

module U = Unzip(Pair)

let l1, l2 = U.f [1,2;3,4]