ZipList in OCaml with Jane Street's Core

zip_list.ml

open Core 

module ZipList = struct 
    type 'a t = ZipList of 'a list 
    
    let return x =
        (* A nice feature of Ocaml; just make sure the operation performed on the list 
           doesn't "recur" over the whole list. It has to be something like `take`, or a 
           `zip` with another finite list *)
        let rec l = x :: l in  
        ZipList l

    let map = `Define_using_apply

    let rec apply fs xs = 
        match fs, xs with 
        | ZipList (f :: fs), ZipList (x :: xs) -> 
            let ZipList almost = apply (ZipList fs) (ZipList xs) in 
            ZipList (f x :: almost)
        | ZipList [], _ | _, ZipList [] -> ZipList []
end

let test () =
    let open Applicative.Make (ZipList) in 
    ZipList [ (+); (-) ] <*> ZipList [ 1;2 ] <*> ZipList [ 2;3 ]