OCaml modules

(* module types are the module-level analogy of types *)
module type Eq = sig
    type t
    val eq : t -> t -> bool
end

(* struct is a module literal *)
module IntEq = struct
    type t = int
    let eq x y = x = y (* OCaml's (=) operator isn't parametric, hence Eq modules are useful for type-safety *)
end

(* module that takes a module! *)
module ListEq (ElementEq : Eq) = struct
    type t = ElementEq.t list
    let rec eq xs ys =
        match (xs, ys) with
        | ([], []) -> true
        | (x :: xs', y :: ys') -> ElementEq.eq x y && eq xs' ys'
        | (_, _) -> false
end

(* module application *)
module IntListEq = ListEq (IntEq)

let () =
    print_endline (if IntListEq.eq [1; 2; 3] [1; 2; 4] then "true" else "false")