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")