OCaml understanding functors with more complicated example

Question

This is my functor ORDTYPE. I want to use compare inside EmptyQueue. I dont knew how to inject functor. All time i get errors that i have invalid signature. I tried to declare functor (Key:ORDTYPE) -> before struct but it was wrong. I dont understand idea of functors. I see some easy examples in OCaml wiki but i dont knew how to deal with something more complicated.

In short i want to use comparator in emptyqueue. But i dont knew how to deal with this to do this more abstract.

module type ORDTYPE =
sig
  type t
  val compare : t -> t -> int
end;;

module Icmp:ORDTYPE with type t= int=
struct
    type t = int
    let compare = fun x y -> if( x< y) then 0 else 1
end;;


module type STH=
sig
  type priority
  type 'a t
  val comp: x -> x->bool
end;;


module Emptyqueue (Comp: ORDTYPE): STH with type priority= int =
    struct
        type priority = int
        type 'a t = Empty 
        let comp = fun x y -> Comp.comp x y

    end;;

I edited how i think i should do this but it doesn't. work.

Answer 1

What you miss is the need to define x in the signature STH. I will use a clearer name elt instead of x. Once STH has elt, we can either add it to Emptyqueue as well, or use "destructive substitution", i.e. signature modification syntax with ... := ...

Pay attention to differences introduced in my "straightening" of your example, because you have some mismatches that are ambiguous to correct.

module type ORDTYPE =
sig
  type t
  val compare : t -> t -> int
end;;
module Icmp:ORDTYPE with type t= int=
struct
  type t = int
  let compare = fun x y -> if( x< y) then 0 else 1
end;;
module type STH=
sig
  type priority
  type 'a t
  type elt
  val comp: elt -> elt -> bool
end;;
module Emptyqueue (Comp: ORDTYPE):
  (STH with type priority= int and type elt := Comp.t) =
struct
  type priority = int
  type 'a t = Empty 
  let comp = fun x y -> Comp.compare x y > 0
end;;