CODEWITHSUNDEEP
typesocamlfunctor
beoliver
beoliver

Reputation: 5759

OCaml functors and type issues

module type FOOable = sig
  type 'a t
  val foo : 'a -> 'a t
end

module type FOO_FUCNTOR =
  functor (Elt : FOOable) ->
    sig
      type 'a t
      val foo_alias : 'a -> 'a t
      (* ... *)
    end

How can I refer to the type 'a t defined by FOOable as it is not possible to use Elt.t?

So in this example it would become type 'a t = 'a list

module MakeFoo : FOO_FUNCTOR =
  functor (Elt : FOOable) ->
  struct
    type 'a t = ??? (* I want the type 'a t belonging to Elt *)
    let foo_alias = Elt.foo
  end

module FooableList = struct 
  type = 'a list
  let foo x = [x]
end

module FooList = MakeFoo(FooableList)

let a = FooList.foo_alias 2

Upvotes: 1

Views: 83

Answers (1)

PatJ
PatJ

Reputation: 6144

You can simply type 'a Elt.t and you'll be sure to refer the right type.

module MakeFoo : FOO_FUNCTOR =
  functor (Elt : FOOable) ->
  struct
    type 'a t = 'a Elt.t
    let foo_alias = Elt.foo
  end

Note that as in the definition of FOO_FUNCTOR, the type equality is hidden, the link between 'a t and 'a Elt.t will not be visible outside the MakeFOO definition (but that may be what you're looking for).

Upvotes: 4

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