Type aliasing

[danabr]

Consider the following example:

module shape

type radius = int

type shape = Circle radius

make_circle r = Circle r

test_circle () = make_circle 1

This unexpectedly fails to typecheck with the error:

exception error: no match of right hand side value 
                 {error,{cannot_unify,shape,9,{adt,"radius",[],[]},t_int}}

That is, radius is considered a distinct type from int.

The same module in OCaml compiles just fine:

type radius = int

type shape = Circle of radius

let make_circle r = Circle r

let test_circle () = make_circle 1

I think it makes sense for MLFE to behave the same way.

If I would like to make the radius type abstract (or opaque in dialyzer terms), I would hide it in a module (example in OCaml):

module Radius : sig
  type radius

  val make_radius: int -> radius
end =
struct
  type radius = int

  let make_radius i = i
end

type shape = Circle of Radius.radius

let make_circle r = Circle r

let test_circle () = make_circle (Radius.make_radius 1)

To achieve the same in MLFE, types should be module local by default, and you would have to export them via a export_type directive or similar. We would also need to be able to mark types as abstract. In Erlang/Dialyzer this is achieved by using the -opaque directive.