Re: [Caml-list] Constraining abstract type to be of a given subtype

["Daniel Bünzli" <daniel.buenzli@erratique.ch>]

> In fact it does compile but I don't think it does what you want
> because the 'a is completly unrelated to the `A in Foo1.kind_t

Oh well it's always the same error : when you are doing phantom types,
the type with phantoms should be abstract. See below.

Daniel

module Kind:
sig
      type t = [ `A | `B | `C ]

      val to_string: t -> string
end =
struct
      type t = [ `A | `B | `C ]

      let to_string = function
              | `A -> "A"
              | `B -> "B"
              | `C -> "C"
end

module type FOO =
sig
      type custom_t
      type 'a kind_t constraint 'a = [< Kind.t]
      type 'a t = int * custom_t * 'a kind_t

      val make: int -> custom_t -> 'a kind_t -> 'a t
      val string_of_custom: custom_t -> string
      val string_of_kind: 'a kind_t -> string
      val kind_to_kind : 'a kind_t -> Kind.t
end


module Foo1 : sig
      type custom_t
      type 'a kind_t constraint 'a = [< Kind.t]
      type 'a t = int * custom_t * 'a kind_t

      val make_custom : float -> custom_t
      val make: int -> custom_t -> 'a kind_t -> 'a t
      val string_of_custom: custom_t -> string
      val string_of_kind: 'a kind_t -> string
      val kind_of_kind : ([ `A ] as 'a) -> 'a kind_t
      val kind_to_kind : 'a kind_t -> Kind.t
end = struct
      type custom_t = float
      type 'a kind_t = [`A] constraint 'a = [< Kind.t]
      type 'a t = int * custom_t * 'a kind_t

      let make_custom f = f
      let make id custom kind = (id, custom, kind)
      let string_of_custom = string_of_float
      let string_of_kind k = Kind.to_string (k :> Kind.t)
      let kind_of_kind k = k
      let kind_to_kind k = (k :> Kind.t)
end

let (v : [`A] Foo1.t) =
  Foo1.make 2 (Foo1.make_custom 2.) (Foo1.kind_of_kind `A)

(* Should not typecheck
let (v : [`B] Foo1.t) =
  Foo1.make 2 (Foo1.make_custom 2.) (Foo1.kind_of_kind `A)
*)

module FOOCheck = (Foo1 : FOO)