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

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

Ok last message I promise.

I don't know exactly what you want but you may prefer the following
FOO (see the signature of kind_to_kind)  :

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 -> 'a
end

In that case the idea is that each implementer declares kind as :

type 'a kind = 'a constraint 'a = [< Kind.t]

but controls which 'a it will actually allow via the kind_of_kind
function (better names to be found)  :

module FooAB : 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 | `B] as 'a) -> 'a kind_t
      val kind_to_kind : 'a kind_t -> 'a
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
end

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

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

let (k : [`A |`B]) = FooAB.kind_to_kind (FooAB.kind_of_kind `A)

(* Should not typecheck
 let v = FooAB.make 2 (FooAB.make_custom 2.) (FooAB.kind_of_kind `C) *)



module FOOCheck = (FooAB : FOO)