[Caml-list] GADT: question about inference

[Mathieu Barbin <mathieu.barbin@gmail.com>]

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Hello,

Playing around with a reduced version of a dynamic types, I ran into some
type errors, and I feel that I'd love to understand more about the way the
inference work:

type _ ty  = Int : int ty | String : string ty

module H : sig
  type t
  val ty : t ty
end = struct
  type t = int
  let ty = Int
end

(* from now, trying various implementation for [f] *)

let f : H.t -> int =
    match H.ty with
    | Int -> (fun (a : H.t) -> (a : int))
    | _ -> assert false
  ;;
Error: This expression has type H.t but an expression was expected of type
         int

let f : H.t -> int = fun (a : H.t) ->
    match H.ty with
    | Int -> (a : int)
    | _ -> assert false
  ;;
Error: This expression has type H.t but an expression was expected of type
         int

let f : H.t -> int =
  let aux : type a. a ty -> a -> int = function
    | Int -> (fun a -> a)
    | _ -> assert false
  in
  aux H.ty
  ;;

(* val f : H.t -> int = <fun> *)

I can't quite come up with a clear mental model of why in-lining the
pattern matching in the first versions of [f]t would not type check. I
tried adding some more type coercions without great success. I'd be very
grateful to get some feed back about this restriction.

Thanks,
Mathieu

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