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