Re: [Caml-list] constructor disambiguation for gadts

[Goswin von Brederlow <goswin-v-b@web.de>]

On Tue, Oct 07, 2014 at 12:14:01PM -0400, Milan Stanojevi?? wrote:
> I was wondering what are technical difficulties that prevent
> constructor disambiguation for GADTs?
> 
> I got an interesting error today, where it is clear that compiler
> knows the type but because it is GADT it is still asking me to qualify
> the constructor.
> 
> Here is the simple example.
> module A : sig
>   type _ t =
>     | I : int t
>     | S : string t
> end = struct
>   type _ t =
>     | I : int t
>     | S : string t
> end
> 
> let add : type a . a A.t -> a -> a -> a =
>   fun w a1 a2 ->
>     match w with
>     | I -> a1 + a2
>     | S -> String.concat "" [a1; a2]
> ;;
> 
> I get File "foo.ml", line 14, characters 6-7:
> Error: The GADT constructor I of type A.t must be qualified in this pattern.
> 
> I'm not sure what extra information I'm giving to the type checker by
> qualifying the constructor.

Shouldn't that be A.I and A.S? You didn't open module A.

MfG
	Goswin