Hi,

It is supposed to work, but it seems thepattern-matching compiler does not
detect that M0.to_string is non exhaustive, thus it does not add a
Match_Failure in case the variant is unknown. With extensible variants, a
pattern matching is only exhaustive if you provide a wildcard or a variable
for variants added afterwards.

You maybe should report it on the bug tracker!

2016-10-18 14:47 GMT+02:00 Andre Nathan <andre@digirati.com.br>:

> Hi
>
> I'm trying to use extensible variants to allow users to extend a type
> from my library, which is a GADT, but it's resulting in a segfault when
> the program is executed. I've managed to reproduce the issue with this
> code:
>
>   module M0 = struct
>     type 'a t = ..
>     type 'a t +=
>       | I : int -> int t
>       | S : string -> string t
>       | P : 'a t * 'b t -> ('a * 'b) t
>
>     let rec to_string : type a. a t -> string = function
>       | I i -> "I:" ^ string_of_int i
>       | S s -> "S:" ^ s
>       | P (x, y) -> to_string x ^ to_string y
>   end
>
>   module M1 = struct
>     include M0
>     type 'a t += T : 'a t -> 'a t
>
>     let to_string = function
>       | T t -> to_string t
>       | x -> to_string x
>   end
>
>   let () =
>     print_endline @@ M1.to_string (M1.P (M1.T (M1.I 42), M1.S "foo"))
>
>
> Is this sort of thing supposed to work?
>
> Thanks,
> Andre
>
>