This is a common problem that has annoyed me as well. The issue is that B
sees foo and bar as abstract types, so cannot be sure they are different,
so cannot be sure that a value of type foo gadt can't be constructed with
Bar. If you add explicit constructors for both currently-empty types, then
their inequality will be exposed and your pattern match should work.

On 27 May 2015 at 16:04, Joris Giovannangeli <joris@giovannangeli.fr> wrote:

> Hi,
>
> The following snippet is compiling without warning :
>
> module A = struct
>
>   type foo
>   type bar
>
>   type 'a gadt =
>       Foo : int -> foo gadt
>     | Bar : int -> bar gadt
>
>   let f = function
>      | Foo i -> i
> end
>
> But if I split the code into two modules :
>
> module A = struct
>
>   type foo
>   type bar
>
>   type 'a gadt =
>       Foo : int -> foo gadt
>     | Bar : int -> bar gadt
>
> end
>
> module B = struct
>   include A
>
>   let f : foo gadt -> int = function
>      | Foo i -> i
> end
>
> I get the following warning :
>
> Warning 8: this pattern-matching is not exhaustive.
> Here is an example of a value that is not matched:
> Bar _
>
> How can i work around this issue ? As far as i can tell, it is not
> possible for Bar to be matched by the function f.
>
> Best regards,
> joris
>
>
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