Matching on a constructor of a GADT may introduce a typing equation. In
your example, matching on `A` introduces the equation `a = unit`.
For this reason, the status of or-patterns (p1 | p2) containing GADT
constructors is delicate: to type-check them we have to decide which
equations from both sides are preserved in the result, computing a sort of
intersection.

Released versions of OCaml avoid this difficulty by not supporting GADTs in
or-patterns at all; you have to expand the pattern into two branches, as
you did in your function `f` above.

In the current trunk, a change from Thomas Réfis and Leo White has been
merged that allows GADTs in or-patterns, but discards the equations. Your
specific example (the function `g`) is now accepted, and will be typeable
in 4.08. Other examples, where you need to use an equation (provided by
both sides of the or-patterns) in the body of the clause, will still be
rejected.


On Thu, Jan 17, 2019 at 10:46 AM Christopher Zimmermann <
christopher@gmerlin.de> wrote:

> Hi,
>
> why does the f type correctly while g fails to type?
>
> Christopher
>
> type 'a t =
>   | A : unit t
>
> let f =
>   fun (type a) ~(p :a t option) () -> match p with
>     | Some A -> ()
>     | None -> ()
>
> let g =
>   fun (type a) ~(p :a t option) () -> match p with
>     | Some A (* TYPING ERROR HERE *)
>     | None -> ()
>
> Error: This pattern matches values of type unit t
>        but a pattern was expected which matches values of type a t
>        Type unit is not compatible with type a
>
>
> --
> http://gmerlin.de
> OpenPGP: http://gmerlin.de/christopher.pub
> CB07 DA40 B0B6 571D 35E2  0DEF 87E2 92A7 13E5 DEE1
>

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