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From: Gabriel Scherer <gabriel.scherer@gmail.com>
To: Christopher Zimmermann <christopher@gmerlin.de>
Cc: caml users <caml-list@inria.fr>
Subject: Re: [Caml-list] matching GADT option types
Date: Thu, 17 Jan 2019 11:12:02 +0100	[thread overview]
Message-ID: <CAPFanBEL9Wdb1aYyOBq=VJWcFyZBuRJiFrkkJtX7qPMHWrpdXg@mail.gmail.com> (raw)
In-Reply-To: <20190117104611.3288dd5a@mortimer.gmerlin.de>

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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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  parent reply	other threads:[~2019-01-17 10:01 UTC|newest]

Thread overview: 6+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-01-17  9:46 Christopher Zimmermann
2019-01-17  9:58 ` Jeremy Yallop
2019-01-17 10:02 ` Christopher Zimmermann
2019-01-17 10:18   ` Gabriel Scherer
2019-01-17 10:12 ` Gabriel Scherer [this message]
2019-01-17 10:08   ` Christopher Zimmermann

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