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From: Christopher Zimmermann <christopher@gmerlin.de>
To: caml-list@inria.fr
Subject: Re: [Caml-list] matching GADT option types
Date: Thu, 17 Jan 2019 11:02:27 +0100	[thread overview]
Message-ID: <20190117110227.3368e34b@mortimer.gmerlin.de> (raw)
In-Reply-To: <20190117104611.3288dd5a@mortimer.gmerlin.de>

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On Thu, 17 Jan 2019 10:46:11 +0100
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



to add to the confusion, why does f type while g fails to type ?

type 'a t =
  | A : [`A] t
  | B : [`A|`B] t

let f =
  fun (type a) ~(p :a t) () -> match p with
    | A -> (A: a t)
    | B -> (B: a t)

let g =
  fun (type a) ~(p :a t option) () -> match p with
    | Some B -> (B: a t)
    | Some A -> (A: a t)
    | None -> (A (* TYPE ERROR HERE *) : a t)

Error: This expression has type [ `A ] t
       but an expression was expected of type a t
       Type [ `A ] is not compatible with type a 

-- 
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  parent reply	other threads:[~2019-01-17 10:02 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 [this message]
2019-01-17 10:18   ` Gabriel Scherer
2019-01-17 10:12 ` Gabriel Scherer
2019-01-17 10:08   ` Christopher Zimmermann

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