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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