Re: [Caml-list] using a restricted subset of a polymorphic variant

[Gabriel Scherer <gabriel.scherer@gmail.com>]

This works if you remove the restrictive annotation (x : dir)
in the get_direction method, or replace it by the more flexible
(x : [< dir]). The type [ `Down | `Up ] of heading is distinct
from dir, but an instance of [< dir ].

On Thu, Sep 10, 2015 at 11:07 AM, Martin DeMello
<martindemello@gmail.com> wrote:
> I'm not sure exactly why this doesn't work:
>
> type dir = [`North | `South | `East | `West | `Up | `Down]
>
> let index = function
>   | `North -> 1
>   | `South -> 2
>   | `East -> 3
>   | `West -> 4
>   | `Up -> 5
>   | `Down -> 6
>
> class foo = object(self)
>   val mutable heading = `Up
>
>   method flip_heading =
>     heading <- match heading with `Up -> `Down | `Down -> `Up
>
>   method get_direction (x : dir) = index x
>
>   method get_heading = self#get_direction heading
> end
>
> it fails with
> File "test.ml", line 28, characters 42-49:
> Error: This expression has type [ `Down | `Up ]
>        but an expression was expected of type dir
>        The first variant type does not allow tag(s)
>        `East, `North, `South, `West
>
> but why is that an error? I'd think that any function that accepts type dir
> should accept type [`Down | `Up] as well. Also, how do I get this to work?
>
> martin
>