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

["Christoph Höger" <christoph.hoeger@tu-berlin.de>]

You explicitly ask for an argument of type dir, but you want to ask for
an argument of any subtype of dir:

method get_direction (x : [< dir])

regards,

Christoph

Am 10.09.2015 um 11:07 schrieb Martin DeMello:
> 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 <http://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
> 


-- 
Christoph Höger

Technische Universität Berlin
Fakultät IV - Elektrotechnik und Informatik
Übersetzerbau und Programmiersprachen

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E-Mail: christoph.hoeger@tu-berlin.de