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

[Jacques Garrigue <garrigue@math.nagoya-u.ac.jp>]

On 2015/09/10 18:07, Martin DeMello 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?


The problem is that OCaml type inference uses unification rather than subtyping.
So [`Down | `Up] and dir are incompatible.

All the answers up to now seem to have assumed that get_direction should only accept [`Up | `Down].
This indeed solves the problem, but may not be what you intended.
If you still want get_direction to acception all directions, then you can either modify get_direction or get_heading
(but no need to change both):

   method get_direction : 'a. ([< dir] as 'a) -> int = index

with this polymorphic method annotation, you can now apply get_direction to heading

  method get_heading = self#get_direction (heading :> dir)

this one uses subtyping to convert heading to dir.

Jacques Garrigue