Thanks all! I was trying to express the fact that heading could only be `Up
or `Down but should act as a direction otherwise; Romain is right that I
was expecting the coercion to happen automatically (too much time in other
languages :)). Annotating

val mutable heading : [`Up | `Down] = `Up

and then coercing it when I pass it as a direction worked nicely.

martin

On Thu, Sep 10, 2015 at 11:03 AM, Martin Jambon <martin.jambon@ens-lyon.org>
wrote:

> I'd like to add that this a good systematic way to avoid confusing error
> messages when dealing with polymorphic variants (or objects). Just add type
> annotations on variables and function parameters which are polymorphic
> variants (or objects).
>
> Here only one extra annotation would be needed, on the heading value:
>
>   val mutable heading : dir = `Up
>   method flip_heading = ...
>   method get_direction (x : dir) = ...
>   method get_heading = ...
>
> The error message becomes closer to the actual error:
>
> File "toto.ml", line 15, characters 15-61:
> Warning 8: this pattern-matching is not exhaustive.
> Here is an example of a value that is not matched:
> (`West|`South|`North|`East)
>
> Martin
>
> On 09/10/2015 02:13 AM, Edouard Evangelisti wrote:
>
>> Dear Martin,
>>
>> In your example, heading is not of type dir.
>> However, this would work :
>>
>> val mutable heading : dir = `Up
>>
>> Regards,
>> Edouard
>>
>>
>> 2015-09-10 10:07 GMT+01:00 Martin DeMello <martindemello@gmail.com
>> <mailto:martindemello@gmail.com>>:
>>
>>     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
>>
>>
>>