Also you might want abbreviation patterns. If your `value` type included
all `color` values, like this:

type value = [color | `Missing]

then, you could write:

let state_of_value v = match v with
  | #color as c -> Color (color_index c)
  | `Missing -> Missing


On Thu, Feb 9, 2017 at 5:03 AM, Martin DeMello <martindemello@gmail.com>
wrote:

> On Thu, Feb 9, 2017 at 1:57 AM, Jeremy Yallop <yallop@gmail.com> wrote:
>
>> >
>> > let state_of_value v = match v with
>> >   | `Red  | `Blue  | `Green -> ?????
>> >   | `Missing -> Missing
>>
>>   let state_of_value v = match v with
>>     | (`Red  | `Blue  | `Green) as c -> Color (color_index c)
>>     | `Missing -> Missing
>>
>
> perfect, thanks!
>
> martin
>