[Haskell-cafe] interesting type families problem

[Gábor Lehel]

2010/9/8 Anthony Cowley <acowley at seas.upenn.edu>:
> 2010/9/8 Gábor Lehel <illissius at gmail.com>:
>> Oh. Hmm. That makes sense. So I gather there's absolutely no way to
>> specify which instance you mean, and hence to use `value` as any
>> concrete type?
>
> Here's one way to indicate which value you are referring to.
>
> Anthony
>
> {-# LANGUAGE EmptyDataDecls, TypeFamilies #-}
> data True
> data False
>
> class TypeValue a where
>    type ValueTypeOf a
>    value :: a -> ValueTypeOf a
>
> instance TypeValue True where
>    type ValueTypeOf True = Bool
>    value _ = True
>
> instance TypeValue False where
>    type ValueTypeOf False = Bool
>    value _ = False
>
> main = print (value (undefined::True))
>

Right. You can also use Tagged :) but I meant specifically with the
formulation I presented originally.

{-# LANGUAGE EmptyDataDecls, TypeFamilies #-}

import Data.Tagged
import Control.Applicative

data True  :: *
data False :: *

class TypeValue a where
    type ValueTypeOf a :: *
    value :: Tagged a (ValueTypeOf a)

instance TypeValue True where
    type ValueTypeOf True = Bool
    value = Tagged True

instance TypeValue False where
    type ValueTypeOf False = Bool
    value = Tagged False

main = untag $ print <$> (value :: Tagged True (ValueTypeOf True))


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