Re: [Haskell-cafe] interesting type families problem

8 Sep 2010


      2010/9/8 Anthony Cowley :
...
2010/9/8 Gábor Lehel :
...
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))


-- 
Work is punishment for failing to procrastinate effectively.