I've needed something similar in the past.
I used it in the reflection library, and its present on its own on hackage
as 'tagged'.
http://hackage.haskell.org/packages/archive/tagged/0.0/doc/html/Data-Tagged....
I talked a bit about using it here:
http://comonad.com/reader/2009/clearer-reflection/
-Edward Kmett
2010/2/17 Jonas Almström Duregård
Hi,
This literate haskell file was intended to be a quick question about a problem i have been pondering, but it developed into a short presentation instead. What i want to know is if there is already something like this (and suggestions for improvement of course).
{-#LANGUAGE GeneralizedNewtypeDeriving#-}
Sometimes i find myself needing to associate a constant with a type or, more precisely, with a type class instance. Something like this would be nice:
class Sized a where size :: Int
instance Sized Int where size = 32
Of course this will not work since there is no way of knowing which instance i refer to when i use "size". A common work-around is to use a dummy parameter:
class SizedDummy a where sizeDummy :: a -> Int
instance SizedDummy Int where sizeDummy = const 32
The size function is typically passed an undefined value. This is not very pretty, and somewhat unsafe. Another workaround is to define a newtype with a type parameter.
newtype SizeOf a = MkSize {toInt :: Int} class SizedNewType a where sizeNewType :: SizeOf a
instance SizedNewType Int where sizeNewType = MkSize 32
If we want the size of a pair to be the sum of it's components, something like this is needed:
instance (SizedNewType a, SizedNewType b) => SizedNewType (a,b) where sizeNewType = sizeNewType' sizeNewType sizeNewType where sizeNewType' :: SizeOf a -> SizeOf b -> SizeOf (a,b) sizeNewType' a b = MkSize $ toInt a + toInt b
This is way to much code say that "size = size a + size b". A more general solution can be achieved by making "Int" another type variable of "SizeOf". I call the resulting type "Related":
newtype Related a b = Related {unrelated :: b} deriving (Eq,Ord,Show,Read,Bounded,Enum,Fractional,Num, Real,Integral,RealFrac,Floating,RealFloat)
This type is highly reusable and the GeneralizedNewtypeDeriving language extension is very practical (although the instances could be written manually). It can also be used as an Identity monad:
instance Functor (Related a) where fmap f (Related a) = Related $ f a
instance Monad (Related a) where return = Related (Related a) >>= f = f a
This allows the Sized class and instances to be specified in a slim fashion using a familiar monadic interface:
class Sized a where size :: Related a Int
instance Sized Int where size = return 32
instance (Sized a, Sized b) => Sized (a,b) where size = do a <- return size :: Sized a => Related (a,b) (Related a Int) b <- return size :: Sized b => Related (a,b) (Related b Int) return $ unrelated a + unrelated b
This still requires a lot of type signatures, some additional magic is required. It is possible to write general versions of the type signatures above, which allows the following instance definition for (,,):
instance (Sized a, Sized b, Sized c) => Sized (a,b,c) where size = do a <- on3 size b <- on2 size c <- on1 size return $ a + b + c
With the derivation of Num, this can be done even more compact:
instance (Sized a, Sized b, Sized c, Sized d) => Sized (a,b,c,d) where size = on1 size + on2 size + on3 size + on4 size
The code for the onN functions:
rerelate :: Related a b -> Related c b rerelate = return . unrelated
on1 :: Related a v -> Related (x a) v on1 = rerelate
on2 :: Related a v -> Related (x a x0) v on2 = rerelate
on3 :: Related a v -> Related (x a x0 x1) v on3 = rerelate
on4 :: Related a v -> Related (x a x0 x1 x2) v on4 = rerelate
Regards, Jonas Almström Duregård _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe