Sorry, I meant to send this to the whole list. Brock Peabody wrote:
Please excuse my newbiness, but in this snippet:
data (Monad m) => DataType m = DataType { f :: Char -> m () }
test_function :: (Monad m) => DataType m -> m () ^^^^^^^^^^^^ test_function d = f d 'C'
Why is "(Monad m) =>" required, when the definition of DataType already implies it? Is there an easier way to do this or will I have to have it in all signatures containing DataType?
Because class constraints on data types are a bit silly. It just restricts the types of the constructors so you can only apply them if m is a Monad. It doesn't actually package up the evidance that m is a monad inside the value to make this stuff work. Existential types do package up the class instance in the value, but they hide the type. data Showable = forall a . (Show a) => Showable a showShowable :: Showable -> String showShowable (Showable x) = show x Getting them both is tricky, but you can do it if you use a GADT to write a type that means "exists a such that a = m and a is a Monad": {-# OPTIONS -fglasgow-exts #-} data TyEq (a :: * -> *) (b :: * -> *) where Refl :: TyEq a a data DataType m = forall m' . (Monad m') => DataType (TyEq m m') (Char -> m' ()) buildDataType :: (Monad m) => (Char -> m ()) -> DataType m buildDataType = DataType Refl test_function :: DataType m -> m () test_function (DataType Refl f) = f 'C' -- try let x = buildDataType putChar -- :t x -- :t test_function -- test_function x Brandon
Thanks,
Brock
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