On Monday 23 August 2010 22:30:03, Oscar Finnsson wrote:
Hi,
I'm wondering why I have to repeat the class constraints at every function.
If I got the data type
data (Eq a, Show a, Data a, Eq b, Show b, Data b, Eq c, Show c, Data c) => Foo a b c = Foo a b c
Type class constraints on datatypes are considered a wart. They don't do what people expect, in particular they don't make the constraints available at the use site. It works if you move the constraints across the '=': {-# LANGUAGE ExistentialQuantification #-} data Foo a b c = (Eq a, Show a, ...) => Foo a b c or with GADT syntax: {-# LANGUAGE GADTs #-} data Foo x y z where Foo :: (Eq a, Show a, ...) => a -> b -> c -> Foo a b c Both make the constraints available at the use site, bar :: Foo a b c -> String bar (Foo a b c) = "Foo " ++ show a ++ " " ++ show b ++ " " ++ show c ++ ", Yay!"
and then a function from Foo to String I have to supply the signature
bar :: (Eq a, Show a, Data a, Eq b, Show b, Data b, Eq c, Show c, Data c) => Foo a b c -> String
even though it should be clear that a, b and c *must* fulfill the constraints already so I should be able to just supply the signature
One would think so. It's a wart.
bar :: Foo a b c -> String