Hi,
I can't figure out why the following program compiles with this
instance declaration
instance Size (Maybe [a]) => Size [a] where size x = size (foo x)
but has GHC loop forever with this one:
instance (Foo a b, Size b) => Size a where size x = size (foo x)
Anybody here know?
Thanks,
- Benja
{-# OPTIONS_GHC -fglasgow-exts -fallow-overlapping-instances
-fallow-undecidable-instances #-}
class Foo a b | a -> b where foo :: a -> b
instance Foo [a] (Maybe [a]) where foo (x:xs) = Just xs; foo [] = Nothing
class Size a where size :: a -> Int
instance Size a => Size (Maybe a) where
size (Just x) = 1 + size x; size Nothing = 0
--instance (Foo a b, Size b) => Size a where size x = size (foo x)
-- OR
--instance Size (Maybe [a]) => Size [a] where size x = size (foo x)
main = print $ size "foo"
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