On November 19, 2012 12:25:57 Tyson Whitehead wrote:
GHC 7.0.4 accepts this with FlexibleInstances and UndecidableInstances, but it seems to still have some issues (features?) as it overrides signatures
*Main> :t pure pure :: Monad f => a -> f a
unless you add other instances that are only at the lowel levels. For example, adding Maybe at the Applicative level to the above [] instance
instance Applicative Maybe where applicative_apply (Just f) (Just x) = Just (f x) applicative_apply _ _ = Nothing applicative_map f (Just x) = Just (f x) applicative_map _ _ = Nothing applicative_pure x = Just x
gives
*Main> :t pure pure :: Monad f => a -> f a *Main> :t Main.fmap Main.fmap :: Applicative f => (a -> b) -> f a -> f b
Cut and paste mistake there. That last bit should have been *Main> :t pure pure :: Applicative f => a -> f a *Main> :t Main.fmap Main.fmap :: Applicative f => (a -> b) -> f a -> f b Adding a Functor only instance will make fmap resolve correctly. Cheers! -Tyson