I think we'd rather like to avoid UndecidableInstances in the prelude!
This does bring up an interesting idea, though, as far as using a flat
hierarchy for these classes. Default method signatures could be used to
provide similar behavior.
http://www.haskell.org/ghc/docs/7.4.2/html/users_guide/type-class-extensions...
I'm not saying that this is a good idea (I think that a good idea would be
closer to my https://github.com/mgsloan/instance-templates proposal - which
I seriously need to rework to be more appealing), but I wonder if this
would work:
class Applicative f where
applicative_map :: (a -> b) -> f a -> f b
default applicative_map :: Monad f => (a -> b) -> f a -> f b
applicative_map = monad_map
-- ...
class Monad m where
monad_map :: (a -> b) -> m a -> m b
default monad_map :: Applicative m => (a -> b) -> m a -> m b
monad_map = applicative_map
-- ...
I haven't seen mutually recursive defaultings like this (not sure if even
works) - but could be a nifty trick for some applications. The defaults in
Applicative are more important, so these could be removed in favor of using
the usual standard defaults in Monad.
-Michael
On Mon, Nov 19, 2012 at 11:00 AM, Tyson Whitehead
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
_______________________________________________ Libraries mailing list Libraries@haskell.org http://www.haskell.org/mailman/listinfo/libraries