On Wed, Jan 5, 2011 at 11:33 AM, Simon Marlow
Furthermore, we have some significant compatibility issues with Haskell 98/2010 code. I wouldn't be in favour of doing this unless we can retain Haskell 98/2010 compatibility somehow (e.g. with superclass defaults or class aliases).
It would indeed be really nice to have something like this: {-# LANGUAGE DefaultInstances #-} class Functor f where fmap :: (a → b) → f a → f b class Functor f ⇒ Applicative f where pure :: a → f a (<*>) :: f (a → b) → f a → f b instance Functor f where fmap f m = pure f <*> m class Applicative m ⇒ Monad m where return :: a → m a (>>=) :: m a → (a → m b) → m b instance Applicative m where pure = return mf <*> mx = do f ← mf x ← mx return (f x) -- The following is nicer -- but may cause a circular definition: mf <*> mx = do f ← mf fmap f mx Now to make a type (for example Maybe) an instance of Monad the only thing to do is to declare: instance Monad Maybe where return = Just Nothing >>= _ = Nothing Just x >>= f = f x And you will get the Applicative and Functor instances for free. The Eq and Ord classes also benefit from this language extension: class Eq a where (==) :: a → a → Bool class Eq a ⇒ Ord a where compare :: a → a → Ordering instance Eq a where x == y = compare x y == Eq Just like default methods, default instances can be overwritten by a user defined instance. There's the question whether a default instance should be required to be a super class of the class that defines the default instance. For example, should the following be allowed: class Foo a where instance Bar a class Bar a I can't see a use of this yet, but I also can't see a reason why it shouldn't be allowed. Now only someone has to implement it :-) Regards, Bas