This is fairly the same as http://hackage.haskell.org/trac/ghc/ticket/8021 but it's rewritten a little Interfaces Interface is a generalization of the class. Now class become just *instances* of interface For backward compatibility compiler should calculate interface by itself, but it should allow to write interfaces to everybody (more detail at "Maybe Problems"). /-- it is calculated by the Compiler or is written directly/ class interface Monoid (a :: k) where /-- like Typeable/ mempty :: * mappend :: * -> * -> * class Monoid a where ... class Monad m => Monoid m where ... class Arrow c => Monoid c where ... It is easy to use them with functions: foo :: (Monad m => Monoid m , Monad m ) => m a bar :: (Arrow c => Monoid c, Arrow c) => c b a These classes are open, you could lately add class MyClass c => Monoid c where ... We use like Typeable (a :: k) because: () => Monoid a :: * Monad m => Monoid m :: * -> * Arrow c => Monoid c :: * -> * -> * Backward compatibility If class have single constraint, you could write without it. If class have many constraints, you could write without it, if it is empty one. bar :: MonadPlus m => m a <<==>> bar :: () => Monad m => MonadPlus m => m a foo :: Monoid a => a <<==>> foo :: () => Monoid a => a baz :: Monad m => Monoid m => m a <<==>> baz :: () => Monad m => Monoid m => m a Better than superclasses we already have class Num a where ... But we wish to generalize it. It's easy now, we just add: class Additive a where addplus = ... class Multiplicative a where multprod = ... class (Additive a, Multiplicative a) => Num a where (+) = addplus (*) = multprod foo :: (Additive a, Multiplicative a) => Num a => a -> a -> a Maybe Problems 1) If we have already class class Foo a where data F :: ... type S :: ... data family G ... what interface do we have ? Possible reply - no data in the interface. 2) What is the interface of Typeable ? 2.1) What is the interface of class with unwritten interface? Possible reply - the same as class, so it is easy to Compiler. 3) Misfeature - to allow write interface manually Why? You can't change it later. So, it must be deprecated from the beginning. -- View this message in context: http://haskell.1045720.n5.nabble.com/Interfaces-the-Golden-Path-of-Haskell-t... Sent from the Haskell - Haskell-prime mailing list archive at Nabble.com.