#8348: Add Super-classes to libraries ------------------------------------+------------------------------------- Reporter: wvv | Owner: Type: feature request | Status: new Priority: normal | Milestone: Component: libraries/base | Version: 7.6.3 Keywords: | Operating System: Unknown/Multiple Architecture: Unknown/Multiple | Type of failure: None/Unknown Difficulty: Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | ------------------------------------+------------------------------------- Haskell community has a long discussion how to implement a superclasses into Haskell. Now it is used `default` method. But it looks ugly! But all their abilities are already implemented! We need just 2 extensions: FlexibleInstances and UndecidableInstances {{{ {-# LANGUAGE FlexibleInstances #-} {-# LANGUAGE UndecidableInstances #-} instance Monad m => Applicative m where pure = return (<*>) = ap instance Monad m => Functor m where fmap = liftM instance Monad m => Bind m where (>>-) = flip (>>=) B.join = M.join }}} this code is valid! I've already defined 3 "superclassses" for Monad: Functor, Applicative and Bind! "superclass' instances" have unique quality from Programming Patterns and typeclasses ideology: do not inherit, extend! We could easily extend "superclasses" and make a lot of class' dependences. We don't need to insert inside the class some ugliness like {{{ default return :: Applicative f => a -> f a return = pure }}} Next is much prettier! {{{ instance Monoid m => Alternative m where (<|>) = mplus empty = mzero }}} We could even use it with `Generic` without any `default`: {{{ class ToJSON a where toJSON :: a -> Value instance (Generic a, GToJSON (Rep a)) => ToJSON a where toJSON = genericToJSON defaultOptions }}} So, I suggest to made true "Applicative and Functor are superclasses of Monad" and add all necessary superclass' instances to base libraries. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/8348 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler