http://hackage.haskell.org/trac/ghc/ticket/997 Add Kleisli composition to Control.Monad. Kleisli composition of monads is a foundational feature missing from the current Control.Monad library. A recent discussion revealed solid support for its inclusion. This patch adds: (>=>) :: (Monad m) => (a -> m b) -> (b -> m c) -> (a -> m c) (<=<) :: (Monad m) => (b -> m c) -> (a -> m b) -> (a -> m c) Along with the useful control combinator: forever :: (Monad m) => m a -> m () Traditionally, >=> has been written as @@, however to support the flipped version, new notation seems to be required. It should be notated that there is overlap with the Kleisli class in Control.Arrow (specifically >>>), however, short of a convenient unifying form for Arrow and Monad, a monad-specific >>> seems reasonable. To mirror >>> and =<<, infixr 1 was chosen. Proposal period: 2 weeks. Deadline: 27th November. -- Don ------------------------------------------------------------------------ hunk ./Control/Monad.hs 40 + , (>=>) -- :: (Monad m) => (a -> m b) -> (b -> m c) -> (a -> m c) + , (<=<) -- :: (Monad m) => (b -> m c) -> (a -> m b) -> (a -> m c) + , forever -- :: (Monad m) => m a -> m () hunk ./Control/Monad.hs 179 +infixr 1 <=<, >=> + +-- | Left-to-right Kleisli composition of monads. +(>=>) :: Monad m => (a -> m b) -> (b -> m c) -> (a -> m c) +f >=> g = \x -> f x >>= g + +-- | Right-to-left Kleisli composition of monads. '(>=>)', with the arguments flipped +(<=<) :: Monad m => (b -> m c) -> (a -> m b) -> (a -> m c) +(<=<) = flip (>=>) + +-- | @'forever' act@ repeats the action infinitely. +forever :: (Monad m) => m a -> m () +forever a = a >> forever a ------------------------------------------------------------------------