[GHC] #997: Add Kleisli composition to Control.Monad

[GHC]

#997: Add Kleisli composition to Control.Monad
-------------------------------+--------------------------------------------
    Reporter:  dons            |       Owner:         
        Type:  proposal        |      Status:  new    
    Priority:  normal          |   Milestone:         
   Component:  libraries/base  |     Version:  6.6    
    Severity:  normal          |    Keywords:         
  Difficulty:  Easy (1 hr)     |    Testcase:         
Architecture:  Multiple        |          Os:  Unknown
-------------------------------+--------------------------------------------
Kleisli composition of monads is a foundational feature missing from the
 current Control.Monad library. A
 [http://thread.gmane.org/gmane.comp.lang.haskell.libraries/5412/focus=5412
 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 ()
 }}}
 [http://www.cse.unsw.edu.au/~dons/data/cc.prelude 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.

 The patch in its entirety:
 {{{
 New patches:

 [Add Kleisli composition
 Don Stewart <dons at cse.unsw.edu.au>**20061113015442] {
 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
 +
 hunk ./Control/Monad.hs 310
 +
 }}}

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/997>
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