id,summary,reporter,owner,description,type,status,priority,milestone,component,version,resolution,keywords,cc,os,architecture,failure,difficulty,testcase,blockedby,blocking,related
997,Add Kleisli composition to Control.Monad,dons,,"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 **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
+
}}}",proposal,closed,normal,,libraries/base,6.6,fixed,,,Unknown/Multiple,Unknown/Multiple,,Easy (less than 1 hour),,,,