Proposal: Adding Kleisli composition to Control.Monad

[Donald Bruce Stewart]

ndmitchell:
> Hi
> 
> >However, it was pointed out that repeatM is next to useless, and it is
> >traditional to use 'forever' for repeatM_ (see for example "Tackling the
> >Awkward Squad", and http://haskell.org/haskellwiki/Roll_your_own_IRC_bot
> >
> >That being said, repeatM_ isn't too bad, should people prefer it.
> 
> If I had to guess what the monadic version of repeat was, I'd guess
> repeatM_ before forever. Forever is a very overloaded word, repeat in
> Haskell has exactly one concrete meaning agreed by everyone (since its
> in the Prelude).

Agreed. But 'forever' is a pretty special control structures, worthy of
a special name. Consider the cuteness of:

    listen = forever $ do
        h <- get
        s <- io (hGetLine h)
        io (putStrLn s)
        if ping s then pong s 
                  else eval (clean s)

versus:

    listen = repeatM_ $ do
        h <- get
        s <- io (hGetLine h)
        io (putStrLn s)
        if ping s then pong s 
                  else eval (clean s)

Or, to quote the awkward squad, s2.4 "Control structures":
    
  > We can easily express an infinite loop as a combinator:
  >
  >     forever :: IO () -> IO ()
  >     forever a = a >> forever a

I'm we're going to use this forever more, then a name more meaningful
than repeatM_ might be appropriate (personally, I have to check every
time whether it is replicate or repeat that is :: Int -> a -> [a]).

-- Don