Proposal: add ifM and whenM to Control.Monad

[Mario Pastorelli]

I would like to propose the addition of two new combinators to 
Control.Monad:

ifM :: (Monad m) => m Bool -> m a -> m a -> m a
whenM :: (Monad m) => m Bool -> m () -> m ()

The reason is that when you work in a `Monad m` the first argument of 
`if` and `when` is often a `m Bool` and not a `Bool`. In those cases, 
you have to write:

monadicOperationReturningBool >>= \b -> when b doAnotherMonadicOperation

or

monadicOperationReturningBool >>= flip when doAnotherMonadicOperation

to accomplish what you want to do. If you use the do-notation this is 
less terrible but you still need to assign a name to the boolean value. 
Take for example:

f = do
     dirDoesntExist <- not <$> doesDirectoryExist path
     when dirDoesntExist $ do
       putStrLn $ "Creating directory " ++ path
       createDirectory path

in this snippet, dirDoesntExist is completely useless and its only 
purpose it to be used in the next expression. With the new combinators 
you could write:

f = whenM (not <$> doesDirectoryExists path) $ do
         putStrLn $ "Creating directory " ++ path
         createDirectory path

Many libraries on hackage already include these functions (I often use 
Control.Conditional). For a list see 
http://holumbus.fh-wedel.de/hayoo/hayoo.html?query=whenM&start=0.