These come up every few months. Historically, I've always been -1 on their inclusion as they are simple compositions of more fundamental operations. However, given that they keep getting reinvented with the exact same names and functionality. I'm finally ready to give in. +1 from me. -Edward On Sun, Apr 20, 2014 at 3:10 PM, Mario Pastorelli < pastorelli.mario@gmail.com> wrote:
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. _______________________________________________ Libraries mailing list Libraries@haskell.org http://www.haskell.org/mailman/listinfo/libraries