Hi Herbert, in general I like pattern matching but not when boolean values are involved. Your code is nice but, in my opinion, still far from the elegance of f = unlessM (doesDirectoryExist path) $ do putStrLn $ "Creating directory " ++ path createDirectory path In particular, note that I don't have to take care of the False case and the code doesn't have boilerplate. While your solution is more general, I would like to point out that when and unless are so useful that they got their own functions in the library instead of relying on pattern matching. I consider ifM, whenM and unlessM as alternate versions of existing functions. On 04/20/2014 09:59 PM, Herbert Valerio Riedel wrote:
Hi Mario,
On 2014-04-20 at 21:10:03 +0200, Mario Pastorelli 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 () [...]
f = do dirDoesntExist <- not <$> doesDirectoryExist path when dirDoesntExist $ do putStrLn $ "Creating directory " ++ path createDirectory path While I'm neutral on this proposal, I'd like to remind that LambdaCase may be useful to avoid temporary variables as well (and is even more useful for types other than Bool):
f = doesDirectoryExist path >>= \case True -> return () False -> do putStrLn $ "Creating directory " ++ path createDirectory path
Cheers, hvr