Proposal: add ifM and whenM to Control.Monad

[Mario Pastorelli]

I see. Another solution is to use if', when' and unless'.

On 04/20/2014 11:42 PM, Edward Kmett wrote:
> My mistake. These rules are still in Control.Monad. I just scrolled 
> right past them.
>
> -Edward
>
>
> On Sun, Apr 20, 2014 at 5:04 PM, Edward Kmett <ekmett at gmail.com 
> <mailto:ekmett at gmail.com>> wrote:
>
>     The principled objection to giving these combinators the "obvious"
>     names in Control.Monad is that that module has historically held
>     to a detailed convention that these proposed names unfortunately
>     don't fit. =/
>
>         The functions in this library use the following naming
>         conventions:
>
>           * A postfix 'M' always stands for a function in the Kleisli
>             category: The monad type constructor m is added to
>             function results (modulo currying) and nowhere else. So,
>             for example,
>
>         filter :: (a -> Bool) -> [a] -> [a]
>         filterM :: (Monad m) => (a -> m Bool) -> [a] -> m [a]
>
>           * A postfix '_' changes the result type from (m a) to (m
>             ()). Thus, for example:
>
>         sequence :: Monad m => [m a] -> m [a]
>         sequence_ :: Monad m => [m a] -> m ()
>
>           * A prefix 'm' generalizes an existing function to a monadic
>             form. Thus, for example:
>
>         sum :: Num a => [a] -> a
>         msum :: MonadPlus m => [m a] -> m a
>
>
>     That said, if we do adopt them, they probably should get the ifM,
>     whenM, unlessM names.
>
>     I don't think the convention has been documented in Control.Monad
>     itself for years.
>
>     -Edward
>
>
>
>     On Sun, Apr 20, 2014 at 4:26 PM, Mario Pastorelli
>     <pastorelli.mario at gmail.com <mailto:pastorelli.mario at gmail.com>>
>     wrote:
>
>         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
>
>
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>

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