Proposal: add ifM and whenM to Control.Monad

[Edward Kmett]

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> 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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