Control.Monad proposal: Add whenJust

[adam vogt]

Another alternative is Data.Foldable.for_, which doesn't clash like
Data.Foldable.forM_

On Fri, May 10, 2013 at 2:28 AM, Ganesh Sittampalam <ganesh at earth.li> wrote:
> -0.5: it looks useful, but it's not that much shorter than flip (maybe
> (return ())), and there's the cost of library clutter.
>
> On 10/05/2013 07:13, Niklas Hambüchen wrote:
>> I would like to propose the addition of
>>
>> whenJust :: Monad m => Maybe a -> (a -> m ()) -> m ()
>> whenJust (Just x) f = f x
>> whenJust _        _ = return ()
>>
>> to Control.Monad, in the section
>>
>>    "Conditional execution of monadic expressions"
>>
>> next to
>>
>>    guard :: MonadPlus m => Bool -> m ()
>>    when :: Monad m => Bool -> m () -> m ()
>>    unless :: Monad m => Bool -> m () -> m ()
>>
>>
>> Why?
>>
>> It would allow us to write more readable code and fit well into the
>> group of similar functions of this style.
>>
>> Compare
>>
>>    mUser <- lookupUser
>>
>>    whenJust mUser email
>>
>> or
>>
>>    whenJust mUser $ \user -> do
>>       putStrLn "Mailing!"
>>       email user
>>
>> with some currently available alternatives:
>>
>>
>>    case mUser of
>>       Just user -> do putStrLn "Mailing!"
>>                       email user
>>       Nothing   -> return ()
>>
>> (Default base case clutter.)
>>
>>
>>    import Data.Foldable
>>
>>    forM_ mUser $ \user -> do
>>      putStrLn "Mailing!"
>>      email user
>>
>> (Not too intuitive/well-named here and "Ambiguous occurrence forM_"
>> clash with Control.Monad.)
>>
>> Some more dissatisfying alternatives:
>>
>>
>>    maybe (return ()) (\user -> do putStrLn "Mailing!"
>>                                   email user
>>                      ) mUser
>>
>>
>>    flip (maybe (return ())) mUser $ \user -> do
>>      putStrLn "Mailing!"
>>      email user
>>
>>
>>    import Control.Monad.Trans.Maybe
>>    import Control.Monad.Trans (lift)
>>
>>    _ <- runMaybeT $ return mUser >>= \user -> lift $ do
>>      putStrLn "Mailing!"
>>      email user
>>    return ()
>>
>>
>> Alternative names:
>>
>>    - withJust, analog to withFile and withForeignPtr
>>
>> Any comments?
>>
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>
>
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