Control.Monad proposal: Add whenJust

[Andreas Abel]

+1

I use whenJust quite frequently and it is much more readable than for_ 
(wrong connotation) or

   flip (maybe $ return ())

Cheers,
Andreas

On 10.05.13 8:13 AM, 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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Theoretical Computer Science, University of Munich
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