Control.Monad proposal: Add whenJust

[Ganesh Sittampalam]

-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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