Control.Monad proposal: Add whenJust

[Evan Laforge]

I like it, I have a local whenJust I use very frequently.

I know about forM_, but I don't use it because it sounds too much like a loop.

But I recall we already had this discussion and it failed to catch on
then, so unless something has changed it might not be worth bringing
it up again.

On Fri, May 10, 2013 at 1:13 PM, Niklas Hambüchen <mail at nh2.me> 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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