-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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