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
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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