Control.Monad proposal: Add whenJust

10 May 2013

      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?

Control.Monad proposal: Add whenJust

Niklas Hambüchen