Control.Monad proposal: Add whenJust

[Ganesh Sittampalam]

For what it's worth, F# has Option.iter, analogous to List.iter,
Array.iter etc: http://msdn.microsoft.com/en-GB/library/ee340387.aspx

I did find it a bit funny initially but it's grown on me.

Ganesh

On 10/05/2013 15:02, Andreas Abel wrote:
> +1
> 
> I use whenJust quite frequently and it is much more readable than for_
> (wrong connotation) or
> 
>   flip (maybe $ return ())
> 
> Cheers,
> Andreas
> 
> On 10.05.13 8:13 AM, 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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>