Re: Libraries Digest, Vol 117, Issue 10

I for one do not view 'monomorphic too' as a feature. Sent from my iPad On May 10, 2013, at 4:36 PM, kudah wrote:

This is nothing but obfuscation, whenJust is clearly more readable than for_. whenJust is monomorphic too.

On Fri, 10 May 2013 11:44:42 -0700 Gabriel Gonzalez wrote:

...

How about I just write a blog post teaching people how to use `for_` (and more generally, how to use `Maybe`'s `Foldable` instance)? I know Oliver Charles wrote a similar post in his 24 days of Hackage, and maybe I could build on that a bit more and perhaps make it as Google-able as possible so it comes up as the top result when people search for keywords like `whenJust` and other `Maybe` idioms.

On Fri, May 10, 2013 at 11:25 AM, wrote:

...

Send Libraries mailing list submissions to libraries@haskell.org

To subscribe or unsubscribe via the World Wide Web, visit http://www.haskell.org/mailman/listinfo/libraries or, via email, send a message with subject or body 'help' to libraries-request@haskell.org

You can reach the person managing the list at libraries-owner@haskell.org

When replying, please edit your Subject line so it is more specific than "Re: Contents of Libraries digest..."

Today's Topics:

1. Re: Control.Monad proposal: Add whenJust (Edward Kmett) 2. Re: Control.Monad proposal: Add whenJust (Evan Laforge) 3. Re: Control.Monad proposal: Add whenJust (Simon Hengel) 4. Re: Control.Monad proposal: Add whenJust (Andreas Abel) 5. Re: Control.Monad proposal: Add whenJust (Ivan Lazar Miljenovic) 6. Re: Control.Monad proposal: Add whenJust (Ganesh Sittampalam) 7. Re: Control.Monad proposal: Add whenJust (Petr Pudl?k)

----------------------------------------------------------------------

Message: 1 Date: Fri, 10 May 2013 07:16:53 -0400 From: Edward Kmett Subject: Re: Control.Monad proposal: Add whenJust To: Niklas Hamb?chen Cc: Haskell Libraries Message-ID: < CAJumaK8XJrtdrXQfVb3pdi193ghz9ZEX8Q-MnVd435tDt5YFbg@mail.gmail.com> Content-Type: text/plain; charset="utf-8"

I'm -1 on this, due to it just further obfuscating the fact that Data.Foldable.for_ already exists.

On Fri, May 10, 2013 at 2:13 AM, Niklas Hamb?chen 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?

_______________________________________________ Libraries mailing list Libraries@haskell.org http://www.haskell.org/mailman/listinfo/libraries