Re: Libraries Digest, Vol 117, Issue 10

Perhaps we should have a dummy module in Hackage that includes commonly searched for functions, but instead of actually implementing them the documentation would say don't use this and suggest an alternative and explains why. Then you could find them in Hoogle and Hackage seems to be highly ranked on Google so they'd be found there as well. So a programmer would Google or Hoogle whenJust, find what they think is a module, take a look at the documentation, and find the alternative suggestion. Chris On 10 May 2013 11:44, 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