Libraries Digest, Vol 117, Issue 10
Gabriel Gonzalez
gabriel439 at gmail.com
Fri May 10 20:44:42 CEST 2013
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, <libraries-request at haskell.org> wrote:
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> 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 <ekmett at gmail.com>
> Subject: Re: Control.Monad proposal: Add whenJust
> To: Niklas Hamb?chen <mail at nh2.me>
> Cc: Haskell Libraries <libraries at haskell.org>
> Message-ID:
> <
> CAJumaK8XJrtdrXQfVb3pdi193ghz9ZEX8Q-MnVd435tDt5YFbg at 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 <mail at nh2.me> 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 at haskell.org
> > http://www.haskell.org/mailman/listinfo/libraries
> >
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>
> ------------------------------
>
> Message: 2
> Date: Fri, 10 May 2013 18:30:14 +0700
> From: Evan Laforge <qdunkan at gmail.com>
> Subject: Re: Control.Monad proposal: Add whenJust
> To: Niklas Hamb?chen <mail at nh2.me>
> Cc: "libraries at haskell.org" <libraries at haskell.org>
> Message-ID:
> <
> CACbaDy5oCQ-xV4-c-gDNzc5L1+XzHaUAqNomCb3ZfAouwyqH3Q at mail.gmail.com>
> Content-Type: text/plain; charset=UTF-8
>
> 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 Hamb?chen <mail at nh2.me> 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 at haskell.org
> > http://www.haskell.org/mailman/listinfo/libraries
>
>
>
> ------------------------------
>
> Message: 3
> Date: Fri, 10 May 2013 14:04:09 +0200
> From: Simon Hengel <sol at typeful.net>
> Subject: Re: Control.Monad proposal: Add whenJust
> To: Niklas Hamb?chen <mail at nh2.me>
> Cc: libraries at haskell.org
> Message-ID: <20130510120409.GA2858 at x200>
> Content-Type: text/plain; charset=iso-8859-1
>
> -1
>
> Personally I think forM_ is the way to go.
>
> On Fri, May 10, 2013 at 02:13:45PM +0800, 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 at haskell.org
> > http://www.haskell.org/mailman/listinfo/libraries
>
>
>
> ------------------------------
>
> Message: 4
> Date: Fri, 10 May 2013 16:02:30 +0200
> From: Andreas Abel <andreas.abel at ifi.lmu.de>
> Subject: Re: Control.Monad proposal: Add whenJust
> To: Niklas Hamb?chen <mail at nh2.me>
> Cc: libraries at haskell.org
> Message-ID: <518CFDF6.2000002 at ifi.lmu.de>
> Content-Type: text/plain; charset=ISO-8859-1; format=flowed
>
> +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 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 at haskell.org
> > http://www.haskell.org/mailman/listinfo/libraries
> >
>
> --
> Andreas Abel <>< Du bist der geliebte Mensch.
>
> Theoretical Computer Science, University of Munich
> Oettingenstr. 67, D-80538 Munich, GERMANY
>
> andreas.abel at ifi.lmu.de
> http://www2.tcs.ifi.lmu.de/~abel/
>
>
>
> ------------------------------
>
> Message: 5
> Date: Sat, 11 May 2013 00:25:04 +1000
> From: Ivan Lazar Miljenovic <ivan.miljenovic at gmail.com>
> Subject: Re: Control.Monad proposal: Add whenJust
> To: Simon Hengel <sol at typeful.net>
> Cc: libraries at haskell.org
> Message-ID:
> <
> CA+u6gbxg6KaXe5etCHcKtEk8sR3-7wAhdCt2mu9S6Y47jTsJqA at mail.gmail.com>
> Content-Type: text/plain; charset=UTF-8
>
> -1
>
> Wasn't there a similar proposal to this last year?
>
> On 10 May 2013 22:04, Simon Hengel <sol at typeful.net> wrote:
> > -1
> >
> > Personally I think forM_ is the way to go.
> >
> > On Fri, May 10, 2013 at 02:13:45PM +0800, 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 at haskell.org
> >> http://www.haskell.org/mailman/listinfo/libraries
> >
> > _______________________________________________
> > Libraries mailing list
> > Libraries at haskell.org
> > http://www.haskell.org/mailman/listinfo/libraries
>
>
>
> --
> Ivan Lazar Miljenovic
> Ivan.Miljenovic at gmail.com
> http://IvanMiljenovic.wordpress.com
>
>
>
> ------------------------------
>
> Message: 6
> Date: Fri, 10 May 2013 18:09:52 +0100
> From: Ganesh Sittampalam <ganesh at earth.li>
> Subject: Re: Control.Monad proposal: Add whenJust
> To: Andreas Abel <andreas.abel at ifi.lmu.de>
> Cc: libraries at haskell.org
> Message-ID: <518D29E0.3070500 at earth.li>
> Content-Type: text/plain; charset=ISO-8859-1
>
> 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 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 at haskell.org
> >> http://www.haskell.org/mailman/listinfo/libraries
> >>
> >
>
>
>
>
> ------------------------------
>
> Message: 7
> Date: Fri, 10 May 2013 20:25:05 +0200
> From: Petr Pudl?k <petr.mvd at gmail.com>
> Subject: Re: Control.Monad proposal: Add whenJust
> To: Evan Laforge <qdunkan at gmail.com>
> Cc: "libraries at haskell.org" <libraries at haskell.org>
> Message-ID:
> <CABSda-fpnWNSLYDnffYDPDuucn4X9+Qbqn7f=
> XUgv6muFBGWiw at mail.gmail.com>
> Content-Type: text/plain; charset="iso-8859-1"
>
> 2013/5/10 Evan Laforge <qdunkan at gmail.com>
>
> > I know about forM_, but I don't use it because it sounds too much like a
> > loop.
> >
>
> I'd say `forM_` is more like "for each" for a collection (rather than
> C-style "for" loop), which makes perfect sense for Maybe. So I prefer
> `forM_` instead of adding a new function.
>
> Petr
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> Libraries mailing list
> Libraries at haskell.org
> http://www.haskell.org/mailman/listinfo/libraries
>
>
> End of Libraries Digest, Vol 117, Issue 10
> ******************************************
>
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