- Haskell-Cafe - Haskell.org

Re: [Haskell-cafe] Using 'lens' for "over"-like computation in Monad
by Nikolay Amiantov 30 Aug '14

30 Aug '14

Thank you! I'm still lost in the forest of all lens functions and generality, and haven't expected this signature. Can someone recommend good reading on all the lens-related types and classes, what they represent and how can they be used? (interested not only in "how", but also "why") 30 авг. 2014 г. 11:20 пользователь "Benno Fünfstück" < benno.fuenfstueck(a)gmail.com> написал: > The lens operator that does this is %%~, also called `traverseOf`: > > λ: ("Hello", "World") & _1 %%~ (\a -> putStrLn a >> return a) > Hello > ("Hello","World") > > Also see the documentation: > http://hackage.haskell.org/package/lens-4.4.0.1/docs/Control-Lens-Traversal… > > -- > Benno > > > 2014-08-30 9:44 GMT+02:00 Nikolay Amiantov <nikoamia(a)gmail.com>: > >> Hello Cafe, >> >> I'm trying to wrap my head around 'lens' library. My current exercise is >> to modify something using Lens in monad. Say, >> >> ("Hello", "World") & _1 `myOp` (\a -> putStrLn a >> return a) >> >> in IO, where myOp would be of type: >> >> myOp :: Monad m => Lens s t a b -> (a -> m b) -> s -> m t >> >> Of course I can write it myself, using combination of "view" and "set": >> >> myOp lens f v = f (view lens v) >>= flip (set lens) v >> >> (have not checked this, but something like that should do), but is there >> a more elegant way? >> >> Nikolay. >> >> _______________________________________________ >> Haskell-Cafe mailing list >> Haskell-Cafe(a)haskell.org >> http://www.haskell.org/mailman/listinfo/haskell-cafe >> >> >

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Using 'lens' for "over"-like computation in Monad
by Nikolay Amiantov 30 Aug '14

30 Aug '14

Hello Cafe, I'm trying to wrap my head around 'lens' library. My current exercise is to modify something using Lens in monad. Say, ("Hello", "World") & _1 `myOp` (\a -> putStrLn a >> return a) in IO, where myOp would be of type: myOp :: Monad m => Lens s t a b -> (a -> m b) -> s -> m t Of course I can write it myself, using combination of "view" and "set": myOp lens f v = f (view lens v) >>= flip (set lens) v (have not checked this, but something like that should do), but is there a more elegant way? Nikolay.

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"Hackathon"
by Richard Eisenberg 29 Aug '14

29 Aug '14

Hi Cafe, In my experience, "hackathon" can refer to two very different sorts of events: hacking marathons (such as jacobsHack), where participants tend to work overnight to accomplish something amazing in a limited time; and hacker weekends (such as Hac Phi), where participants work on projects, socialize, and then (presumably) rest at night. Both of these sorts of events have their place in the world, and I'm in no way suggesting one is "better" than the other. But, I do think it be good for all of us to name them differently, so folks know what they are signing up for. In particular, I'm worried that calling hacker weekends "hackathons" may discourage those of us with outside, inflexible commitments (e.g. kids; the need for 8 hours of sleep) from attending. Conversely, folks looking for the higher-energy environment of an all-night marathon might be disappointed to show up at a hacker weekend. What do you think? Is this distinction pointless? Would being consistent about this difference help? Here are some proposed new names for hacker weekends: - Hacker weekend - Hacker meetup - Community Hack - Weekend of Haskell I personally favor reserving the term "hackathon" for the marathon events. Richard

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Problem finding rewrite rules
by Dominik Peteler 29 Aug '14

29 Aug '14

Dear Cafe, I'm currently looking at the optimization GHC is doing and I cannot find the rewrite rules it fires. When I run my test code with ghc -O2 -ddump-simpl-stats -ddump-rule-firings Main.hs GHC shows the rules which are fired: ... Rule fired: Class op + ... Rule fired: +## ... and so on. Nothing new, nothing special. However, where do I find the definitions of these rules ? I grepped[1] the GHC code base and found nothing so far. I didn't find any documentation on it either. Can anyone point me to some place where I can find further information ? Thank you folks and have a nice day Dominik PS.: Since I'm working on numerical stable code with directed rounding I'm only interested in these two particular rules. I suspect them to break parts of my code. [1] http://jamie-wong.com/2013/07/12/grep-test

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Master's thesis topic and mentor
by Konstantin Saveljev 29 Aug '14

29 Aug '14

Hello, *short summary*: I am looking for someone who might have a topic for Master's thesis and will be interested in mentoring me along the way (giving pointers, helping here and there but not doing everything for me) *more detailed*: I have been introduced to Haskell about a year ago. It's been a slow ride, but I am gaining knowledge step by step. I am having some difficulty in finding a topic for my Master's degree and thought it would benefit me to try doing something Haskell related (be it a library, some part of a project or something similar). Unfortunately I do not believe I am able to pick something up by myself. Therefore I am looking for someone out there who might just have something to do they would be interested in delegating to somebody else and helping a bit throughout that work (and it cannot be something too easy, as there are some expectations from the work done for Master's degree) . It might be a long shot but I believe it is worth trying. Thank you for your time. Best regards, Konstantin Saveljev

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Does web-based email harm mailman lists?
by Kim-Ee Yeoh 29 Aug '14

29 Aug '14

Beautiful haskell people, Ever noticed the lacunae on some list threads? Someone hits reply and instead of reflecting via mailman, it goes direct to OP. OP notices absence of To:haskell-cafe and adds it back in in their reply to the reply. End result? The thread looks like OP having a convo with themselves. Unless you look at the quoted parts, which you have to click to reveal in web-based email. The convention, say with a google-groups based mailing list, is that conversations in mailing list are public by default. With some manual C&P, you can email responses in private. For cafe participants using web-based email, the situation is reversed, through no fault of their own. Approx 18 months ago, the haskell-beginners list suffered the same problem [1]. After some digging, it looks like there's a configurable option to Do The Right Thing: http://ccit.mines.edu/Mailman-FAQ#25 I was also privately emailed that there are downsides I wasn't aware of: Reply-To Munging Considered Harmful http://www.unicom.com/pw/reply-to-harmful.html Reply-To Munging Considered Useful http://www.metasystema.org/essays/reply-to-useful.html So what happened in the aftermath? Not much that I can tell. Everything just purred along. Now you might argue that haskell-cafe will just purr along grandly as well given the status quo. But this is a wrinkle that probably needs some awareness at least, hence this email. Would you like this fixed? The list owner would need to be obliged upon. [1] http://www.haskell.org/pipermail/beginners/2013-January/011304.html -- Kim-Ee

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ANN: Yampa 0.9.6
by Ivan Perez 29 Aug '14

29 Aug '14

Hi Café, I'm happy to announce that a new version of the Arrowized Functional Reactive Programming (FRP) library Yampa has been released. This new version includes a lot of haddock documentation, finally addressing the old issue of having to browse a large collection of papers and the code to find information about the API. We would like to encourage users to submit issues on github if they find this documentation insufficient, unclear or otherwise unsuitable in any way. There are no changes to the API itself, so this version remains backwards compatible. Future releases will seek to improve the documentation and introduce new optimisations as mandated by users' needs. As you may have heard, the current version is now working on Android, so we expect new releases to be published relatively soon. Additionally, Henrik Nilsson will give a tutorial on Declarative Game Programming at PPDP 2014, which will take place in Canterbury (UK) on September 8-10 (http://users-cs.au.dk/danvy/ppdp14/) We would like to encourage everyone to attend (see PPDP web page for instructions, venue, etc.). Links and more: * Hackage package: http://hackage.haskell.org/package/Yampa-0.9.6. * Github project page: https://github.com/ivanperez-keera/Yampa * Haskell Wiki page (examples, papers): http://www.haskell.org/haskellwiki/Yampa * Official announcement: http://fplab.bitbucket.org/posts/2014-08-29-yampa-0_9_6-released.html * (Another) Release announcement: http://keera.co.uk/blog/2014/08/29/new-version-yampa-0-9-6. Cheers Ivan -- Facebook: https://www.facebook.com/keerastudios Twitter: https://twitter.com/KeeraStudios Web: http://www.keera.co.uk

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ANN: HacBerlin - Haskell Hackathon in Berlin, 26-28 Sep 2014
by Stefan Wehr 29 Aug '14

29 Aug '14

Hi everyone, this is just a quick reminder: The Haskell Hackathon in Berlin is coming soon and there are still some places left. Please register now: http://goo.gl/aLfnWu The first keynote is also fixed; it will be given by Andres Löh (http://www.andres-loeh.de/) Thanks, Andres! Where: Berlin, Germany When: Fri 26 - Sun 28 September 2014 Meet in Berlin, discuss, hack together and improve the Haskell infrastructure. We welcome all programmers interested in Haskell, beginners and experts! For all details, visit our wiki page (http://www.haskell.org/haskellwiki/HacBerlin2014) and make sure to register now! Cheers, Stefan

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Conduit+GHC high memory use for simple Sink
by Bryan Vicknair 28 Aug '14

28 Aug '14

Hello Cafe, First I'd like to thank Michael Snoyman and Gabriel Gonzalez for the work they've done on the conduit and pipes stream processing libraries, and all the accompanying tutorial content. I've been having fun converting a text processing app from C to Haskell. I'm seeing unexpectedly high memory usage in a stream-processing program that uses the conduit library. I've created a example that demonstrates the problem. The program accepts gzip files as arguments, and for each one, classifies each line as either Even or Odd depending on the length, then outputs some result depending on the Sink used. For each gzip file: action :: GzipFilePath -> IO () action (GzipFilePath filePath) = do result <- runResourceT $ CB.sourceFile filePath $$ Zlib.ungzip =$ CB.lines =$ token =$ sink2 putStrLn $ show result The problem is the following Sink, which counts how many even/odd Tokens are seen: type SinkState = (Integer, Integer) sink2 :: (Monad m) => SinkState -> Sink Token m SinkState sink2 state@(!evenCount, !oddCount) = do maybeToken <- await case maybeToken of Nothing -> return state (Just Even) -> sink2 (evenCount + 1, oddCount ) (Just Odd ) -> sink2 (evenCount , oddCount + 1) When I give this program a few gzip files, it uses hundreds of megabytes of resident memory. When I give the same files as input, but use the following simple Sink, it only uses about 8Mb of resident memory: sink1 :: MonadIO m => Sink Token m () sink1 = awaitForever (liftIO . putStrLn . show) At first I thought that sink2 performed so poorly because the addition thunks were being placed onto the heap until the end, so I added some bang patterns to make it strict. That didn't help however. I've done profiling, but I'm unable to figure out exactly what is being added to the heap in sink2 but not sink1, or what is being garbage collected in sink1, but not sink2. The full source is here: https://bitbucket.org/bryanvick/conduit-mem/src/HEAD/hsrc/bin/mem.hs Or you can clone the repo, which contains a cabal file for easy building: git clone git@bitbucket.org:bryanvick/conduit-mem.git cd comduit-mem cabal sandbox init cabal install --only-dependencies cabal build mem ./dist/build/mem/mem [GIVE SOME GZIP FILES HERE] You can change which sink is used in the 'action' function to see the different memory usage.

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Haskell Platform 2014 build problems on OS X
by Nicola Gigante 28 Aug '14

28 Aug '14

Hello! I just want to point to your attention this issue of Homebrew: https://github.com/Homebrew/homebrew/issues/31609 It seems that they’re forced to remove the formula for the Haskell Platform because, it seems, the build process has become extremely complicated and they can’t figure out how to automate it. Is there someone that know enough internals of the platform to help them or to give some advice? Thanks, Nicola

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