- Haskell-Cafe - Haskell.org

Just found GHCI can be used as a debugger
by Yves Parès 25 Jan '12

25 Jan '12

Hello, By randomly typing :<Tab> under GHCI, I discovered it could be used as a debugger (:breakpoint, :step, etc.) Three questions in that regard: 1) Is there some documentation about it? 2) I haven't investigated much, is that fairly thorough and convenient? Does some people over here use those functions on a regular basis? 3) Is it possible to save the state (notably to save the position of the breakpoints)? Thanks!

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XMonad.Shell.Prompt and unicode input
by Eugene Dzhurinsky 25 Jan '12

25 Jan '12

Hello, community! I have some strange issue with recent XMonad: with using of standard Shell module it is not possible to use unicode input. It simply freezes up and in console I can observe: Enum.toEnum{Word8} : tag (1092) is outside of bounds (0,255). What function may cause such error indirectly? I looked through the sources - but have no idea where such conversion may have place and why Word8 is used. -- Eugene N Dzhurinsky

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STM: "nested atomically" error
by Johan Brinch 24 Jan '12

24 Jan '12

Hi all, I'm seeing the "Control.Concurrent.STM.atomically was nested" error, but I just can't figure out what's happening. I'm not using any unsafe IO (only for debug printing), and the test program is only running one thread. Where is a transaction being nested? What are the scenarios where this error is reported? The behaviour is consistent on GHC 7.4.0 RC and GHC 7.2.2 (stable). That function that's being run can be found here: https://gist.github.com/30b94760abc27b05ec7c And here is the last output from the runtime system: https://gist.github.com/4356ae541895becb4169 Any ideas to track this down will be greatly appreciated! -- Johan Brinch, Dept. of Computer Science, University of Copenhagen

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bindings for libvirt
by Michael Litchard 24 Jan '12

24 Jan '12

Due to the direction things are going at work, I have become interested in Haskell bindings for libvirt. Noticed that this hasn't been done yet. I was wondering if this was due to lack of motivation, or if there were some difficult hurdles with libvirt that make the project cost-prohibitive. If it's the former, I don't see a problem proceeding with exploration. If it's the latter, I'd like to know what the hurdles are.

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Data & newtype differences. Today: strictness
by Yves Parès 24 Jan '12

24 Jan '12

Hello, I had for long thought that data and newtype were equivalent, but then I spotted some differences when it comes to strictness. Those can be summed up as: data Test = Test Int newtype TestN = TestN Int pm (Test _) = 12 -- Strict (pm undefined = undefined) pm2 t = t `seq` 12 -- Strict pmN (TestN _) = 12 -- Non strict (pm undefined = 12) pmN2 t = t `seq` 12 -- Strict When I think about it, pm and pmN are logical, as newtype layers are removed by the compiler and then do not exist at runtime. Some would maybe expected pmN2 to behave just like pmN (as I don't 'seq' the inner Int but the outer TestN), but if you follow the same logic (TestN layer doesn't exist at runtime), then you ask to evaluate the inner Int. This can however be misleading when you use an opaque type, can't it? As you don't know if it's declared using data or newtype... These make me think that pattern matching against a newtype is always lazy (irrefutable). Am I right? Is there some litterature expliciting in a less empiric way than I did the differences like this between data and newtype? I've never come against such documentation through all my learning of Haskell, yet I think it's an important point.

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interest in Irish Haskell User's Group?
by John Lato 24 Jan '12

24 Jan '12

Hello, Some other Haskellers and I have been discussing starting an Irish Haskell User's Group. I think we're close to critical mass, and need just a few more interested people to give it a run. To that end, I'd appreciate hearing from anyone who would be interested in attending or participating. Our initial meeting would likely be in Maynooth as that's where we're based, however we'd like to see involvement across at least the greater Dublin area, and possibly further afield. So, please let me know if you're interested in participating. I'll post further details when we've established an initial meeting. Thanks, John L.

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Haskellers in Delhi, India
by Anupam Jain 24 Jan '12

24 Jan '12

Hi all, Are there any haskellers in Delhi or nearby areas interested in a meetup? I've been dabbling in Haskell for a long time but only recently became interested in creating large "real world" programs with it. It would be interesting to meet folks who already have such experience (or are looking to get such experience). Thanks, Anupam Jain

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Efficient temporary file storage??
by Nick Rudnick 24 Jan '12

24 Jan '12

Dear all, if you want to temporarily store haskell data in a file – do you have a special way to get it done efficiently? In an offline, standalone app, I am continuously reusing data volumes of about 200MB, representing Map like tables of a rather simple structure, key: (Int,Int,Int) value: [((Int,Int),LinkId)] which take quite a good deal of time to produce. Is there a recommendation about how to 'park' such data tables most efficiently in files – any format acceptable, quick loading time is the most desirable thing. Thanks a lot in advance, Nick

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ANNOUNCE: hxournal-0.6.3
by Ian-Woo Kim 24 Jan '12

24 Jan '12

Hi, I am very happy to announce hxournal-0.6.3. (To learn about hxournal, see http://ianwookim.org/hxournal ) Compared with the previous version (0.6.2), it has now a big enhancement in rendering, implementing resizing selected elements and highlighter pen implementation. Now, after selection, you can see nice visual feedback of selection handle and selected strokes. When moving or resizing, it smoothly shows intermediate steps. Enhancement in double buffer rendering also enables smoother scrolling. I think this program is now in a pretty usable state as I use it as my daily notetaking program. Installation must be simple as always. Type "cabal install hxournal" after updating your cabal database. (you need to have gtk2hs installed. ) Last improvement has been done much while I was participating in Hac Boston activity. Thank Hac Boston organizers and participants very much! Thank you for your interest. Enjoy hxournaling! Ian-Woo

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Munich Haskell Meeting
by Heinrich Hördegen 24 Jan '12

24 Jan '12

Dear all, I hope, all of you had a good start into this year. For Munich's Haskellers, it's time for our next get-together. With a number of participants between 12 and 14, our last meetings have been a real success. Let's continue this joyful events. Even if you don't live at Munich, if you can make it one day, we will be happy to receive you there. Check out the dates! So, this year's first get-together will take place next week, Monday, 30. January at Cafe Puck, our usual venue. If you plan to join, please go to the site and click the button: http://www.haskell-munich.de/dates This will help us to reserve tables. With hope of a large attendance, have a nice time, Heinrich -- -- hoerdegen(a)funktional.info www.funktional.info --

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Happy holidays Haskell
by R J 24 Jan '12

24 Jan '12

hello Haskell ever since I started this my life has been better than ever http://www.news13open.com

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[C][enums][newbie] What is natural Haskell representation of such enum?
by Данило Глинський 24 Jan '12

24 Jan '12

What is natural Haskell representation of such enum? enum TypeMask { UNIT, GAMEOBJECT, CREATURE_OR_GAMEOBJECT = UNIT | GAMEOBJECT }; More sophisticated question is: and what data structures must be used when converting this naturally one to Haskell? // 1-byte flaged enum enum TypeMask { // ... UNIT = 0x0004, GAMEOBJECT = 0x0008, // ... CREATURE_OR_GAMEOBJECT = UNIT | GAMEOBJECT WORLDOBJECT = UNIT | PLAYER | GAMEOBJECT | DYNAMICOBJECT | CORPSE // ... even more enum combos ... };

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Finding longest common prefixes in a list
by Gwern Branwen 24 Jan '12

24 Jan '12

Recently I wanted to sort through a large folder of varied files and figure out what is a 'natural' folder to split out, where natural means something like >4 files with the same prefix. (This might be author, genre, subject, whatever I felt was important when I was naming the file.) Now usually I name files with hyphens as the delimiters like the hypothetical '1998-wadler-monads.pdf', and it would be easy to write a stdin/stdout filter to break Strings on hyphens and sort by whatever is most common. But this is rather hardwired, can I solve the more general problem of finding the longest common prefixes, whatever they are? This turns out to be much more difficult than simply finding 'the' longest common prefix (which is usually ""). I found an algorithm of sorts at http://stackoverflow.com/a/6634624 but it was easier described than implemented. Eventually I wrote what I *think* is a correct program, but it's definitely of the write-only sort. Perhaps people have better implementations somewhere? I saw a lot of discussion of tries, but I didn't go that route. The code, followed by an example: #!/usr/bin/env runhaskell import Data.List (intercalate, isPrefixOf, nub, sort) main :: IO () main = interact (unlines . intercalate [""] . chunkFiles . lines ) -- basic algorithm from <http://stackoverflow.com/a/6634624> chunkFiles :: Ord a => [[a]] -> [[[a]]] chunkFiles f = map (\(_,b) -> filter (isPrefixOf b) f) $ sort $ map (\x -> (countPrefixes x f,x)) (e $ bar f) sharedPrefixes :: Ord a => [[a]] -> [a] sharedPrefixes [] = [] sharedPrefixes s = foldr1 sp2 s where sp2 l1 l2 = map fst . takeWhile (uncurry (==)) $ zip l1 l2 traverse :: Ord a => [[a]] -> [[a]] traverse [] = [] traverse x = sharedPrefixes (take 2 x) : traverse (drop 1 x) bar :: Ord a => [[a]] -> [[a]] bar = nub . sort . traverse . sort countPrefixes :: (Ord a) => [a] -> [[a]] -> Int countPrefixes x xs = length $ filter (x `isPrefixOf`) xs e :: Eq a => [[a]] -> [[a]] e y = map fst $ filter snd $ map (\x -> (x, (==) 1 $ length . filter id $ map (x `isPrefixOf`) y)) y {- Example input from `ls`: chorus-kiminoshiranaimonogatari.ogg chorus-mrmusic.ogg choucho-lastnightgoodnight.ogg dylanislame-aikotoba.ogg electriclove-エレクトリック・ラブ-korskremix.ogg gumi-bacon8-justhangingaround.ogg gumi-iapologizetoyou.ogg gumi-montblanc.ogg gumi-mozaikrole.ogg gumi-ハッピーシンセサイザ.ogg gumi-showasengirl.ogg gumi-sweetfloatflatsスイートフロートアパート.ogg gumi-timewarpedafterchoppingmystagbeetle.ogg gumi-オリジナル曲-付きホシメグリ.ogg gumi-ミクオリジナル親友.ogg kaito-byakkoyano.ogg kaito-flowertail.ogg kasaneteto-tam-ochamekinou重音テト吹っ切れたおちゃめ機能.ogg len-crime-timetosaygoodbye.ogg len-fire◎flower.ogg len-ponponpon.ogg lily-prototype.ogg luka-apolxcore-waitingforyou.ogg luka-dimトロイ.ogg luka-dion-myheartwillgoon.ogg luka-dirgefilozofio-dirgeasleepinjesus.ogg luka-アゴアニキ-doubelariatダブルラリアット.ogg luka-emon-heartbeats.ogg luka-emonloid3-ハローハロー.ogg luka-everybreathyoutake.ogg luka-オリジナル-garden.ogg luka-justbefriends.ogg lukameiko-gemini.ogg luka-milkyway.ogg luka-やみくろ-かいぎ.ogg luka-tic-tick.ogg luka-torinouta.ogg luka-zeijakukei-shounenshoujo.ogg luka-勝手にアニメ-nologic-作ってみた.ogg luka-駄目人間.ogg meiko-artemis-awake.ogg miku-9ronicleプラチナ.ogg miku-acolorlinkingworld-この世界の下で.ogg miku-acolorlinkingworld-青い花.ogg miku-a+jugos-lullabyforkindness.ogg miku-akayaka-beacon.ogg miku-akayakap-sunrise.ogg miku-aoihana.ogg miku-arabianresponse.ogg miku-avtechno-tear.ogg miku-こえをきかせてcicci.ogg miku-cleantears-remind2011natsu-greenhillzonecrystiararemix.ogg miku-cleantears-remind2011natsu-夏影summerwindremix.ogg miku-clocklockworks.ogg miku-dancedancevol2-runner.ogg miku-daniwellp-chaoticuniverse.ogg miku-dixieflatline-shinonomescrumble.ogg miku-electricloveエレクトリックラヴ.ogg miku-elegumitokyo-kissmebaby.ogg miku-galaxyodyssey-cryingirl.ogg miku-galaxyodyssey-galaxyspacelines.ogg miku-hakamairi.ogg miku-haruna.ogg miku-heartshooter.ogg miku-hoshikuzutokakera.ogg miku-innes.ogg miku-innocence初音ミク.ogg miku-jemappelle-motion-likeyou.ogg miku-jemappelle-motion-ohwell.ogg miku-jevannip-myfavoritesummer.ogg miku-kakokyuudance-過呼吸ダンス.ogg miku-kz-packaged.ogg miku-kz-tellyourworld.ogg miku-lastscene.ogg miku-lostmemories付き-初音ミク.ogg miku-lovelyday.ogg miku-いいわけlove_song.ogg mikulukagumi-prayfor.ogg miku-maple-初音ミク楓-オリジナル曲.ogg miku-more1.5.ogg miku-m(a)rk-eklosion.ogg miku-m(a)rk-kirch.ogg miku-nana-ボーナストラック-ハッピー般若コア.ogg miku-nekomimiswitch.ogg miku-nightrainbow.ogg miku-noyounome.ogg miku-むかしむかしのきょうのぼくオリジナル.ogg miku-pandolistp-neverendinghammertime.ogg miku-ジラートP-birthdayofeden-deepsleep.ogg miku-ジラートP-birthdayofeden-水中読書.ogg miku-plustellia-dear.ogg miku-plustellia-壁の彩度-crazygirl.ogg miku-plustellia-壁の彩度-discoradio.ogg miku-ぽわぽわP-ストロボライト.ogg miku-rabbitforgets.ogg miku-re:package-lastnightgoodnight.ogg miku-re:package-ourmusic.ogg miku-re:package-sutorobonaitsu.ogg miku-rollinggirl.ogg miku-ryo-メルト-melt.ogg miku-senseiniitteyaro.ogg miku-sevencolors-レモネード.ogg miku-shoukinosatadenia.ogg miku-stratosphere.ogg miku-supernova.ogg miku-tam-lastnightgoodnight.ogg miku-tanatofobia.ogg miku-thearmyforyourenvy-スーパー・ノヴァ.ogg miku-theendlesslove.ogg miku-tinyparadise-snowflake.ogg miku-tinyparadise-tinyparadise.ogg miku-unfragment.ogg miku-worldismine-ルドイズマイン.ogg miku-yakiimo.ogg miku-文学少年の憂鬱-オリジナル.ogg miku-カラフルポップビートオリジナル曲.ogg miku-杯本選life.ogg miku-杯本選初音ミクどういうことなのダンス.ogg miku-般若心経beautyfloor-buddhamix.ogg miku-般若心経ポップ.ogg niconicochorus-blackrockshooter.ogg niconicochorus-justbefriends.ogg rin-dixieflatline-gemini.ogg rin-elegumitokyo-二人、恋してgirlsside.ogg rin-helloworld.ogg rin-jutenija.ogg rin-lastnightgoodnight.ogg rin-ripples-evergreen.ogg rin-っ´ω｀c.ogg rollinggirl-piano.ogg seeu-gagain-따라리라ddadada.ogg utau-雪歌ユフbeyondオリジナル曲.ogg yuki-discochocolatheque.ogg yuki-shouwasenhosiga^ru.ogg yuki-shouwasenhosiga^ru.ogg Example output from `ls | ~/lcp.hs`: chorus-kiminoshiranaimonogatari.ogg chorus-mrmusic.ogg gumi-montblanc.ogg gumi-mozaikrole.ogg gumi-showasengirl.ogg gumi-sweetfloatflatsスイートフロートアパート.ogg kaito-byakkoyano.ogg kaito-flowertail.ogg luka-emon-heartbeats.ogg luka-emonloid3-ハローハロー.ogg luka-tic-tick.ogg luka-torinouta.ogg miku-acolorlinkingworld-この世界の下で.ogg miku-acolorlinkingworld-青い花.ogg miku-akayaka-beacon.ogg miku-akayakap-sunrise.ogg miku-cleantears-remind2011natsu-greenhillzonecrystiararemix.ogg miku-cleantears-remind2011natsu-夏影summerwindremix.ogg miku-dancedancevol2-runner.ogg miku-daniwellp-chaoticuniverse.ogg miku-electricloveエレクトリックラヴ.ogg miku-elegumitokyo-kissmebaby.ogg miku-galaxyodyssey-cryingirl.ogg miku-galaxyodyssey-galaxyspacelines.ogg miku-hakamairi.ogg miku-haruna.ogg miku-innes.ogg miku-innocence初音ミク.ogg miku-jemappelle-motion-likeyou.ogg miku-jemappelle-motion-ohwell.ogg miku-kz-packaged.ogg miku-kz-tellyourworld.ogg miku-lostmemories付き-初音ミク.ogg miku-lovelyday.ogg miku-m(a)rk-eklosion.ogg miku-m(a)rk-kirch.ogg miku-plustellia-壁の彩度-crazygirl.ogg miku-plustellia-壁の彩度-discoradio.ogg miku-senseiniitteyaro.ogg miku-sevencolors-レモネード.ogg miku-tam-lastnightgoodnight.ogg miku-tanatofobia.ogg miku-thearmyforyourenvy-スーパー・ノヴァ.ogg miku-theendlesslove.ogg miku-tinyparadise-snowflake.ogg miku-tinyparadise-tinyparadise.ogg miku-ジラートP-birthdayofeden-deepsleep.ogg miku-ジラートP-birthdayofeden-水中読書.ogg miku-杯本選life.ogg miku-杯本選初音ミクどういうことなのダンス.ogg miku-般若心経beautyfloor-buddhamix.ogg miku-般若心経ポップ.ogg niconicochorus-blackrockshooter.ogg niconicochorus-justbefriends.ogg len-crime-timetosaygoodbye.ogg len-fire◎flower.ogg len-ponponpon.ogg luka-dimトロイ.ogg luka-dion-myheartwillgoon.ogg luka-dirgefilozofio-dirgeasleepinjesus.ogg miku-re:package-lastnightgoodnight.ogg miku-re:package-ourmusic.ogg miku-re:package-sutorobonaitsu.ogg miku-nana-ボーナストラック-ハッピー般若コア.ogg miku-nekomimiswitch.ogg miku-nightrainbow.ogg miku-noyounome.ogg rin-dixieflatline-gemini.ogg rin-elegumitokyo-二人、恋してgirlsside.ogg rin-helloworld.ogg rin-jutenija.ogg rin-lastnightgoodnight.ogg rin-ripples-evergreen.ogg rin-っ´ω｀c.ogg -} -- gwern http://www.gwern.net

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Monads, do and strictness
by Victor S. Miller 23 Jan '12

23 Jan '12

The "do" notation translates do {x <- a;f} into a>>=(\x -> f) However when we're working in the IO monad the semantics we want requires that the lambda expression be strict in its argument. So is this a special case for IO? If I wanted this behavior in other monads is there a way to specify that? Victor Sent from my iPhone

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ANN: OpenCL 1.0.3.0 package
by Luis Cabellos 23 Jan '12

23 Jan '12

Hello, I update the OpenCL package with the last changes. The most important thing, currently it fully implements OpenCL 1.0. IMPORTANT, some functions change signature (e,g: clSetKernelArg, and clCreateContext* ) Thanks all comments in the previous version, issues and pulls in github. # Where to get it * Hackage page (http://hackage.haskell.org/package/OpenCL) * Repository (https://github.com/zhensydow/opencl) * Bugs (https://github.com/zhensydow/opencl/issues) # Changes * Move to Control.Parallel.OpenCL (thanks Jason Dagit) * Added Image functions * Added clExecuteNativeKernel * Fixes on Windows, Macos and Linux versions (thanks Anthony Cowley, ehird, axman6) * Fix bug with preprocessor code in OpenCL source code * Fix clSetKernelArg to allow cannot specify size of local memory arrays Thanks, Luis Cabellos

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Can't install hspec
by Erik de Castro Lopo 23 Jan '12

23 Jan '12

Hi all, I'm trying to cabal install hspec and I get the following: Resolving dependencies... /tmp/hspec-0.9.04062/hspec-0.9.0/Setup.lhs:2:10: Could not find module `System' It is a member of the hidden package `haskell98-2.0.0.0'. Use -v to see a list of the files searched for. cabal: Error: some packages failed to install: hspec-0.9.0 failed during the configure step. The exception was: ExitFailure 1 This is with ghc-7.2.2. Any clues how I can get around this? Cheers, Erik -- ---------------------------------------------------------------------- Erik de Castro Lopo http://www.mega-nerd.com/

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Idris
by Ryan Ingram 23 Jan '12

23 Jan '12

Has anyone played with Idris (http://idris-lang.org/) at all? It looks interesting, and I'd love to play with it, but unfortunately I only have windows machines up and running at the moment and the documentation seems to imply it only builds on unixy systems. I'm curious how difficult it would be to get a win32 implementation up and running. -- ryan

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Compiling dph package with ghc-7.4.0.20111219
by mukesh tiwari 23 Jan '12

23 Jan '12

Hello all I have installed ghc-7.4.0.20111219 and this announcement<http://www.haskell.org/pipermail/glasgow-haskell-users/2011-December/021310…>says that "The release candidate accidentally includes the random, primitive, vector and dph libraries. The final release will not include them". I tried to compile a program [ntro@localhost src]$ ghc-7.4.0.20111219 -c -Odph -fdph-par ParallelMat.hs ghc: unrecognised flags: -fdph-par Usage: For basic information, try the `--help' option. [ntro@localhost src]$ ghc-7.2.1 -c -Odph -fdph-par ParallelMat.hs Also in ghc-7.4.0.20111219 --help , i don't see any option related data parallel haskell. I search the ghc manual <http://www.haskell.org/ghc/dist/stable/docs/html/users_guide/> and got one reference for data parallel haskell<http://www.haskell.org/ghc/dist/stable/docs/html/users_guide/lang-parallel.…>. This link again redirects to data parallel haskell <http://www.haskell.org/haskellwiki/GHC/Data_Parallel_Haskell> wiki page using ghc-7.2.1. Could some one please tell me how to compile haskell program using dph with ghc-7.4.0.20111219. Regards Mukesh Tiwari

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Right tree structure for complicated problem
by Pierre Penninckx 22 Jan '12

22 Jan '12

Hello everyone, So here is what I want to achieve: I'd like a program that calculates the time needed for water to flow out of a circuit made out of tube. The rules are : - There are multiple sources of water and only one exit. - The water can only take one path from a source to the exit. - Of course, a source of water contains a certain amount of water at the beginning. ### Algorithm Here's the concept of how it should be working: #1 The tubes are connected together : 1 parent with n childs. This looks really like a tree: data Tree = Leaf { getVolume::Int } | Path { getMaxFlow::Int , getFlow::Int , getOpen::Bool , getChilds::[Tree} } But this structure doesn't work well :( (see later why) When I determine the time taken by the water to flow down to the exit, I need to take water jams (haha) into account. To do this, the algorithm begins by the exit node and "distribute" it's maximum flow through the childs, but only to open tubes. And the childs distribute their flow to their own child. Repeat until you reach a Leaf. example : Path 5 0 True [Path 4 0 True [], Path 3 0 True [], Path 3 0 False []] after distribution: Path 5 5 True [Path 4 4 True [], Path 3 2 True [], Path 3 0 False []] #2 After the distribution, you must get the water out. Easy, the time taken by the water to flow down is, in pseudo code: max( for all leafs : getVolume * getFlow ) ### Problem My problem, is this : I must be able to change the "distribution" the flow of the parent to its childs: in my example, I said "take the biggest child, give it all you can and give the rest to the other childs, repeat" but I could have said "give first to the lowest child" or "give a certain percentage depending of the sum of the childs max capacity" So I can't map simply over the childs: distribute :: (Int -> Int -> Int) -> Tree -> Tree distribute _ (Leaf a) = Leaf a distribute f path@(Path _ _ False _) = path distribute f (Path max cur open childs) = Path max (f cur) open (map f childs) I must indeed let the distribution function choose to which childs it wants to give: distribute :: (Int -> [Tree] -> [Tree]) -> Tree -> Tree distribute f (Path max use open child) = Path max use open $ map (distribute (f use)) newChilds where newChilds = f use childs And I find this quite awful... I know Hakell is everything about data structure, so I suppose my data structure is awful too. Do you have any suggestions/remarks to help me? Sorry I made it so long, but you needed to know what I wanted to acheive to help me (I hope I made it crystal clear ^^). Thanks, Ibiz

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GLFW-0.5.0.0 is released
by Paul Liu 22 Jan '12

22 Jan '12

There is now a new GLFW package on hackage with version number 0.5.0.0. Maintainers of packages that depend on GLFW may want to update your packages since (1) GLFW-0.4.2 is mostly broken on OS X Lion (10.7); (2) there are incompatible API changes. Notable changes include (but not limited to): 1. It now uses GLFW C library version 2.7.2 on all platforms. 2. It still tries to compile and build a static version of the C library on all platforms. If this is not what you want, use '--flags dynamic' to dynamically link to pre-installed glfw C library on your system. 3. It nows does a much better job at configuring itself on Linux and FreeBSD. 4. There is an outstanding bug preventing GLFW programs to be invoked from GHCi on OS X. One has to compile before running GLFW programs. 5. Documentation has been improved, and there are also API changes affecting window params and hints. See the hackage documentation for details. 6. An example program and detail change log are included if you unpack the source. Many thanks to Marc Sunet and a lot of other people for sending in bug reports and helping out with this upgrade. Marc is now also a maintainer of this package. Please send your bug report to either of us or to the GLFW mailing list <glfw(a)projects.haskell.org>. For new users to GLFW or GLFW-b, there are some style and API differences between these two packages (see https://github.com/bsl/GLFW-b/blob/master/README.md) and in particular, GLFW includes some basic texture support and a "quick-and-dirty" font rendering hack (not found in GLFW C library). Now, more on the outstanding bug with GHCi on OS X. Here is what happens when invoking initialize from GHCi on OS X: Prelude> import Graphics.UI.GLFW Prelude Graphics.UI.GLFW> initialize Loading package OpenGLRaw-1.1.0.1 ... linking ... done. Loading package GLURaw-1.1.0.0 ... linking ... done. Loading package ObjectName-1.0.0.0 ... linking ... done. Loading package StateVar-1.0.0.0 ... linking ... done. Loading package Tensor-1.0.0.1 ... linking ... done. Loading package OpenGL-2.4.0.1 ... linking ... done. Loading package GLFW-0.5.0.0 ... linking ... done. objc[18039]: Object 0x105e00a70 of class __NSCFString autoreleased with no pool in place - just leaking - break on objc_autoreleaseNoPool() to debug 2012-01-21 03:23:58.574 ghc[18039:1303] -[NSAutoreleasePool init]: unrecognized selector sent to instance 0x105e00740 2012-01-21 03:23:58.577 ghc[18039:1303] *** Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: '-[NSAutoreleasePool init]: unrecognized selector sent to instance 0x105e00740' On the other hand, if GLFW is installed with the dynamic flag to link to a pre-installed system libglfw.dylib, then initialize would work fine but again GLFW applications still fail to run properly as the window fails to receive user input and seems to hang. The enableGUI hack wouldn't help either (and is broken on 10.7). I believe this situation is the same when running GLFW-b applications from GHCi on OS X, since it builds and installs dynamic library on OS X too. Compiled applications are not affected by this bug. Any help is greatly appreciated! Thanks! -- Regards, Paul Liu

2 2

Re: [Haskell-cafe] Fwd: Monads, do and strictness
by David Barbour 21 Jan '12

21 Jan '12

On Sat, Jan 21, 2012 at 2:24 PM, Roman Cheplyaka <roma(a)ro-che.info> wrote: > * David Barbour <dmbarbour(a)gmail.com> [2012-01-21 12:18:09-0800] > > `undefined` is not a value in any domain. It isn't a value at all. It's > > certainly not part of my monad language or algebra. Up to the semantic > > level of comparing observable and legally defined behaviors, we can have > > the identity law. That's sufficient for the letter of the law, even if > not > > ideal from Haskell's operational perspective. > > First, can we please agree on what kind of semantics we are discussing? > > In the beginning you are talking about domains, so it sounds like > denotational semantics. But then you move to behaviours, and that sounds > like operational semantics. > Can you first agree what *layer* of semantics you are discussing? It's useful to understand programs in layers. Your argument seems to depend on confusing the semantics of the monad abstraction with the semantics of the Haskell representation of the abstraction. We could even look at underlying layers, such as the implementation of Haskell - including allocation and GC. The important point is that the ONLY relevant layer for monad laws is the monad algebra itself - not the representation in Haskell, and certainly not the representation of Haskell on a Von Neumann machine. Since strictness does not affect the *values* of any elements in the monad's algebra, it is trivial to meet the monad laws. My argument did switch from denotational to operational as I crossed layers, but that was a conscious decision on my part. > If we are talking about the denotational semantics, then we choose a > domain and need to map every legal Haskell expression to something in > that domain. > We can discuss denotational semantics of many languages. And we can discuss the representations of those languages in Haskell. And we can discuss the denotational semantics of Haskell. Layers upon layers. Denotational semantics certainly doesn't imply the Haskell layer. Regards, Dave

1 0

Fwd: Monads, do and strictness
by David Barbour 21 Jan '12

21 Jan '12

`undefined` is not a value in any domain. It isn't a value at all. It's certainly not part of my monad language or algebra. Up to the semantic level of comparing observable and legally defined behaviors, we can have the identity law. That's sufficient for the letter of the law, even if not ideal from Haskell's operational perspective. Implicit use of laziness or undefined to model and reason about data or algebras is a mistake that mixes semantic layers. Even in Haskell. There's a reason every robust library for processing signals or streams will model them explicitly. Regards, Dave On Sat, Jan 21, 2012 at 11:22 AM, Roman Cheplyaka <roma(a)ro-che.info> wrote: > * David Barbour <dmbarbour(a)gmail.com> [2012-01-21 11:09:43-0800] > > Logically only has meaning when `=` applies to values in the domain. > > `undefined` is not a value in the domain. > > > > We can define monads - which meet monad laws - even in strict languages. > > In strict languages 'undefined' is not a value in the domain indeed, but > it is in non-strict languages, exactly because they are non-strict. > > I think that's what Robert Harper meant by saying that Haskell doesn't > have a type of lists, while ML has one [1]. > > [1]: > http://existentialtype.wordpress.com/2011/04/24/the-real-point-of-laziness/ > > -- > Roman I. Cheplyaka :: http://ro-che.info/ >

2 1

Serializing UTCTimes
by Bas van Dijk 21 Jan '12

21 Jan '12

Hello, What's the recommended way for serializing (with the cereal package) an UTCTime? It's easy to give Serialize instances for UTCTime and Day: instance Serialize UTCTime where get = liftM2 UTCTime get get put (UTCTime day time) = put day >> put time instance Serialize Day where get = liftM Day get put = put . toModifiedJulianDay However I have no idea how to serialize the DiffTime stored in an UTCTime: instance Serialize DiffTime where get = ? put = ? Regards, Bas

3 4

Parsing workflow
by Nils Schweinsberg 21 Jan '12

21 Jan '12

Hi! I'm having a really hard time to write a correct parser for a small language I've developed. I have been trying to write a parser using parsec, but always get a lot of error messages like "unexpected "\n", expected ..., new-line or..." when trying to run the parser. Then I read about the happy parser and really liked the separation of lexing the text into tokens and parsing the actual logic behind those tokens. Since I couldn't get familiar with the lexer "alex" I gave up on the alex-happy-approach again and went back to parsec. But with that lexer->parser idea on my mind, my parser currently looks a lot like a lexer. So I came up with the idea of using a combination of parsec and happy, where I generate a list of tokens for my text via parsec and analyse it with happy. My questions would be: - Is this a valid approach? - What is your workflow on parsing complex data structures? - What about performance? Since my project is going to be an interpreted language parsing performance might be interesting aswell. I've read that happy is in general faster than parsec, but what if I combine both of them as I said above? I guess that parsing a simple list of tokens without any nested parser structures would be pretty fast? - Do you have any other ideas on how to improve my parser? - What are your general thoughts on happy vs. parsec? Thanks for any replies, Nils

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generating parens for pretty-printing code in haskell-src-exts
by Conal Elliott 21 Jan '12

21 Jan '12

I'm using haskell-src-exts together with SYB for a code-rewriting project, and I'm having difficulty with parenthesization. I naïvely expected that parentheses would be absent from the abstract syntax, being removed during parsing and inserted during pretty-printing. It's easy for me to remove them myself, but harder to add them (minimally) after transformation. Rather than re-inventing the wheel, I thought I'd ask for help. Has anyone written automatic minimal parens insertion for haskell-src-exts? -- Conal

4 3

Combining Regions and Iteratees
by oleg＠okmij.org 21 Jan '12

21 Jan '12

Regions is an automatic resource management technique that statically ensures that all allocated resources are freed and a freed resource cannot be used. Regions also promote efficiency by helping to structure the computation so that resources will be freed soon, and en masse. Therefore, regions are particularly suitable for scarce resources such as file handles, database or network connections, etc. A lightweight monadic region library is available on Hackage. Iteratee IO also aims, among other things, to encapsulate resources such as file handles and network connections, ensuring their safe use and prompt disposal. One may wonder how much Iteratees and Regions have in common and if that commonality can be factored out. There seem to be several ways to combine regions and iteratees. This message describes the most straightforward attempt, combining a monadic region library (mostly as it is) with an Iteratee IO library, also mostly as it is. We use monadic regions to manage file handles or file descriptors, ensuring that file handles are always closed even in case of IO and other asynchronous exceptions. An enumerator like enumFile provided similar guarantees for its handles. (Since an enumerator keeps its handles to itself, there is no danger of iteratees' misusing them.) With the monadic region library, the enumerator code becomes simpler: we no longer have to worry about exceptions. The main benefit of monadic region library is to manage files opened by iteratees. The latter being passed around, and so their resources are harder to keep track. We thus demonstrate enumFile and iterFile for incremental file reading and writing, with the same safety guarantees. All opened files are *always* closed, regardless of any (asynchronous) exceptions that may arise during opening, reading, writing or transforming. The code has many examples of throwing errors from various stages of the pipeline and at various times. All files are closed. The commented code is available at http://okmij.org/ftp/Haskell/Iteratee/IterReg.hs which uses the lightweight monadic regions library from http://okmij.org/ftp/Computation/resource-aware-prog/ Since lightweight monadic library needs rank-2 types (now standard), it seemed appropriate to avail ourselves to common GHC extensions. We can clearly see that enumerators and enumeratees unify, both being instances of a general type forall a. Iteratee e mi a -> mo (Iteratee e mi a) which is a Monoid. To compose enumerators or enumeratees, we use the standard mappend. An alias in the code type R e m = Iteratee e m suggests that Iteratees are the view from the right -- the System.IO, getChar-like view. From that point of view, Iteratee IO is hardly different from System.IO (getChar, getLine, peekChar, etc). The dual newtype L e mi mo = L{unL :: forall a. R e mi a -> mo (R e mi a)} is the view from the left. Here are a few examples from the IterReg code. The first simply copies one file to another, block-by-clock. tIF1 = runSIO $ run =<< unL (enumFile "/etc/motd") (iterFile "/tmp/x") According to the trace opened file /etc/motd iterFile: opening /tmp/x Closing {handle: /etc/motd} Closing {handle: /tmp/x} the files are indeed closed, but _not_ in the LIFO order. That is important, so to let an iteratee write data coming from several sources. For example: tIF3 = runSIO $ run =<< unL (enumFile "/etc/motd" `mappend` enumFile "/usr/share/dict/words") (iterFile "/tmp/x") opened file /etc/motd iterFile: opening /tmp/x Closing {handle: /etc/motd} opened file /usr/share/dict/words Closing {handle: /usr/share/dict/words} Closing {handle: /tmp/x} The files will be closed even in case of exceptions: tIF4 = runSIO $ run =<< unL (enumFile "/etc/motd" `mappend` enumFile "/nonexistent") (iterFile "/tmp/x") opened file /etc/motd iterFile: opening /tmp/x Closing {handle: /etc/motd} opened file /nonexistent Closing {handle: /tmp/x} *** Exception: /nonexistent: openFile: does not exist All monadic region monads all support shCatch, so we can write our own exception-handling code. Other examples in IterReg.hs raise errors during data transformation. Monadic regions plus GHC extensions simplify code. For example, here are iterFile and enumFile (the signatures could be omitted; they will be inferred) iterFile :: (SMonad1IO m, m ~ (IORT s' m')) => FilePath -> R ByteString m () iterFile fname = lift (newSHandle fname WriteMode) >>= loop where loop h = getChunk >>= check h check h (Chunk s) = lift (mapM (shPut h) s) >> loop h check h e = return () enumFile :: (SMonadIO m) => FilePath -> L ByteString m m enumFile filepath = L $ \iterv -> do newRgn $ do h <- newSHandle filepath ReadMode unL (enumHandle h) iterv

1 0

How to get Cabal to spit out a .a library suitable for linking into C/Objective-C
by David Pollak 21 Jan '12

21 Jan '12

Howdy, I'm trying to figure out how to get Cabal configured to compile and link my Haskell code such that the code can be part of C and/or Objective-C code such that all the Haskell dependencies are rolled into a .a file and can be linked by a normal C linker (e.g., ld). I've been through http://haskell.org/ghc/docs/6.12.2/html/users_guide/ffi-ghc.html#using-own-… the associated linked, but I'm unable to find out the Cabal incantation to output a library that's linkable into my other code. Any pointers or examples would be greatly appreciated. Thanks, David -- Visi.Pro, Cloud Computing for the Rest of Us http://visi.pro Lift, the simply functional web framework http://liftweb.net Follow me: http://twitter.com/dpp Blog: http://goodstuff.im

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Re: [Haskell-cafe] How to get Cabal to spit out a .a library suitable for linking into C/Objective-C
by Donn Cave 20 Jan '12

20 Jan '12

Sorry about the belated response, but this shouldn't be a problem since it isn't going to be very helpful anyway! I've managed to follow the process described on this page: http://www.haskell.org/haskellwiki/Using_Haskell_in_an_Xcode_Cocoa_project to link Haskell code to a non-Haskell main program, via Xcode. You've probably already seen this, it's mostly about a few iterations of trial and error linking, to get the list of GHC library dependencies, which is what you need whether you're using Xcode or not. If you really need a single library with your code and all those dependencies, that may be technically feasible, though awfully tedious. The only clear problem I encountered while experimenting with it is that GHC run time options need ghc -rtsopts, so they're unavailable if your program isn't built by ghc. I believe I worked around that with a special rtsmain.o, could probably recover the details if that's of interest. Donn

2 1

In-memory Handle for testing
by Simon Hengel 20 Jan '12

20 Jan '12

For testing I want to stub handles, performing all reads and writes in memory (and in process, so no mmap). From looking at the documentation of mkFileHandle[1], I think this should be possible. But it requires some work. Is there already something out there? Cheers, Simon [1] http://hackage.haskell.org/packages/archive/base/4.4.1.0/doc/html/GHC-IO-Ha…

2 2

instance (Enum a) => IArray UArray a
by Mikhail Arefiev 20 Jan '12

20 Jan '12

Is there a reason why there is no instance of (Enum a) => IArray UArray a (other than that it will require OverlappingInstances and/or IncoherentInstances if e. g. UArray of Bools is used in the same code)? I have written one with the help of StackOverflow (http://stackoverflow.com/questions/8941386/) here is the source code: - UArrays.hs: http://hpaste.org/56728 - BenchmarkUArray.hs: http://hpaste.org/56729 - Makefile: http://hpaste.org/56727 Running the benchmark with +RTS -s on the UArray version shows 1,283,705,968 bytes allocated in the heap 113,304 bytes copied during GC 80,027,864 bytes maximum residency (2 sample(s)) 744,128 bytes maximum slop 155 MB total memory in use (0 MB lost due to fragmentation) versus 567,014,904 bytes allocated in the heap 962,651,888 bytes copied during GC 324,111,008 bytes maximum residency (3 sample(s)) 2,887,992 bytes maximum slop 545 MB total memory in use (0 MB lost due to fragmentation) on the boxed (Array Int Color) version. Also, boxed enum time is 1.75s and unboxed is 0.45s. The benchmark may be silly (I am not experienced in testing for Haskell performance), and I would be grateful for any tips on how to improve it to achieve more accurate measurements. Does having such thing make any sense? -- Best regards, Arefiev

2 1

Re: [Haskell-cafe] Why were unfailable patterns removed and "fail" added to Monad?
by Michael Snoyman 20 Jan '12

20 Jan '12

On Jan 20, 2012 8:31 AM, "John Meacham" <john(a)repetae.net> wrote: > > > As expected, no warnings. But if I change this "unfailable" code above > > to the following failable version: > > > > data MyType = Foo | Bar > > > > test myType = do > > Foo <- myType > > return () > > > > I *still* get no warnings! We didn't make sure the compiler spits out > > warnings. Instead, we guaranteed that it *never* will. > > This is actually the right useful behavior. using things like > > do > Just x <- xs > Just y <- ys > return (x,y) > > will do the right thing, failing if xs or ysresults in Nothing. for > instance, in the list monad, it will create the cross product of the > non Nothing members of the two lists. a parse monad may backtrack and > try another route, the IO monad will create a useful (and > deterministic/catchable) exception pointing to the exact file and line > number of the pattern match. The do notation is the only place in > haskell that allows us to hook into the pattern matching mechanism of > the language in a general way. > > John I mention later that this is a "feature, not a bug" to some people, but I'm not one of them. The convenience of having this feature is IMO far outweighed by the cost of the runtime errors it can produce if you use the pattern matching in the wrong monad (e.g., IO, Reader, Writer...). Michael

2 1

partial type annotations
by Nicholas Tung 20 Jan '12

20 Jan '12

Dear all, I wanted to voice support for a partial type annotations. Here's my usage scenario: I have a monad for an imperative EDSL, which has an associated expression data type, class (Monad m, Expression (ExprTyp m)) => MyDSLMonad m where data ExprTyp m :: * -> * and you write imperative EDSL code like so, my_code_block = do x <- instruction1 y <- instruction2 (x + x) ... I want the user to be able to annotate "x is an Integer". However, to do that now, one has to now add a type signature for my_code_block like so, so that the $m$ variable is in scope, my_code_block :: forall m. MyDSLMonad m => m () my_code_block = do x :: ExprTyp m Integer <- instruction1 ... If such a feature were available, one could write a nice type synonym "Expr" and use it like so, type Expr a = ExprTyp _ a my_code_block = do x :: Expr Integer <- instruction1 Suggestions for workarounds are appreciated. I created an `asExprTypeOf`, similar to Prelude's `asExprTyp`, but I don't like the syntax as much. Some previous discussion * http://www.haskell.org/pipermail/haskell/2002-April/009409.html * (a reply) http://www.haskell.org/pipermail/haskell/2002-April/009413.html * http://hackage.haskell.org/trac/haskell-prime/wiki/PartialTypeAnnotations cheers, Nicholas — https://ntung.com — 4432-nstung

6 7

Haddock 2.8.1 and alex 3.0.1
by cwr＠netcom.co.uk 20 Jan '12

20 Jan '12

Building haddock 2.8.1 with alex 3.0.1 I get the error message: [ 7 of 33] Compiling Haddock.Lex ( dist/build/Haddock/Lex.hs, dist/build/Haddock/Lex.o ) src/Haddock/Lex.x:150:17: Couldn't match expected type `AlexInput' against inferred type `(t, t1, t2)' In the first argument of `alexScan', namely `inp' In the expression: alexScan inp sc In the expression: case alexScan inp sc of { AlexEOF -> [] AlexError _ -> error "lexical error" AlexSkip inp' _ -> go inp' sc AlexToken (inp'@(pos', _, _)) len act -> Haddock 2.8.1 builds correctly (or at least completely) with alex 2.3.5 I'm not sure if this is a bug or a feature - if it's a feature, then the dependencies for haddock 2.8.1 need updating. Will

1 0

Not an isomorphism, but what to call it?
by Sean Leather 20 Jan '12

20 Jan '12

I have two types A and B, and I want to express that the composition of two functions f :: B -> A and g :: A -> B gives me the identity idA = f . g :: A -> A. I don't need g . f :: B -> B to be the identity on B, so I want a weaker statement than isomorphism. I understand that: (1) If I look at it from the perspective of f, then g is the right inverse or section (or split monomorphism). (2) If I look at from g, then f is the left inverse or retraction (or split epimorphism). But I just want two functions that give me an identity on one of the two types and I don't care which function's perspective I'm looking at it from. Is there a word for that? Regards, Sean

5 5

Vector (or List) to CArray?
by Dominic Espinosa 19 Jan '12

19 Jan '12

Hello, I'm trying to use the fftw binding, and its functions operate on CArrays of Complex. My data is coming from hsndfile, so it starts out as a Vector of Double. How do I convert this data to CArray? The API functions in the CArray module don't seem to indicate how. Thanks.

4 3

cabal-install bootstrap.sh
by cwr＠netcom.co.uk 19 Jan '12

19 Jan '12

I've just spent most of a morning trying to get bootstrap.sh from the cabal-install package to work. The trick is to use "ghc-pkg init pathname" to initialise the package file - simply adding an empty package file or directory doesn't work. Whoever is responsible for cabal-install, could you please update the README to reflect this requirement. Thanks - Will

1 0

Is there a way to do *allocation* profiling with a breakdown by type?
by Eugene Kirpichov 19 Jan '12

19 Jan '12

Hi, I've got a program that seems to spend much of its time allocating short-lived objects, which therefore don't show up in +RTS -hy or alike. Is there a way to get a breakdown by type of the objects that are being *allocated* but not necessarily retained (disappear after gen0)? -- Eugene Kirpichov Principal Engineer, Mirantis Inc. http://www.mirantis.com/ Editor, http://fprog.ru/

1 0

Conduit versions of wai and warp?
by Erik de Castro Lopo 19 Jan '12

19 Jan '12

Hi Michael, The current versions of wai and warp: http://hackage.haskell.org/package/wai http://hackage.haskell.org/package/warp still seem to be the versions that use enumerator. Any idea when the Conduit versions might show up on Hackage? Until then, should I be grabbing these packages from Github? Cheers, Erik -- ---------------------------------------------------------------------- Erik de Castro Lopo http://www.mega-nerd.com/

4 9

PhD program at Portland State accepting applications for Fall 2012
by Nathan Collins 19 Jan '12

19 Jan '12

Hi, Portland State University has a lot going on in functional programming. The Fall 2012 PhD program application deadline is March 1 for US students and February 1 for international students: http://cs.pdx.edu/programs/admissions FP related work at Portland: - Tim Sheard is working on the Trellys project: design and implementation (in Haskell) of a "practical" dependently typed programming language. Joint project with Aaron Stump at UIowa and Stephanie Weirich at UPenn. Here "practical" means intended for programming more than for theorem proving. Supports theorem proving, but also logically dubious features like general recursion and Type in Type. The key design issue is the interplay between safe and unsafe features. Project repo: https://code.google.com/p/trellys/ - Andrew Tolmach, Mark Jones, and James Hook are working on HASP: design and implementation (in Haskell) of a functional programming language for high-assurance systems programming. Project page: http://hasp.cs.pdx.edu/ - Sergio Antoy is working on narrowing in functional logic programming and the Curry FLP language. Intro to functional logic programming: http://web.cecs.pdx.edu/~antoy/homepage/publications/cacm/paper.pdf PAKCS Curry implementation: http://www.informatik.uni-kiel.de/~pakcs/ - Arthur Peters, a student of Sergio’s, is working on a new implementation of Curry based on a simplified graph rewriting model of functional logic computation. A paper about it and the prototype implementation are available at: http://web.cecs.pdx.edu/~amp4/vialois - Andrew Black is a co-author on a recent paper on Haskell for the Cloud: https://research.microsoft.com/en-us/um/people/simonpj/papers/parallel/ Other FP resources in Portland: - Galois, located a half mile from the computer science department, hosts many "Tech Talks", open to the public: http://corp.galois.com/blog/ - Functional programming study group: http://pdxfunc.org/ Living in Portland: - Portland is a very popular US city, known for beer, bikes, music, and street food: http://en.wikipedia.org/wiki/Portland_Oregon (wikipedia is blacked out today) Cheers, -nathan (PhD student in programming languages)

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Haskell Weekly News: Issue 211
by Daniel Santa Cruz 19 Jan '12

19 Jan '12

Welcome to issue 211 of the HWN, a newsletter covering developments in the Haskell community. This release covers top stories found around the net between January 01 to 14, 2012. This is going to be a shorted version, as I have been out of time. I did not want to let more time go by without a heartbeat from your friendly HWN. Happy coding! You can find an HTML version at: http://contemplatecode.blogspot.com/2012/01/haskell-weekly-news-issue-211.h… Quotes of the Week * <ddarius> xtalmath: #math or #not-math would probably be a more appropriate place to ask. <shachaf> ddarius: Those two channels pretty much cover it all. <ddarius> shachaf: You're clearly not an intuitionist. * monochrom: in fact, freedom is the freedom to write 2.2 = 4 * Jafet: caleskell is the standard golf course. * byorgey: Escaped skolem! Authorities mount massive search. News at 11. * BMeph: loves swapping function arguments, he said flippantly * kmc: "Haskell is great, because Curry-Howard! Proving things in the type system. We can prove that, uh, Ints exist, unless they're [bottom]." * Jafet: In Soviet Russia, State executes you * byorgey: that would be known as "low-dimensional linear algebra", where "low-dimensional" is said in as condescending a tone as possible * monochrom: at a crossroad you meet an angel (always tells the truth) or demon (always lies). the crossroad gives you two choices of roads, one to heaven, one to hell, and only the angel or demon knows which is which. if you ask "how would you answer if I asked 'what is a monad?'", what will happen? Top Reddit Stories * Dropbox tech blog » Using the Dropbox API from Haskell Domain: tech.dropbox.com, Score: 68, Comments: 4 On Reddit: [1] http://goo.gl/rsndV Original: [2] http://goo.gl/g3RyF * A new approach to iteratees Domain: self.haskell, Score: 48, Comments: 99 On Reddit: [3] http://goo.gl/ZRbGW Original: [4] http://goo.gl/ZRbGW * Haskell / Repa Real-Time Ray Tracing Demo Domain: youtube.com, Score: 45, Comments: 7 On Reddit: [5] http://goo.gl/rzyZs Original: [6] http://goo.gl/15P2V * Order theory for computer scientists Domain: matt.might.net, Score: 39, Comments: 1 On Reddit: [7] http://goo.gl/nTOfr Original: [8] http://goo.gl/HAVjQ * Generalizing Streams into Coroutines (with an introduction to functors, applicative functors and arrows) Domain: github.com, Score: 38, Comments: 15 On Reddit: [9] http://goo.gl/hUopY Original: [10] http://goo.gl/avTUf * Well-Typed are hiring: Haskell consultant Domain: well-typed.com, Score: 38, Comments: 71 On Reddit: [11] http://goo.gl/Chu7f Original: [12] http://goo.gl/VHPfJ * Why iteratees are hard to understand Domain: blog.ezyang.com, Score: 34, Comments: 11 On Reddit: [13] http://goo.gl/LrEoV Original: [14] http://goo.gl/d1scG * Modern reall-world Haskell? Domain: self.haskell, Score: 33, Comments: 21 On Reddit: [15] http://goo.gl/l0Aud Original: [16] http://goo.gl/l0Aud * FRP: reactive-banana is getting dynamic event switching Domain: apfelmus.nfshost.com, Score: 30, Comments: 3 On Reddit: [17] http://goo.gl/dGblT Original: [18] http://goo.gl/ovRE5 * wxHaskell and wxWidgets 2.9 Domain: wewantarock.wordpress.com, Score: 29, Comments: On Reddit: [19] http://goo.gl/sxiBx Original: [20] http://goo.gl/878Aj * Updated Yesod blog example: i18n, authentication, authorization, and database Domain: yesodweb.com, Score: 28, Comments: 22 On Reddit: [21] http://goo.gl/4eDAT Original: [22] http://goo.gl/duzSG * Experiments in Haskell Packaging Domain: justhaskell.org, Score: 27, Comments: 5 On Reddit: [23] http://goo.gl/y2vHk Original: [24] http://goo.gl/0nADv * http-conduit Domain: yesodweb.com, Score: 26, Comments: 26 On Reddit: [25] http://goo.gl/gisUJ Original: [26] http://goo.gl/mMhaf * Reasoning about Stream Processing with Effects Domain: personal.cis.strath.ac.uk, Score: 26, Comments: 6 On Reddit: [27] http://goo.gl/3Ckzr Original: [28] http://goo.gl/Uw8ZO * Simon Marlow's Parallel and Concurrent Programming Haskell (slides) Domain: self.haskell, Score: 26, Comments: 3 On Reddit: [29] http://goo.gl/Rg0Lz Original: [30] http://goo.gl/Rg0Lz * Who is my Community? -- wxHaskell maintainer's thoughts on C bindings Domain: wewantarock.wordpress.com, Score: 26, Comments: On Reddit: [31] http://goo.gl/tcqZN Original: [32] http://goo.gl/mRwQa Top StackOverflow Questions * Fun with repeated fmap votes: 26, answers: 1 Read on SO: [33] http://goo.gl/T2FPm * Can a compiler automatically detect pure functions without the type information about purity? votes: 23, answers: 2 Read on SO: [34] http://goo.gl/wgYim * What does “floated out” mean? votes: 21, answers: 2 Read on SO: [35] http://goo.gl/8J3no * Haskell projects to study and learn from? votes: 19, answers: 2 Read on SO: [36] http://goo.gl/SHwsZ * How do you identify monadic design patterns? votes: 19, answers: 4 Read on SO: [37] http://goo.gl/dE3EJ * how do I get good (small) shrinks out of QuickCheck? votes: 16, answers: 0 Read on SO: [38] http://goo.gl/X3ZOs * Which applicative functor is used for passing shared parameters? votes: 14, answers: 1 Read on SO: [39] http://goo.gl/UxmZK * Double map in haskell? votes: 12, answers: 2 Read on SO: [40] http://goo.gl/uWpuz * Higher ranked and impredicative types votes: 12, answers: 1 Read on SO: [41] http://goo.gl/zpkXn * Handling events in Haskell votes: 12, answers: 1 Read on SO: [42] http://goo.gl/e7sV9 * Does the term “monadic” in J have anything to do with its Haskell use? votes: 12, answers: 4 Read on SO: [43] http://goo.gl/hTa2j * Any good articles on optimizing Haskell code? votes: 12, answers: 2 Read on SO: [44] http://goo.gl/FzRrf * What is a “free variable”? votes: 11, answers: 2 Read on SO: [45] http://goo.gl/Og5SC * Haskell Thread Communication Pattern Scenario votes: 10, answers: 1 Read on SO: [46] http://goo.gl/3Rzq0 * Defining a function by equations with different number of arguments votes: 10, answers: 1 Read on SO: [47] http://goo.gl/hejqO * What functionality do you get for free with Functors or other type-classes? votes: 10, answers: 5 Read on SO: [48] http://goo.gl/9hSlv * Why are types with no data constructors valid? votes: 10, answers: 1 Read on SO: [49] http://goo.gl/Pp6Ze * Folding flatMap/bind over a list of functions (a.k.a. Name That Combinator!) votes: 9, answers: 2 Read on SO: [50] http://goo.gl/bBtBI Until next time, Daniel Santa Cruz References 1. http://tech.dropbox.com/?p=129 2. http://www.reddit.com/r/haskell/comments/o0j2a/dropbox_tech_blog_using_the_… 3. http://www.reddit.com/r/haskell/comments/ohjg7/a_new_approach_to_iteratees/ 4. http://www.reddit.com/r/haskell/comments/ohjg7/a_new_approach_to_iteratees/ 5. https://www.youtube.com/watch?v=jBd9c1gAqWs 6. http://www.reddit.com/r/haskell/comments/o0im2/haskell_repa_realtime_ray_tr… 7. http://matt.might.net/articles/partial-orders/ 8. http://www.reddit.com/r/haskell/comments/o6nnp/order_theory_for_computer_sc… 9. https://github.com/leonidas/codeblog/blob/master/2012/2012-01-08-streams-co… 10. http://www.reddit.com/r/haskell/comments/o87z5/generalizing_streams_into_co… 11. http://www.well-typed.com/blog/63 12. http://www.reddit.com/r/haskell/comments/odv3t/welltyped_are_hiring_haskell… 13. http://blog.ezyang.com/2012/01/why-iteratees-are-hard-to-understand/ 14. http://www.reddit.com/r/haskell/comments/o2p3p/why_iteratees_are_hard_to_un… 15. http://www.reddit.com/r/haskell/comments/o5iov/modern_reallworld_haskell/ 16. http://www.reddit.com/r/haskell/comments/o5iov/modern_reallworld_haskell/ 17. http://apfelmus.nfshost.com/blog/2012/01/01-frp-api-0-5.html 18. http://www.reddit.com/r/haskell/comments/nz0af/frp_reactivebanana_is_gettin… 19. http://wewantarock.wordpress.com/2012/01/11/wxhaskell-and-wxwidgets-2-9/ 20. http://www.reddit.com/r/haskell/comments/oc8qi/wxhaskell_and_wxwidgets_29/ 21. http://www.yesodweb.com/blog/2012/01/blog-example 22. http://www.reddit.com/r/haskell/comments/of4g9/updated_yesod_blog_example_i… 23. http://justhaskell.org/2012/01/02/experiments-in-haskell-packaging/ 24. http://www.reddit.com/r/haskell/comments/nzv56/experiments_in_haskell_packa… 25. http://www.yesodweb.com/blog/2012/01/http-conduit 26. http://www.reddit.com/r/haskell/comments/o5b4w/httpconduit/ 27. http://personal.cis.strath.ac.uk/~raa/posts/2012-01-06-streams.html 28. http://www.reddit.com/r/haskell/comments/o5ioy/reasoning_about_stream_proce… 29. http://www.reddit.com/r/haskell/comments/oaqsa/simon_marlows_parallel_and_c… 30. http://www.reddit.com/r/haskell/comments/oaqsa/simon_marlows_parallel_and_c… 31. http://wewantarock.wordpress.com/2012/01/12/who-is-my-community/ 32. http://www.reddit.com/r/haskell/comments/ogtos/who_is_my_community_wxhaskel… 33. http://stackoverflow.com/questions/8736995/fun-with-repeated-fmap 34. http://stackoverflow.com/questions/8760956/can-a-compiler-automatically-det… 35. http://stackoverflow.com/questions/8779390/what-does-floated-out-mean 36. http://stackoverflow.com/questions/8703319/haskell-projects-to-study-and-le… 37. http://stackoverflow.com/questions/8777216/how-do-you-identify-monadic-desi… 38. http://stackoverflow.com/questions/8788542/how-do-i-get-good-small-shrinks-… 39. http://stackoverflow.com/questions/8762943/which-applicative-functor-is-use… 40. http://stackoverflow.com/questions/8735072/double-map-in-haskell 41. http://stackoverflow.com/questions/8739741/higher-ranked-and-impredicative-… 42. http://stackoverflow.com/questions/8780183/handling-events-in-haskell 43. http://stackoverflow.com/questions/8807539/does-the-term-monadic-in-j-have-… 44. http://stackoverflow.com/questions/8832624/any-good-articles-on-optimizing-… 45. http://stackoverflow.com/questions/8857038/what-is-a-free-variable 46. http://stackoverflow.com/questions/8705332/haskell-thread-communication-pat… 47. http://stackoverflow.com/questions/8745597/defining-a-function-by-equations… 48. http://stackoverflow.com/questions/8785311/what-functionality-do-you-get-fo… 49. http://stackoverflow.com/questions/8829194/why-are-types-with-no-data-const… 50. http://stackoverflow.com/questions/8716668/folding-flatmap-bind-over-a-list…

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Error in converting List of Lists into PArray ( Parray a )
by mukesh tiwari 19 Jan '12

19 Jan '12

Hello all I am trying to convert List of Lists ( [[(Int , Double )]] ) into PArray ( PArray ( Int , Double )) but getting run time error. This code works fine and print list of PArray ( Int , Double ) but when i put print $ P.fromList ( map P.fromList c ) then i am getting runtime error. It says "Main: Data/Array/Parallel/PArray/PDataInstances.hs:337:10-30: No instance nor default method for class operation Data.Array.Parallel.PArray.PData.fromListPR". Could some one please tell me how to resolve this issue. Thank you --import ParallelMat import Data.List import System.Environment import Data.Array.Parallel import qualified Data.Array.Parallel.PArray as P processMatrix :: [ [ Double ] ] -> [ [ Double ] ] -> [ ( [ ( Int , Double ) ] , [ ( Int , Double ) ]) ] processMatrix [] [] = [] processMatrix ( x : xs ) ( y : ys ) | ( all ( == 0 ) x ) Prelude.|| ( all ( == 0 ) y ) = processMatrix xs ys | otherwise = ( filter ( \( x , y ) -> y /= 0 ) . zip [ 1..] $ x ,filter ( \( x , y ) -> y /= 0 ) . zip [1..] $ y ) : processMatrix xs ys main = do [ first , second ] <- getArgs a <- readFile first b <- readFile second let a' = transpose . tail . map ( map ( read :: String -> Double ) . words ) . lines $ a b' = tail . map ( map ( read :: String -> Double ) . words ) . lines $ b ( c , d ) = unzip $ processMatrix a' b' print $ ( map P.fromList c ) --print d Macintosh-0026bb610428:Haskell mukesh$ ghc --make -Odph -fdph-par Main.hs [1 of 1] Compiling Main ( Main.hs, Main.o ) Linking Main ... Macintosh-0026bb610428:Haskell mukesh$ ./Main A.in A.in [fromList<PArray> [(1,1.0),(6,1.0)],fromList<PArray> [(4,11.0),(9,11.0)],fromList<PArray> [(1,4.0),(4,2.0),(6,4.0),(9,2.0)]] Putting print $ P.fromList ( map P.fromList c ) Macintosh-0026bb610428:Haskell mukesh$ ghc --make -Odph -fdph-par Main.hs [1 of 1] Compiling Main ( Main.hs, Main.o ) Linking Main ... Macintosh-0026bb610428:Haskell mukesh$ ./Main A.in A.in Main: Data/Array/Parallel/PArray/PDataInstances.hs:337:10-30: No instance nor default method for class operation Data.Array.Parallel.PArray.PData.fromListPR Input file A.in 10 10 1 2 3 0 0 0 0 0 0 4 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 2 11 2 0 1 2 0 0 0 0 0 0 0 1 2 3 0 0 0 0 0 0 4 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 2 11 2 0 1 2 0 0 0 0 0 0 0

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Pattern-matching & substitution for haskell-src-exts?
by Conal Elliott 18 Jan '12

18 Jan '12

Has anyone implemented pattern-matching & substitution for haskell-src-exts? - Conal

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REMINDER: Hac Boston from January 20-22 at MIT
by Edward Z. Yang 18 Jan '12

18 Jan '12

I'd like to remind everyone that Hac Boston, a Haskell hackathon is being held January 20-22, 2012 at MIT (rooms 4-159 and 4-261) in Cambridge, MA. The hackathon will officially kick off at 2:30 Friday afternoon, and go until 5pm on Sunday with the occasional break for sleep. Everyone is welcome -- you do not have to be a Haskell guru to attend! Helping hack on someone else's project could be a great way to increase your Haskell skills. If you plan on coming, please officially register<http://haskell.org/haskellwiki/Hac_Boston/Register>, even if you already put your name on the wiki. Registration, travel, some information about lodging and many other details can now be found on the Hac Boston wiki <http://haskell.org/haskellwiki/Hac_Boston>. Edward Kmett tells me we still have space. We're also looking for a few people interested in giving short (15-20 min.) talks, probably on Saturday afternoon. Anything of interest to the Haskell community is fair game---a project you've been working on, a paper, a quick tutorial. If you'd like to give a talk, add it on <http://goog_145539007>the wiki <http://haskell.org/haskellwiki/Hac_Boston/Talks>. We look forward to seeing you at MIT! Cheers, Edward

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Encouraging performance with ghc 7.4
by Eugene Kirpichov 18 Jan '12

18 Jan '12

Hi cafe, Just wanted to inform you that I've been benchmarking my compute-intensive stuff on ghc 7.0.4 vs 7.4.0, and 7.4.0 gave a large speedup - one program that took 2.9s on a particular input is now taking 2.2s. This result is repeatable. So I encourage people to try out GHC 7.4.0. Some stuff may stop compiling (I had to do 3 basically one-line fixes in 3 packages before I got mine to compile), but I didn't encounter other problems. Below is +RTS -s of two runs: one compiled with ghc 7.4.0 and another with 7.0.4. I can make a more detailed comparison if it's useful and if someone tells me how - I thought about including +RTS -p, but Simon Marlow wrote recently that it has started giving meaningful results only in 7.4, so comparison with 7.0.4 would be unfair. However in this case I'm sure that the program is heavily compute-bound by the following two functions (because it's always compute-bound by them): 1) https://github.com/jkff/timeplot/blob/master/Tools/TimePlot/Plots.hs#L226 2) https://github.com/jkff/timeplot/blob/master/Tools/TimePlot/Conf.hs - localToUTC and strptime. jkff@jkff-laptop ~/projects/logs/... $ tplot -if e.trace -dk 'within[.] cumsum 10' -o e.png +RTS -s tplot -if e.trace -dk within[.] cumsum 10 -o e.png +RTS -s 2,751,756,296 bytes allocated in the heap 135,129,112 bytes copied during GC 33,149,720 bytes maximum residency (22 sample(s)) 1,755,868 bytes maximum slop 56 MB total memory in use (0 MB lost due to fragmentation) Generation 0: 4994 collections, 0 parallel, 0.22s, 0.23s elapsed Generation 1: 22 collections, 0 parallel, 0.08s, 0.09s elapsed INIT time 0.01s ( 0.00s elapsed) MUT time 2.61s ( 2.91s elapsed) GC time 0.30s ( 0.32s elapsed) EXIT time 0.00s ( 0.00s elapsed) Total time 2.91s ( 3.22s elapsed) %GC time 10.1% (9.8% elapsed) Alloc rate 1,051,262,083 bytes per MUT second Productivity 89.6% of total user, 80.9% of total elapsed jkff@jkff-laptop ~/projects/logs/... $ tplot -if e.trace -dk 'within[.] cumsum 10' -o e.png +RTS -s 2,161,811,620 bytes allocated in the heap 107,589,660 bytes copied during GC 34,799,400 bytes maximum residency (22 sample(s)) 1,721,152 bytes maximum slop 58 MB total memory in use (0 MB lost due to fragmentation) Tot time (elapsed) Avg pause Max pause Gen 0 3899 colls, 0 par 0.13s 0.14s 0.0000s 0.0003s Gen 1 22 colls, 0 par 0.08s 0.09s 0.0043s 0.0482s INIT time 0.00s ( 0.00s elapsed) MUT time 2.03s ( 2.28s elapsed) GC time 0.21s ( 0.23s elapsed) EXIT time 0.00s ( 0.00s elapsed) Total time 2.26s ( 2.51s elapsed) %GC time 9.3% (9.1% elapsed) Alloc rate 1,056,397,390 bytes per MUT second Productivity 90.7% of total user, 81.5% of total elapsed -- Eugene Kirpichov Principal Engineer, Mirantis Inc. http://www.mirantis.com/ Editor, http://fprog.ru/

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Lifting IO (IO a) -> IO a to m (m a) -> m a with monad-control
by Edward Z. Yang 18 Jan '12

18 Jan '12

Hello folks, I was curious whether or not it is possible to lift an arbitrary IO (IO a) -> IO a function to MonadBaseControl IO m => m (m a) -> m a. That is, implement a function: liftJoin :: MonadBaseControl mb m => (mb (mb (StM m a)) -> mb (StM m a)) -> m (m a) -> m a The difficulty seems to be that we can't extract the resumable state from the inner base monad action. If this is not possible, is there a suitable strengthening of MonadBaseControl that achieves this effect? Cheers, Edward [1] http://hackage.haskell.org/packages/archive/monad-control/latest/doc/html/C…

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Re: [Haskell-cafe] Need advice: Haskell in Web Client
by dokondr 18 Jan '12

18 Jan '12

On Wed, Jan 18, 2012 at 10:47 AM, John Lenz <lenz(a)math.uic.edu> wrote: > > > I don't see a great need of developing something like GWT for haskell, > since we already have good support for all sorts of existing tools that > span more than just haskell, like extjs, yui, and jqueryui. > Haskell makes my work doable in many areas where other PLs will take enormous efforts and time to achieve the same result. It would be great if I could write Web client code in pure Haskell (no HTML, no DOM, no Ajax, ..) compile it to Javascript and just run it in the browser and do all GUI and backend communication work. Simple as that. Regards, Dmitri.

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strict, lazy, non-strict, eager
by Albert Y. C. Lai 18 Jan '12

18 Jan '12

Most individuals of the Haskell community have long been maintaining a cognitive dissonance; some cases turn into plain hypocrisy. You might excuse it for its ancient and prominent origin: Richard Bird and/or Philip Wadler themselves wrote like "it is too lazy", "make it more strict" 13 years ago and surely more. But perpetuating it is not helping. I have not written this complaint until now because I have been waiting for unmistakable evidence, a smoking gun, a red hand so caught that you cannot explain away, for example you cannot explain that "one sentence is from one person, the other sentence is from a different person". So, on IRC in #haskell, from the same person, speaking on the same topic in the same context, in the same interval of 3 minutes (the first two sentences in the same minute): 1. a function f is strict if f ⊥ = ⊥ 2. ⊥ represents any computation which does not terminate, i.e. an exception or an infinite loop 3. "strict" describes the denotational semantics People, could you please make up your mind already? It has been more than 13 years. Denotational semantics: A. There are no computational steps. There is no termination, and there is no non-termination, since there are no steps to finish, and no steps to keep going. B. ⊥ represents "no information", not "non-termination". There is no "non-termination" to represent. C. fix id = ⊥ because ⊥ is the least fixed point of id, not because fix id non-terminates. There is nothing to terminate or non-terminate. D. You say strict, more strict, less strict; non-strict, more non-strict, less non-strict. You don't say eager, and you don't say lazy. Operational semantics: A. There is no ⊥; it does not appear in any sequence of computational steps, finitely long or infinitely long. B. You say eager, more eager, less eager; lazy, more lazy, less lazy; or speculative, more speculative, less speculative; or any other adjectives for evaluation strategies. You don't say strict, and you don't say non-strict. "Semantics", "semantically speaking": A. Which semantics, which semantically? There are two. B. Actually there are more, but apparently two is already enough to cause all kinds of incoherent statements. If I draw your attention to algebraic semantics, will you start saying "it is too lazy, need to make it more idempotent"?

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implementing a text editor swap file
by Martin DeMello 18 Jan '12

18 Jan '12

I'm writing a gtk2hs-based text editor, and would like to implement continuous (swap-file based) autosave the way vim and emacs do it. Any suggestions for how to implement this in a cross-platform manner? Also, is there a library that returns standard config file locations on a per-platform basis? martin

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haskell tcp server
by Alexander V Vershilov 18 Jan '12

18 Jan '12

Hello. I'm interested if there exists some library like warp but only for tcp. Or maybe some web page with skeleton for such server or with some variants, eg. concurent/single-threaded. I know that each service can have it's specific properties and realization, but there are many common things left. If it matter — my main task is to create server for protobuf protocol, maybe for such a task there are already created solutions. -- Alexander V Vershilov.

2 1

How to make asynchronous I/O composable and safe?
by Joey Adams 18 Jan '12

18 Jan '12

I'm not happy with asynchronous I/O in Haskell. It's hard to reason about, and doesn't compose well. At least in my code. I'm currently trying to build a networking layer for my application using Network.TLS. Here is a rather minimalist API: newtype Connection = Connection (TLSCtx Handle) connectClient :: Handle -- ^ Connection handle, as returned by 'connectTo' -> X509 -- ^ TLS certificate (i.e. public key) -> IO Connection connectServer :: Handle -- ^ Connection handle, as returned by 'accept' -> X509 -- ^ TLS certificate (i.e. public key) -> TLS.PrivateKey -- ^ TLS private key -> IO Connection close :: Connection -> IO () sendMessage :: Connection -> Message -> IO () recvMessage :: Connection -> ByteString -> IO (Message, ByteString) The module provides little more than connection initialization and message serialization. I don't try to use locks or STM to multiplex the connection or, in the case of recvMessage, hide connection state. I just be sure to only use sendMessage in one thread at a time, only use recvMessage in one thread at a time, and marshal the "extra bytes" parameter of recvMessage from call to call (with the help of StateT). I wrote a simple "chat server" to test it. The client turned out okay: main :: IO () main = do cert <- getCertificate handle <- connectTo "localhost" (PortNumber 1337) conn <- connectClient handle cert _ <- forkIO $ forever $ do s <- getLine sendMessage conn $ TestMessage s forever $ flip runStateT B.empty $ do msg <- StateT $ recvMessage conn case msg of TestMessage s -> liftIO $ putStrLn s _ -> liftIO $ hPrintf stderr "Warning: unrecognized message from server: %s\n" (messageTypeName msg) The only glaring problem is that, if the user presses Ctrl+D, the forked (sending) thread dies, but the main (receiving) thread lingers. I'd have to add exception handlers to ensure that when one thread dies, the other thread dies too. However, the server is an abomination (see attachment). Unfortunately, it's not as simple as "spawn one thread per client". We need at least two threads, one to listen for messages from the client, and another to send messages to the client. GHC won't let us simultaneously, in the same thread, wait for input from a connection and wait for an STM transaction to succeed. Another source of complexity is: what if we throw an exception at a thread while it is in the middle of sending a packet? Then we can't shut down the connection properly (i.e. Network.TLS.bye), because the receiver might think the close_notify packet is part of the interrupted packet. Having a thread for each client is good, as it: * Lets us think about each client separately. No need to turn our code inside out or write one big loop that juggles all the clients. * Isolates exceptions. If sendMessage or recvMessage throws an exception, it doesn't bring the whole server down. On the other hand, having multiple threads interact with a single client is hard to think about: * We have to synchronize the threads (e.g. when one dies, kill the other one) * Multiple places where an exception can arise * Can multiple threads interact with the connection handle simultaneously? So why don't I make my connection API handle some of this? Well, I tried. There are so many ways to do it, and I couldn't find a way that simplified usage much. The approach used by Handle and by Network.TLS is to use MVars and IORefs to ensure that, if two threads access the same connection, the connection doesn't become totally corrupt. If I do the same, then I'll have *three* layers of locking under the hood. Worse, the locking done by Handle and Network.TLS doesn't guarantee much. I don't know if it's safe to have one thread sending and another thread receiving. Especially in the case of Network.TLS, where 'recvData' automatically handshakes in some cases, which sends packets. Since I don't know how much thread safety to expect, I can't write networking code and know for sure that it is safe. I'm certainly not protected from interleaved data if multiple threads send on the same handle. For example: import Control.Concurrent import System.IO main :: IO () main = do hSetBuffering stdout NoBuffering _ <- forkIO $ putStrLn "One sentence." putStrLn "Another sentence." produces: AnOonteh esre nsteenntceen.c e. That is, I can't rely on putStrLn being "atomic". To produce intelligible output (without changing the buffering mode), I have to "lock" the output each time I write something. putStrLn doesn't do it for me. === Summary === In Haskell, sound logic and a great type system lead to elegant, composable code in a variety of domains, such as: * Expression evaluation * Parsing * Concurrent programming (thanks to STM) Asynchronous I/O is tricky. However, Haskell currently does little to alleviate the complexity (at least for me). How can we structure network protocol APIs so that they stack well (e.g. only lock once, rather than locking each layer's connection state)? How can we deal with I/O errors without having to think about them at every turn? For now, how can I structure my application's communication API so it's less messy to use? Thanks, - Joey

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