- Haskell-Cafe - Haskell.org

CFG specification and analysis directly in Haskell
by Anton Tayanovskyy 30 Sep '11

30 Sep '11

Hi, As a weekend hack, I just realized that Haskell has this wonderful DoRec syntax that among other things seems to be able to let the user express context-free grammars together with their processing rules in normal Haskell code, without template Haskell or anything like that, just like parser combinators. I am just wondering if this is this a known and useful result? Are there libraries doing similar stuff? I wrote up the Earley algorithm to demonstrate that one can in principle analyze the complete grammar (https://github.com/toyvo/haskell-earley) The result derivations `cheat` by using Data.Dynamic, but the result is quite pleasing, for example one can do: grammar :: G.Grammar (G.Rule Char E) grammar = do nat <- G.rule "NAT" [ fmap (\_ -> 0) (G.term '0') , fmap (\_ -> 1) (G.term '1') ] rec expr <- G.rule "EXPR" [ fmap Nat $ G.var nat , pure (\x _ y -> Add x y) <*> G.var expr <*> G.term '+' <*> G.var expr ] return expr Thanks, Anton

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Turn GC off
by Andreas Voellmy 30 Sep '11

30 Sep '11

Hi everyone, Is there a way to completely turn garbage collection off in the Haskell runtime system? I'm aware of the -A runtime option, but I'd like to completely turn it off, if possible. I'm OK with running the program until it runs out of memory, and I'm willing to recompile GHC if needed. Regards, Andreas

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Programming with arrows, exercises
by Sergey Mironov 29 Sep '11

29 Sep '11

Hello. I am reading "Programming with Arrows" by John Hughes (very helpful and interesting!), the book has an exercises requiring a module called Circuits for checking the answer. There should be things like class ArrowCircuit and various functions related to digital logic circuits simulation. Does anybody know where can I find one? Thanks, Sergey

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Cabal rebuilding of the C++ code for wxHaskell
by Dave Tapley 29 Sep '11

29 Sep '11

Hi all, I've been trying to resolve a compile time issue[1] with wxHaskell, and I thought I'd throw it open to see if anyone on cafe can help. Here's the crux of the issue: The Setup.hs for wxcore (the major component of wxHaskell) uses "simpleUserHooks", overriding only "confHook". However there is also "cleanHook", which is defined by simpleUserHooks to be: cleanHook = \p _ _ f -> clean p f, If you consult the source for clean[2] you'll see that it tries to "remove the whole dist/ directory rather than tracking exactly what files we created in there". I presume that's why we have to do a full re-build every time? To try and circumvent this I modified the definition of "main" in Setup.hs to this: main = defaultMainWithHooks simpleUserHooks { confHook = myConfHook, cleanHook = (\_ _ _ _ -> return ())} Unfortunately it still seems to re-build all the C++ on each 'install' from cabal. Not sure why? Dave, [1] http://sourceforge.net/mailarchive/message.php?msg_id=28099997 [2] Taken from http://www.haskell.org/ghc/docs/6.10.4/html/libraries/Cabal/src/Distributio… -- Cleaning clean :: PackageDescription -> CleanFlags -> IO () clean pkg_descr flags = do let distPref = fromFlag $ cleanDistPref flags notice verbosity "cleaning..." maybeConfig <- if fromFlag (cleanSaveConf flags) then maybeGetPersistBuildConfig distPref else return Nothing -- remove the whole dist/ directory rather than tracking exactly what files -- we created in there. chattyTry "removing dist/" $ do exists <- doesDirectoryExist distPref when exists (removeDirectoryRecursive distPref) -- these live in the top level dir so must be removed separately removeRegScripts -- Any extra files the user wants to remove mapM_ removeFileOrDirectory (extraTmpFiles pkg_descr) -- If the user wanted to save the config, write it back maybe (return ()) (writePersistBuildConfig distPref) maybeConfig where removeFileOrDirectory :: FilePath -> IO () removeFileOrDirectory fname = do isDir <- doesDirectoryExist fname isFile <- doesFileExist fname if isDir then removeDirectoryRecursive fname else if isFile then removeFile fname else return () verbosity = fromFlag (cleanVerbosity flags)

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HackageDB User Account
by Jonathan Frywater 29 Sep '11

29 Sep '11

How does one go about getting an account? I sent an email to the address provided at http://hackage.haskell.org/packages/accounts.html but haven't received any response yet. Since it's been over 3 weeks, I decided to try my luck here.

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LLVM: function pointer in global struct
by Tom Schouten 29 Sep '11

29 Sep '11

Hi everyone, Using the LLVM bindings, I'm trying to create an initialized global struct variable containing a pointer to a function. > {-# LANGUAGE ScopedTypeVariables #-} > import LLVM.Core > import Data.Word > import LLVM.Util.File(writeCodeGenModule) > sm_module = do > tick :: Function (Word32 -> IO Word32) <- createFunction ExternalLinkage $ \x -> ret x > info <- createNamedGlobal False ExternalLinkage "sm_info" $ constStruct (tick & ()) > return info I run into a complaint that this pointer isn't constant when it's part of a global variable initializer. No instance for (llvm-0.10.0.1:LLVM.Core.CodeGen.IsConstStruct (Function (Word32 -> IO Word32) :& ()) a0) arising from a use of `constStruct' Possible fix: add an instance declaration for (llvm-0.10.0.1:LLVM.Core.CodeGen.IsConstStruct (Function (Word32 -> IO Word32) :& ()) a0) In the second argument of `($)', namely `constStruct (tick & ())' In a stmt of a 'do' expression: info <- createNamedGlobal False ExternalLinkage "sm_info" $ constStruct (tick & ()) In a stmt of a 'do' expression: tick :: Function (Word32 -> IO Word32) <- createFunction ExternalLinkage $ \ x -> ret x I suppose this is because of > -- |A function is simply a pointer to the function. > type Function a = Value (Ptr a) being a Value instead of ConstValue. Is there a way around this? Cheers Tom

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PhD studentship in Nottingham
by Graham Hutton 29 Sep '11

29 Sep '11

Dear all, I am currently advertising a PhD Studentship in Functional Programming at the University of Nottingham in the UK: http://tinyurl.com/phd-nottingham Due to the nature of the funding, the position is only to UK/EU students who have resided in the UK for the last three years. If you are interested in applying yourself, please drop me a note by email. If you know of any good candidates who may be interested in applying, or there is a local mailing list for advertising such things, I'd be much obliged if you could pass on the above link. Many thanks, Graham Hutton -- Prof Graham Hutton Functional Programming Lab School of Computer Science University of Nottingham, UK http://www.cs.nott.ac.uk/~gmh This message and any attachment are intended solely for the addressee and may contain confidential information. If you have received this message in error, please send it back to me, and immediately delete it. Please do not use, copy or disclose the information contained in this message or in any attachment. Any views or opinions expressed by the author of this email do not necessarily reflect the views of the University of Nottingham. This message has been checked for viruses but the contents of an attachment may still contain software viruses which could damage your computer system: you are advised to perform your own checks. Email communications with the University of Nottingham may be monitored as permitted by UK legislation.

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network 2.3.05 on windows 7 with ghc-7.2.1
by Sai Hemanth K 29 Sep '11

29 Sep '11

Hi, Apologies if this is an off-topic for the list, If someone here managed to build network 2.3.05 on windows 7 with ghc-7.2.1 , could you kindly pass me the trick? The config step fails with missing header. I tried running it with mingw (tried with the latest one too - just in case : http://sourceforge.net/projects/mingw/files/Automated%20MinGW%20Installer/) I did not face any issue for the same thing on linux. Many thanks, Hemanth K

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Haskell Weekly News: Issue 201
by Daniel Santa Cruz 29 Sep '11

29 Sep '11

Welcome to issue 201 of the HWN, a newsletter covering developments in the Haskell community. This release covers the week of September 18 to 24, 2011. You can find the HTML version of this issue at: http://contemplatecode.blogspot.com/2011/09/haskell-weekly-news-issue-201.h… Announcements Jasper Van der Jeugt sent more details about the 9th GhentFPG meeting, which will take place on October the 4th, at 19:30 in the Technicum building of Ghent University. "You don't have to be a Functional Programming guru to attend, everyone eager to learn is welcome." [1] http://goo.gl/hAa5Zn New and Updated Projects * usb, bindings-libusb, usb-iteratee (Bas van Dijk; 1.0, 1.4.4, 0.4) Wrapers around the portable C library libusb-1.0 [2] http://goo.gl/lTMNJn * ivy-web (James Deng; 0.1) A lightweight web framework, with type-safe routes, based on invertible syntax, and i18n support, influenced by Django, Snap and Yesod. [3] http://goo.gl/dTcPEn * lushtags (Bit Connor) ctags compatible tags files for Haskell programs. [4] http://goo.gl/30TYen * lightweight monadic regions family (Bas van Dijk; multiple versions) Updates to regios, regions-mtl, regions-monadstf, regional-pointers, safer-file-handles, safer-file-handles-bytestring, safer-file-handles-text, and usb-safe. [5] http://goo.gl/g8P6Kn * skein (Felipe Almeida Lessa; 0.1) A family of fast secure cryptographic hash functions. [6] http://goo.gl/xwafmn Quotes of the Week * dafis: I thought a C++ friend class was one you allowed to touch your private parts * edwardk: any time i can help, just ask. i may not actually help, but i'll be sure to note your suffering. * edwardk: i have a lot fewer [haddock problems] now since i stopped bothering trying to document ;) * edwardk: Had i thought of it earlier i could have named trifecta kessel run. at this point it looks like it'll contain at least 12 parsecs. * ddarius: The history of iteratees is making a simple idea complex. * shachaf: You can never escape having learned monads. If you learn two monads, though, you can go back to only knowing one. Top Reddit Stories * GHCi now supports all top-level declarations Domain: github.com, Score: 104, Comments: 35 On Reddit: [7] http://goo.gl/GfTc9n Original: [8] http://goo.gl/Md5B3n * The best ghc bug ever Domain: twitter.com, Score: 93, Comments: 27 On Reddit: [9] http://goo.gl/3sCHUn Original: [10] http://goo.gl/N7IGdn * Running a startup on Haskell Domain: bos.github.com, Score: 73, Comments: 33 On Reddit: [11] http://goo.gl/HevD3n Original: [12] http://goo.gl/IsXedn * No chrome, Haskell is not Danish. Domain: i.imgur.com, Score: 73, Comments: 26 On Reddit: [13] http://goo.gl/AYyq4n Original: [14] http://goo.gl/whKijn * Did Haskell Platform miss the July release? It's already September. Domain: trac.haskell.org, Score: 46, Comments: 12 On Reddit: [15] http://goo.gl/6m8Yln Original: [16] http://goo.gl/r9k6tn * Iteratees at Tsuru Capital: various contributions to the hackage iteratee library used in a production trading system Domain: blog.kfish.org, Score: 36, Comments: 12 On Reddit: [17] http://goo.gl/rCTO6n Original: [18] http://goo.gl/PikaAn * Apprendre Haskell vous fera le plus grand bien ! -- LYAH in French Domain: lyah.haskell.fr, Score: 33, Comments: 15 On Reddit: [19] http://goo.gl/pV3jvn Original: [20] http://goo.gl/ixiXWn * Would You Bet $100,000,000 on Haskell? Domain: digitalmars.com, Score: 28, Comments: 44 On Reddit: [21] http://goo.gl/oPI5Kn Original: [22] http://goo.gl/YYeOln * Galois offers Haskell course: The Tao of Functional Programming Domain: corp.galois.com, Score: 26, Comments: 9 On Reddit: [23] http://goo.gl/6HX8Kn Original: [24] http://goo.gl/n7urdn Top StackOverflow Questions * What are Haskell's strictness points? votes: 59, answers: 7 Read on SO: [25] http://goo.gl/5PiDHn * Can someone explain the traverse function in Haskell votes: 14, answers: 3 Read on SO: [26] http://goo.gl/P5WtFn * Why do we have map, fmap and liftM? votes: 13, answers: 1 Read on SO: [27] http://goo.gl/TCRKwn * Does a function like this already exist? (Or, what's a better name for this function?) votes: 12, answers: 4 Read on SO: [28] http://goo.gl/QY6i8n * Question about the ~ and @ operators in Haskell votes: 11, answers: 2 Read on SO: [29] http://goo.gl/pXnV6n * Folding across Maybes in Haskell votes: 8, answers: 5 Read on SO: [30] http://goo.gl/y1ALtn * How is foldl lazy? votes: 7, answers: 4 Read on SO: [31] http://goo.gl/LQAxNn * avoid explicit passing of lookup table votes: 7, answers: 4 Read on SO: [32] http://goo.gl/tKh2wn * Why is [Char]-based input so much slower than the [Char]-based output in Haskell? votes: 7, answers: 1 Read on SO: [33] http://goo.gl/2nuqNn Until next time, Daniel Santa Cruz References 1. http://permalink.gmane.org/gmane.comp.lang.haskell.cafe/92680 2. http://permalink.gmane.org/gmane.comp.lang.haskell.general/18935 3. http://permalink.gmane.org/gmane.comp.lang.haskell.general/18936 4. http://permalink.gmane.org/gmane.comp.lang.haskell.cafe/92480 5. http://permalink.gmane.org/gmane.comp.lang.haskell.general/18940 6. http://permalink.gmane.org/gmane.comp.lang.haskell.cafe/92528 7. https://github.com/ghc/ghc/commit/3db757241ce7fb99c096c30481aefa86bb9855a1 8. http://www.reddit.com/r/haskell/comments/kmxf2/ghci_now_supports_all_toplev… 9. http://twitter.com/#!/bos31337/status/116372971509121025 10. http://www.reddit.com/r/haskell/comments/kmk6q/the_best_ghc_bug_ever/ 11. http://bos.github.com/strange-loop-2011/talk/talk.html#(1) 12. http://www.reddit.com/r/haskell/comments/km2b8/running_a_startup_on_haskell/ 13. http://i.imgur.com/D4QEO.png 14. http://www.reddit.com/r/haskell/comments/koom1/no_chrome_haskell_is_not_dan… 15. http://trac.haskell.org/haskell-platform/wiki/ReleaseTimetable 16. http://www.reddit.com/r/haskell/comments/kk9hl/did_haskell_platform_miss_th… 17. http://blog.kfish.org/2011/09/iteratees-at-tsuru.html 18. http://www.reddit.com/r/haskell/comments/kjem1/iteratees_at_tsuru_capital_v… 19. http://lyah.haskell.fr/ 20. http://www.reddit.com/r/haskell/comments/km1l6/apprendre_haskell_vous_fera_… 21. http://www.digitalmars.com/d/archives/digitalmars/D/Would_You_Bet_100_000_0… 22. http://www.reddit.com/r/haskell/comments/klhln/would_you_bet_100000000_on_h… 23. http://corp.galois.com/haskell-course 24. http://www.reddit.com/r/haskell/comments/kp6as/galois_offers_haskell_course… 25. http://stackoverflow.com/questions/7490768/what-are-haskells-strictness-poi… 26. http://stackoverflow.com/questions/7460809/can-someone-explain-the-traverse… 27. http://stackoverflow.com/questions/7463500/why-do-we-have-map-fmap-and-liftm 28. http://stackoverflow.com/questions/7518589/does-a-function-like-this-alread… 29. http://stackoverflow.com/questions/7506684/question-about-the-and-operators… 30. http://stackoverflow.com/questions/7464208/folding-across-maybes-in-haskell 31. http://stackoverflow.com/questions/7487455/how-is-foldl-lazy 32. http://stackoverflow.com/questions/7504012/avoid-explicit-passing-of-lookup… 33. http://stackoverflow.com/questions/7510078/why-is-char-based-input-so-much-…

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instance Enum Double considered not entirely great?
by Evan Laforge 28 Sep '11

28 Sep '11

So I've been excising occurrences of the [n..m] syntax from my code. The reason is that I had been using [0..5] as a shorthand for [0, 1, 2, 3, 4, 5], mostly in tests. This works for integral types and it appears to work for floating point types. Then I tried switching to a fixed point format, and discovered my mistake. Enum is supposed to enumerate every value between the two points, and the result is memory exhaustion. I had already been switching from enumFrom because they accumulate floating point errors, in favor of a couple of more appropriate functions: -- | Enumerate an inclusive range. Uses multiplication instead of successive -- addition to avoid loss of precision. -- -- Also it doesn't require an Enum instance. range :: (Num a, Ord a) => a -> a -> a -> [a] range start end step = go 0 where go i | val > end = [] | otherwise = val : go (i+1) where val = start + (i*step) -- | Like 'range', but includes the end. range_end :: (Num a, Ord a) => a -> a -> a -> [a] range_end start end step = go 0 where go i | val >= end = [end] | otherwise = val : go (i+1) where val = start + (i*step) -- | Infinite range. range_ :: (Num a) => a -> a -> [a] range_ start step = go 0 where go i = start + (i*step) : go (i+1) So I think the Enum instances for Float and Double are sort of a trap. They misled me into thinking the .. syntax is the range function defaulting to a step of 1, led to precision errors I had to debug then fix (and I've noticed lead to the odd stack overflow question), then led to more disaster when I switched to an Enum instance that lived up to Enum's real purpose. The problem is that the [..] syntax is convenient and tempting, especially for tests, wanting a numeric range (rather than enumeration) is very common, and as far as I know the 'range' function isn't in the standard library. So it seems like one of these things that seem convenient in the short term but come around and bite you later. Using the right functions from the start avoids all that. Is there any support for the idea of removing Enum instances for floating point numbers?

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