October 2011 - Haskell-Cafe

MonadPlus versus Alternative
by Gregory Crosswhite 30 Oct '11

30 Oct '11

Hey everyone, What is the difference between MonadPlus and Alternative? In my mind, it would make sense for the difference to be that the former provides "and" semantics (i.e., x `mplus` y means do both x and y) whereas the latter provides "or" semantics (i.e., x <|> y means do x or y but not both). However, when I look at the instances defined for List I see that it is exactly the same as MonadPlus. So is there any difference between the interpretation of MonadPlus and Alternative, or is the only difference between them that the former applies to Monad whereas the latter applies to Applicative? Also, along similar lines, why does MonadPlus exist when it is essentially just a special case of Monoid? (That is, any MonadPlus instance could be equivalently cast as a Monoid instance.) Thanks! Greg

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control-c only caught once -- bug?
by Brian Johnson 30 Oct '11

30 Oct '11

Hi, The second time I press control-c, it isn't caught -- the program exits instead. Why? (The context is, I'm writing an interactive program where calculations may take a long time. Control-c during a calculation should return the user to a prompt. As things stand, this can only be done once -- the second calculation so interrupted causes the whole program to exit.) $ ./ctrlctest ^Cuser interrupt ^C -- program exits! $ cat ctrlctest.hs module Main where import Control.Concurrent (threadDelay) import qualified Control.Exception as C main :: IO () main = do (threadDelay 1000000 >> return ()) `C.catch` (\e -> print (e::C.AsyncException)) main $ ghc --version The Glorious Glasgow Haskell Compilation System, version 7.0.3 $ uname -mrsv Darwin 11.2.0 Darwin Kernel Version 11.2.0: Tue Aug 9 20:54:00 PDT 2011; root:xnu-1699.24.8~1/RELEASE_X86_64 x86_64 $ file ctrlctest ctrlctest: Mach-O executable i386

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ANN: Haskell OpenGL package updates
by Jason Dagit 30 Oct '11

30 Oct '11

Several months ago I announced my intent to take over maintainership of the Haskell OpenGL bindings and associated packages. Today I have pushed new minor revisions to hackage. I have updated the following packages: * OpenGL * OpenGLRaw * GLURaw * GLUT The change log is very minor for each of these. For my first release as the new maintainer I opted to be very conservative about changes. Consider this a bug fix release with updated metadata. Please report bugs to either the Haskell OpenGL mailing list or the appropriate issue tracker on the github page: http://www.haskell.org/mailman/listinfo/hopengl https://github.com/haskell-opengl I would like to give a big thank you to the following individuals: Sven Panne Lars Corbjin Dylan Simon Alexey Karakulov Balazs Komuves If you contributed to this release by giving advice, filing a bug report, sending a patch, or in some other way, and I've not listed your name then I apologize for the omission. Going forward I could use help with testing, development, and documentation so if you're interested in OpenGL and Haskell let me know and also keep your eyes out for calls for volunteers. Thank you! Jason

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Package documentation complaints -- and a suggestion
by Max Rabkin 30 Oct '11

30 Oct '11

Hi all Following a link from the Yesod book, I arrived at [1], curious to find out what groundhog was. Once there, I learned... nothing: "This library provides just the general interface and helper functions. You must use a specific backend in order to make this useful." [1] http://hackage.haskell.org/package/groundhog Hoping to find more, I clicked on the top-level module. Once again, the text here was uninformative, though from the example it seems to be something to do with databases (and it is to be applauded for including an example in the docs). So, package authors: PLEASE, tell me what your package does in the description. Tell me *again* in the top-level haddocks (I may have come directly there by a link). And then, tell me where to look for more information. Instead of "This module defines the functions and datatypes used throughout the framework. Most of them are for internal use", tell me which are *not* for internal use. Write documentation for people who don't know how to use your package. Some people prefer to have this outside of the haddocks (I don't like this because papers and blogs are less likely to stay up to date than what is embedded in the file, but for tutorials etc. a website might be more appropriate) but if so link to it from both the Haddocks and the package description. But I also have a concrete suggestion for Hackage: include the package synopsis on the package's page. The distinction between synopsis and description can be confusing, and sometimes it seems to violate DRY to have the same info in both. To the author of groundhog: I hope you are not offended by my picking on your package. It's not the only culprit -- just the one that pushed me over the edge. Of course, I would be pleased if this email encouraged you to fix it, but I am addressing this rant to the Haskell community because the problem is a cultural one. --Max

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What library package fulfills these requirements?
by Joosten, Han 29 Oct '11

29 Oct '11

I am planning to give a workshop on FP using Haskell. The audience will be programmers with quite a bit of experience with conventional languages like Java and .net . I want to give them some feeling about FP. And hopefully, they will become interested so they want more... A couple of years ago, Simon Peyton Jones gave a splendid presentation<http://www.bestechvideos.com/2007/08/09/oscon-2007-simon-peyton-jones-a-tas…> with a comparable goal. He used Xmonad as an example to show some 'real code'. I like this idea, because Xmonad contains some code that is suitable to explain (not too hard to understand), and still gives a feeling about the power of Haskell. However, Xmonad needs X11 to run. Me and most people in the audience are stuck to windows (no flames please ;-s ) , and don't want to fiddle around with cygwin or the likes) I am wondering what package is suitable to be used as an example too? It needs to fulfill at least the following requirements: + I have to be able to explain the purpose of the software in no more than 1 or 2 minutes + There should be parts of the code that can be easily linked to the purpose /use of the package + These code parts must show the 'prettiness' of Haskell + It would be nice if there is something GUI-like to demo, which runs under windows. + I prefer not to use some kind of a compiler as an example. Now the question is: What package would you suggest, and of course, why? Thanks! Han Joosten. Dit bericht is vertrouwelijk en kan geheime informatie bevatten enkel bestemd voor de geadresseerde. Indien dit bericht niet voor u is bestemd, verzoeken wij u dit onmiddellijk aan ons te melden en het bericht te vernietigen. Aangezien de integriteit van het bericht niet veilig gesteld is middels verzending via internet, kan Atos Nederland B.V. niet aansprakelijk worden gehouden voor de inhoud daarvan. Hoewel wij ons inspannen een virusvrij netwerk te hanteren, geven wij geen enkele garantie dat dit bericht virusvrij is, noch aanvaarden wij enige aansprakelijkheid voor de mogelijke aanwezigheid van een virus in dit bericht. Op al onze rechtsverhoudingen, aanbiedingen en overeenkomsten waaronder Atos Nederland B.V. goederen en/of diensten levert zijn met uitsluiting van alle andere voorwaarden de Leveringsvoorwaarden van Atos Nederland B.V. van toepassing. Deze worden u op aanvraag direct kosteloos toegezonden. This e-mail and the documents attached are confidential and intended solely for the addressee; it may also be privileged. If you receive this e-mail in error, please notify the sender immediately and destroy it. As its integrity cannot be secured on the Internet, the Atos Nederland B.V. group liability cannot be triggered for the message content. Although the sender endeavours to maintain a computer virus-free network, the sender does not warrant that this transmission is virus-free and will not be liable for any damages resulting from any virus transmitted. On all offers and agreements under which Atos Nederland B.V. supplies goods and/or services of whatever nature, the Terms of Delivery from Atos Nederland B.V. exclusively apply. The Terms of Delivery shall be promptly submitted to you on your request. Atos Nederland B.V. / Utrecht KvK Utrecht 30132762

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Libraries to compare trees?
by dokondr 28 Oct '11

28 Oct '11

Please advise on Haskell libraries to compare trees in textual representation. I need to compare both structure and node contents of two trees, find similar sub-trees, and need some metric to measure distance between two trees. Also need advice on simple parser to convert textual tree representation into a data type convenient for tree manipulation (comparison, matching, etc.) What data type to use for trees with arbitrary structure? Example trees: *** Tree 1: (ROOT (S (NP (DT The) (NN voice) (NN quality)) (VP (VBD was) (ADJP (JJ clear) (RB too))) (. .))) *** Tree 2: (ROOT (S (SBAR (IN Although) (S (NP (DT the) (NN battery) (NN life)) (VP (VBD was) (RB not) (ADJP (JJ long))))) (, ,) (NP (DT that)) (VP (VBZ is) (VP (VBN ok) (PP (IN for) (NP (PRP me))))) (. .))) Thanks! Dmitri

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What is the status of GPU-accelerated Haskell?
by Rafael Gustavo da Cunha Pereira Pinto 28 Oct '11

28 Oct '11

Hi folks, I just bought a NVidia Fermi-based card and remembered reading a few months (years?) ago about some effort to accelerate array processing in Haskell using GPUs. How is this going on? Any progresses? Do we have GPU based DPH already? (the last one is a joke...) I keep thinking on the advantages of such GPU data processing on the signal processing field. Best regards, Rafael Gustavo da Cunha Pereira Pinto

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A catch-all MonadIO instance
by Bas van Dijk 28 Oct '11

28 Oct '11

Hello, Is it unsafe to add the following catch-all MonadIO instance to transformers' Control.Monad.IO.Class module? {-# LANGUAGE FlexibleInstances, FlexibleContexts, UndecidableInstances #-} instance (MonadTrans t, Monad (t m), MonadIO m) => MonadIO (t m) where liftIO = lift . liftIO It could get rid of all the similarly looking instances: instance (MonadIO m) => MonadIO (ReaderT r m) where liftIO = lift . liftIO instance (MonadIO m) => MonadIO (StateT s m) where liftIO = lift . liftIO instance (Monoid w, MonadIO m) => MonadIO (WriterT w m) where liftIO = lift . liftIO ... The reason I ask is that I want to do something similar for monad-control's MonadControlIO type class. But I'm not sure if I don't introduce any undecidability in the type-checker. Regards, Bas

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Reminder Munich Haskell
by Heinrich Hördegen 28 Oct '11

28 Oct '11

Hi all, this evening, Haskeller meet at Cafe Puck at 19h30: www.haskell-munich.de See you, Heinrich -- -- hoerdegen(a)funktional.info www.funktional.info --

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When does the UNPACK pragma work?
by Eugene Kirpichov 28 Oct '11

28 Oct '11

Hi, I'm trying to speed up Data.Time (the time package) 'cause my program is spending a very substantial fraction of time manipulating dates. I decided to start with strictifying and unpacking the date/time types, as there's no point in having them be lazy and I indeed had a lot of allocation in date/time arithmetic in my performance profile. data UTCTime = UTCTime { utctDay :: {-# UNPACK #-} !Day, utctDayTime :: {-# UNPACK #-} !DiffTime } (unpacks and strictness mine) newtype Day = ModifiedJulianDay {toModifiedJulianDay :: Integer} newtype DiffTime = MkDiffTime Pico And Pico is also essentially a newtype for Integer. So, I'm getting warnings on this definition of UTCTime. QUESTION: Is it the case that I can only UNPACK primitive fields, and not even their newtypes? Data\Time\Clock\UTC.hs:30:16: Warning: Ignoring unusable UNPACK pragma on the first argument of `UTCTime' In the definition of data constructor `UTCTime' In the data type declaration for `UTCTime' Data\Time\Clock\UTC.hs:30:16: Warning: Ignoring unusable UNPACK pragma on the second argument of `UTCTime' In the definition of data constructor `UTCTime' In the data type declaration for `UTCTime' -- Eugene Kirpichov Principal Engineer, Mirantis Inc. http://www.mirantis.com/ Editor, http://fprog.ru/

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