- Haskell-Cafe - Haskell.org

Need comments on a libusb asynchronous select/poll loop
by John Obbele 20 Apr '11

20 Apr '11

I was reading the recent thread about select/poll, events handling and why forkIO is the only thing you should need (sorry if this is a horrible summary ;) and I'm realizing I could use some advice concerning my last project. I'm trying to implement an asynchronous interface to libusb, re-using the raw bindings-usb (by Maurício C. Antunes) and partially copying what can be found in the (great but synchronous-only) usb package (from Bas van Dijk). The issues are: 1) I don't know how to expose the asynchronous API to Haskell. (How would you 'functionalize' it ?) 2) I am messing with lots of FFI calls around epoll and libusb and would appreciate if someone could sanitize my approach. A pre-alpha-use-at-your-own-risk code preview is available there[1]. [1]: https://patch-tag.com/r/obbele/usb-asynchronous/home So far, my answer to both issues was: Issue 1) Create an 'AsyncManager' data type to use in code like the following: >readAsyncIO = do $ > dev <- getDeviceByVIDPID > amanager <- newAsyncManager dev iface > -- AsyncManager set a background thread to poll libusb for new events. > > sendURB amanager dummyURB -- non-blocking call > urb <- getURB amanager -- blocking > putStr . pprintURB $ urb > > closeAsyncManager amanager > > where iface = 0 > dummyURB = newControlTransfer … Any opinions on this model ? (N.B.: to keep things simple, I do not yet plan to add a 'cancel' feature to this binding). Issue 2) What libusb asynchronous API requires is the following: 1. initialize resources, get device handles, etc. 2. get a set of file descriptors and select/poll them for read or write access. 3. register USB transfers 4. when the transfers are completed, libusb will signal it on one of the file descriptors. (but we don't know which one) 5. when your select/poll awakes from one event (it doesn't matter which one), you call libusb_handle_events which will flush every pending transfer by calling its associated callback function. This results in the C runtime calling back an Haskell function (thus the RTS will spawn an OS thread to handle it -- which doesn't bug anymore since GHC 7.0.2.x :) 6. GOTO 3 or 4 Complete libusb idiosyncrasies are explained in [2] and [3]: [2]: http://libusb.sourceforge.net/api-1.0/group__asyncio.html [3]: http://libusb.sourceforge.net/api-1.0/mtasync.html So far, I didn't find a 'blessed' FFI interface to select/poll the set of file descriptors and I'm currently using Toralf Wittner's Epoll package. (which is very fine albeit a linux-only hack). I am wondering if anyone know a magic wand (or abstraction model) to ease my headaches in Haskell. Something that could be more cross-platform ? Something that could be well-integrated with the (GHC) FFI ? Something that could be ... simple ? any help appreciated ,) /John P.S.: and third question : how the heck am I supposed to debug threaded FFI code in Haskell ? With GDB ? Yes I'm particularly upset about background Haskell threads which die silently (e.g. when encountering a segfault) and dead-lock the whole RTS.

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hackage not building documentation?
by Doug Burke 20 Apr '11

20 Apr '11

I uploaded a package to hackage last Saturday (16th) and the docs are not displayed yet (Wed, 20th), but it doesn't appear to be because of a build failure: http://hackage.haskell.org/package/swish Is this a known issue? Thanks, Doug

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Re: [Haskell-cafe] An example of enumerator processing some data between input and output
by oleg＠okmij.org 20 Apr '11

20 Apr '11

Paul Sujkov wrote: > I played a bit with the enumerator package, and I'm quite stuck with the > question how to duplex data to two (or more) consumers Many packages have the combinator named enumPair (or something like that). Here is one example http://okmij.org/ftp/ftp/Streams.html#1enum2iter The combinator is quite useful for logging, for dumping the contents of the stream as it is being processed. > actually I need to pipeline output of one consumer to another > (outputting text after making some internal representation - with Show > instance - with the parseData). You need enumeratees, which convert one stream into another. There are many examples of those: e.g., converting a stream of bytes into a stream of lines, or a stream of compressed/encoded data into the plaintext stream. You enumerator package probably has them. The Stream.html page also describes many examples of enumeratees.

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Message could not be delivered
by nanardon＠nanardon.zarb.org 20 Apr '11

20 Apr '11

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Unicode normalization
by wren ng thornton 20 Apr '11

20 Apr '11

Hello all, I'm in need of a Unicode normalization function, Utf8 NFC for ByteString in particular. From some quick Googling around it looks like the only available option is to use ICU in some form. The text-icu package has a nice binding to it, but unfortunately that means a lot of redundant conversions (Utf8 ByteString -> Text; Text -> Utf8 ByteString) and an additional rather large non-Haskell dependency[1]. Is ICU really the only available implementation of normalization? The TR15 doesn't really give a complete algorithm and only hints at the "numerous opportunities for optimization" implicit in the complexity of the spec. [1] Which is especially annoying on OSX since OSX does ship with libicu in a public location, but it doesn't provide header files and apparently it's incomplete somehow, meaning you'd have to reinstall it for text-icu to use (and hilarity ensues when your copy gets out of sync with the OS's). -- Live well, ~wren

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CFT -- Haskell Implementors' Workshop 2011
by Ben Lippmeier 20 Apr '11

20 Apr '11

Call for Talks ACM SIGPLAN Haskell Implementors' Workshop http://haskell.org/haskellwiki/HaskellImplementorsWorkshop/2011 Tokyo, Japan, September 23rd, 2011 The workshop will be held in conjunction with ICFP 2011 http://www.icfpconference.org/icfp2011/ Important dates Proposal Deadline: 22nd July 2011 Notification: 8th August 2011 Workshop: 23rd September 2011 The Haskell Implementors' Workshop is to be held alongside ICFP 2011 this year in Tokyo, Japan. There will be no proceedings; it is an informal gathering of people involved in the design and development of Haskell implementations, tools, libraries, and supporting infrastructure. This relatively new workshop reflects the growth of the user community: there is a clear need for a well-supported tool chain for the development, distribution, deployment, and configuration of Haskell software. The aim is for this workshop to give the people involved with building the infrastructure behind this ecosystem an opportunity to bat around ideas, share experiences, and ask for feedback from fellow experts. We intend the workshop to have an informal and interactive feel, with a flexible timetable and plenty of room for ad-hoc discussion, demos, and impromptu short talks. Scope and target audience ------------------------- It is important to distinguish the Haskell Implementors' Workshop from the Haskell Symposium which is also co-located with ICFP 2011. The Haskell Symposium is for the publication of Haskell-related research. In contrast, the Haskell Implementors' Workshop will have no proceedings -- although we will aim to make talk videos, slides and presented data available with the consent of the speakers. In the Haskell Implementors' Workshop we hope to study the underlying technology. We want to bring together anyone interested in the nitty gritty details necessary to turn a text file into a deployed product. Having said that, members of the wider Haskell community are more than welcome to attend the workshop -- we need your feedback to keep the Haskell ecosystem thriving. The scope covers any of the following topics. There may be some topics that people feel we've missed, so by all means submit a proposal even if it doesn't fit exactly into one of these buckets: * Compilation techniques * Language features and extensions * Type system implementation * Concurrency and parallelism: language design and implementation * Performance, optimisation and benchmarking * Virtual machines and run-time systems * Libraries and Tools for development or deployment Talks ----- At this stage we would like to invite proposals from potential speakers for a relatively short talk. We are aiming for 20 min talks with 10 mins for questions and changeovers. We want to hear from people writing compilers, tools, or libraries, people with cool ideas for directions in which we should take the platform, proposals for new features to be implemented, and half-baked crazy ideas. Please submit a talk title and abstract of no more than 200 words to benl(a)cse.unsw.edu.au We will also have a lightning talks session which will be organised on the day. These talks will be 2-10 minutes, depending on available time. Suggested topics for lightning talks are to present a single idea, a work-in-progress project, a problem to intrigue and perplex Haskell implementors, or simply to ask for feedback and collaborators. Organisers ---------- * Rebekah Leslie (Portland State University) * Ben Lippmeier - co-chair (University of New South Wales) * Andres Loeh (Well-Typed LLP) * Oleg Lobachev (University of Marburg) * Neil Mitchell - co-chair (Standard Chartered) * Dimitrios Vytiniotis (Microsoft Research)

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HXT and xhtml page encoded in cp1251
by Dmitry V'yal 20 Apr '11

20 Apr '11

Greetings, I'm writing a small webcrawler. Usually I used tagsoup for such tasks but this time I decided to give hxt a try. Unfortunately, I ran into the troubles with character encodings. The site I'm targeting uses cp1251, which is the one of the most popular among sites in Russian. Pages contain the following meta tag <meta http-equiv="content-type" content="text/html; charset=windows-1251" /> The readDocument arrow fails with the following message: fatal error: encoding scheme not supported: "WINDOWS-1251" Can someone suggest a workaround for my use case? Best regards, Dmitry

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Painless parallelization.
by Grigory Sarnitskiy 20 Apr '11

20 Apr '11

Hello, I'm searching a way to benefit from modern hardware in my programs. I consider parallel programing to be actually easier than sequential one. Parallel computation allows to avoid sophisticated algorithms that were developed to gain performance on sequential architecture. It should also allow to stop bothering about using immutable objects --- immutable parallel arrays should be as fast as mutable ones, right? (provided there is enough cores) So what are the options to write a pure functional parallel code with the level of abstraction I used in Haskell? So far I've found Data Parallel Haskell for multicore CPU's and Data.Array.Accelerate for GPU's. It would be nice to have something at the release state, rather than some beta.

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An example of enumerator processing some data between input and output
by Paul Sujkov 19 Apr '11

19 Apr '11

Hi haskellers, I played a bit with the enumerator package, and I'm quite stuck with the question how to duplex data to two (or more) consumers using combinators from xml-enumerator (for example) package. What I mean is: main = withFile "out.xml" WriteMode $ \h -> parseFile "in.xml" decodeEntities $ (joinI $ renderText $$ iterHandle h) >> (force "data required" parseData) where parseData is a simple xml-enumerator parser. Combining consumers in such a way I can get text being output to the file "out.xml" (like in the example given) or parsed with parseData (if I switch the order of the consumers), but not both. Can anybody tell me how to make a pipe (input -> process -> output) to have text both parsed and then put back to the file? While pretty straightforward with the arrow approach, it doesn't seem obvious to me with the iteratees :( -- Regards, Paul Sujkov

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Stack overflow in simple code with getStdRandom / randomR (GHC)
by Volker Stolz 19 Apr '11

19 Apr '11

The following three-liner creates a stack overflow (on my machine, MacOS), and I'm a bit puzzled as to why: > import System.Random > > main = do > mapM (const (getStdRandom (randomR (0, 50000::Int)))) [0..10000000] botanix:~ stolz$ ./a.out Stack space overflow: current size 8388608 bytes. Use `+RTS -Ksize -RTS' to increase it. Yes, the list is quite long, but somehow I'd expected mapM to chug slowly along...printing "Hello World" doesn't create a stack overflow either, after all. My Haskell-foo has become a bit rusty, but I was wondering where I am missing a point or two here (and I couldn't find the correct profiler incantation that would give me more than a big black blob for the main-CAF). GHC is 6.12.3 on MacOS. Cheers, Volker

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ANNOUNCE: cereal-0.3.1.0
by Trevor Elliott 19 Apr '11

19 Apr '11

Hi Everyone, I'm proud to announce version 0.3.1.0 of the cereal binary-(de)serialization library [1]! This version includes some new functionality contributed by Lemmih, that adds support for partial parses in the Get monad. The changes don't modify the API presented in 0.3.0.0, instead adding a new runPartialGet function for the Get monad when partial parses are desired. [1] http://hackage.haskell.org/package/cereal-0.3.1.0

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Another(!) PhD Position at Strathclyde
by Conor McBride 19 Apr '11

19 Apr '11

Hot on the heels of Patricia Johann's advertisement, here's another PhD Position in the Mathematically Structured Programming Group, Deparment of Computer and Information Sciences at the University of Strathclyde to be supervised by Dr. Conor McBride and Prof. Neil Ghani on something related to Designing Precision with Dependent Types. We invite applications for one PhD position within the Mathematically Structured Programming group at the University of Strathclyde. The group comprises Prof. Neil Ghani, Dr. Patricia Johann, Dr. Conor McBride, Dr. Peter Hancock, Dr. Robert Atkey, and five PhD students. The PhD project involves THEORETICAL and PRACTICAL issues in FUNCTIONAL PROGRAMMING with DEPENDENT TYPES. Dependent type systems allow us to construct precise variations on general-purpose datatypes which address the specific needs of particular programming problems, thus opening a new "precision" axis in the design space of programs and data. We seek equipment to help programmers explore this axis, determining what is needed to shift the level of precision at which existing functions operate and which properties are guaranteed in return. The project thus represents an opportunity to study mathematical abstractions with a concrete engineering motivation. The PhD position is for 3 years, starting in October 2011. The position is a fully-funded post for a UK or EU student, and includes coverage both of fees and an EPSRC-level stipend for each of the three years. More information about the department is available at http://www.strath.ac.uk/cis The University of Strathclyde (http://www.strath.ac.uk) is slap bang in the middle of Glasgow, a thronging metropolis of wit and daring. Scotland is a hive of activity in Computer Science: we have active collaborations with researchers at Edinburgh, Heriot-Watt, Glasgow and St. Andrews. This is the time and the place to make an impact. Requests for further information and other informal enquiries can be sent to: Conor McBride conor at cis.strath.ac.uk Please get in touch as soon as you can. We hope to appoint in early May.

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Adjoint Folds and Unfolds Or: Scything through the Thicket of Morphisms
by caseyh＠istar.ca 19 Apr '11

19 Apr '11

Just tripped over this: Adjoint Folds and Unfolds Or: Scything through the Thicket of Morphisms Folds and unfolds are at the heart of the algebra of programming. They allow the cognoscenti to derive and manipulate programs rigorously... Ralf Hinze Lecture Notes in Computer Science, 2010, Volume 6120, Mathematics of Program Construction, Pages 195-228

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Re: [Haskell-cafe] [Haskell] select(2) or poll(2)-like function?
by Henning Thielemann 19 Apr '11

19 Apr '11

On Sun, 17 Apr 2011, Matthias Kilian wrote: > is there something like select(2) or poll(2) available in the > standard (HP) libraries? I hoogled around a little bit but didn't > find anything. (Something like this will be crucial for networking > stuff listening on v4 and v6 sockets at the same time) A very basic thing to start from: http://code.haskell.org/~thielema/poll/ It is still subject to change. We are planning to use this in alsa-seq.

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Errata for "Pearls of Functional Algorithm Design" by Richard Bird, 2010, page 25 #Haskell
by caseyh＠istar.ca 19 Apr '11

19 Apr '11

-- Errata for "Pearls of Functional Algorithm Design" by Richard Bird, -- 2010, page 25 #Haskell -- I don't like this solution it suffers from "indexitis" which -- Bird talks about in the chapter on Sudoku. -- I can see why lists are liked, they avoid "indexitis" at -- the cost of chopping and concatenating them. -- One of the advantages of functional programming is supposed to be less bookkeeping. smallest :: (Ord a) => Int -> (Array Int a, Array Int a) -> a smallest k (xa,ya) = ~~~~search k (0,m+1) (0,n+1) ~~~~~~~~where ~~~~~~~~(0,m) = bounds xa ~~~~~~~~(0,n) = bounds ya ~~~~~~~~search k (lx,rx) (ly,ry) ~~~~~~~~~~~~| lx == rx = ya ! (k+ly) ~~~~~~~~~~~~| ly == ry = xa ! (k+lx) ~~~~~~~~~~~~| otherwise = case (xa ! mx < ya ! my, k <= mx-lx+my-ly) of ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~(True, True) -> search k (lx,rx) (ly,my) ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~(True, False) -> search (k-(mx-lx)-1) (mx+1,rx) (ly,ry) ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~(False, True) -> search k (lx,mx) (ly,ry) ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~(False, False)-> search (k-(my-ly)-1) (lx,rx) (my+1,ry) ~~~~~~~~~~~~~~~~~~~~~~~~~~where ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~mx = (lx+rx) `div` 2 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~my = (ly+ry) `div` 2

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A paper relating to Data.Set and Data.Map
by Kazu Yamamoto 19 Apr '11

19 Apr '11

Hello libraries and cafe, We (Hirai and I) would like to tell you our paper relating to Data.Set and Data.Map. "Balancing Weight-Balanced Trees" is now accepted and will appear in Journal of Functional Programming. The camera-ready version of the paper is available from: http://www.mew.org/~kazu/proj/weight-balanced-tree/ Please recall that Taylor Campbell reported a bug of Data.Map in last Summer. In some cases, the balance of Data.Map is broken after delete operations. This triggered our research. http://article.gmane.org/gmane.comp.lang.haskell.libraries/13444 Though Data.Set/Data.Map are based of a variant Weight-Balanced tree by Adams, we target the original Weight-Balanced tree by Nievergelt and Reingold. They are parameterized algorithms and the difference of two algorithms is quite small. But the original has smaller conditions which are gentle for proof. We identified the valid area of two parameters of the original Weight-Balanced tree and showed <3,2> is only one integer solution. Soundness is proved in Coq and completeness is verified with four algorithms to generate counterexamples for the outside of the valid area. "wttree.scm" of MIT/GNU Scheme and slib have already incorporated our fixes. When I offered our fixes to MIT/GNU Scheme, Taylor Campbell appeared again. I understand that he is an MIT/GNU Scheme guy and found the bug of Data.Map when re-implementing "wttree.scm" consulting Data.Map. :-) We think it's Haskell community turn now. Data.Set/Data.Map are still based on the variant whose soundness is not proved yet. If interested, let's discuss whether or not we should replace the variant with the original in Data.Set/Data.Map. We guess that Milan Straka, a big contributer of the container package, has his opinion. We welcome opinions from other people, two. Regards, --Kazu

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RFC: Possible improvements to System.Random and Control.Monad.Random API
by Krzysztof Skrzętnicki 19 Apr '11

19 Apr '11

Today I answered a question on stackoverflow.com about behaviour of random data generation [1]<http://stackoverflow.com/questions/5714873/weird-performance-with-evalrandi…>. The code in question was as follows: import Control.Monad.Random inf :: (RandomGen g, Random a) => Rand g [a] inf = sequence $ repeat $ getRandom many :: (RandomGen g, Random a) => Int -> Rand g [a] many n = sequence $ replicate n $ getRandom main = do m <- evalRandIO $ many 1000000 :: IO [Bool] i <- evalRandIO $ inf :: IO [Bool] putStrLn $ show $ take 5 m putStrLn $ show $ take 5 i It triggered stack overflow if the stack wasn't set high enough. This was because to calculate i one has to first calculate the seed that results from evaluating many 1000000 :: IO [Bool]. Even worse, if we swap the order of execution like this: main = do i <- evalRandIO $ inf :: IO [Bool] m <- evalRandIO $ many 1000000 :: IO [Bool] putStrLn $ show $ take 5 m putStrLn $ show $ take 5 i We will get space leak and our code will never finish nor provide single bit of output. One possible solution is to make cell holding std gen strict. This would make behaviour more clear to the unsuspecting user. I think it might have other uses, but unfortunately this would also break some code. So I think another solution might be applicable here. In my response I provided solution based on the fact that StdGen is splittable. The essence of the solution is to have each call to evalRandIO run on different seed. main = do newGen' <- newStdGen m <- evalRandIO $ many 1000000 :: IO [Bool] setStdGen newGen' i <- evalRandIO $ inf :: IO [Bool] putStrLn $ show $ take 5 m putStrLn $ show $ take 5 i This code works as expected, i.e. very runs fast. It dawns on me that this solution might be useful as a general pattern. There are several ways to do that. One might define additional functions for random values generation. For example for Control.Monad.Random: randomSplitIO :: (Random a) => IO a randomSplitIO = do newGen <- newStdGen val <- randomIO setStdGen newGen return val But this is slightly unfortunate, as it grows the API considerably. I estimate that overall 5-10 new functions will be needed (both internal and external, that is). The best idea I have now is to add this function to System.Random: withSplitGen :: IO a -> IO a withSplitGen act = do newGen <- newStdGen val <- act setStdGen newGen return val Additional function will be needed for Control.Monad.Random, but I don't know how to write it. getSplit [2]<http://hackage.haskell.org/packages/archive/MonadRandom/0.1.6/doc/html/Cont…>will likely be involved in such function. The original program becomes this, and works as expected. main = do i <- withSplitGen $ evalRandIO $ inf :: IO [Bool] m <- withSplitGen $ evalRandIO $ many 1000000 :: IO [Bool] putStrLn $ show $ take 5 i putStrLn $ show $ take 5 m As a bonus this function could draw attention to the problem that otherwise might be overlooked. People would get better understanding of how StdGen works just by reading documentation of this function. Please comment on this idea. Perhaps there are some problems I didn't anticipate or some naming could be improved. Best regards, Krzysztof Skrzętnicki [1] http://stackoverflow.com/questions/5714873/weird-performance-with-evalrandi… [2] http://hackage.haskell.org/packages/archive/MonadRandom/0.1.6/doc/html/Cont…

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Trac configuration error
by Andrew Pennebaker 19 Apr '11

19 Apr '11

Try to visit http://trac.haskell.org/ghc. <type 'exceptions.IOError'> Python 2.5.2: /usr/bin/pythonTue Apr 19 15:02:32 2011A problem occurred in a Python script. Cheers, Andrew Pennebaker www.yellosoft.us

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Confusing performance variation while optimising a tight loop
by Patai Gergely 19 Apr '11

19 Apr '11

Hello all, I've been trying to improve the performance of Hemkay, my MOD player. The mixing routines are in a separate library with no special dependencies. Most of the time is spent in a small loop in the mixToBuffer function [1] (note that this version is not the master branch in case you give it a try). Each iteration does the following: it advances a pointer in a waveform (float vector) with a fractional amount (depending on the frequency at which it's played back), multiplies it by the desired volume and distributes it over the two channels (two more multiplication). The case where the step is less than zero is special cased for efficiency (lines 170-183), but commenting that out won't affect the output, just make it take about 15% more time. The isEmpty and stepWave' functions come from the Music module. The confusion is the following: a waveform can be of three kinds (empty, single-shot or looped), which is represented by a plain sum type. In the fastest version I match this pattern twice for every sample output (through the isEmpty and stepWave' functions). Since the kind of waveform doesn't change during this tight loop, I figured I could speed it up by moving the pattern match out of the loop and only increasing the index into the vector, thereby doing strictly less work per sample. This is the version on lines 204-234. However, this modification makes the program twice as slow as the original, and I simply don't understand what I'm doing wrong or what could possibly cause this! The speed test is a self-contained program in the test directory, you can compile it after installing the library. If you pass it a MOD music file, it'll perform mixing and quit immediately afterwards. Gergely [1] https://github.com/cobbpg/hemkay-core/blob/vector/Sound/Hemkay/Mixer.hs -- http://www.fastmail.fm - The professional email service

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deriving instances in GHC 7.*
by Alberto G. Corona 19 Apr '11

19 Apr '11

By my little experience with GHC 7.* it seems that this version is more "deterministic" when deriving instances. For example if there is a general instance declaration such is: instance C1 x=> C2 x where .... then the absence of the C1 instance becomes an error. C1 instance becomes a prerequisite for any C2 instance, where in previous versions, that was just a overlapping instance in collision with any other less general instance of the C2 class, so it could be overcomed with the "OverlappingInstances" flag. Given that is not possible to hide instances, this is annoying. I´m not saying that this is wrong or even If this is so, because this could be a misinterpretation on my side based on the tests I have done. Where can I read about the rationale of this change?. General Instance derivations rules, as any rule system seem to me naturally flexible and with different degrees of generalization rather tan rigid.

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Errata for "Pearls of Functional Algorithm Design" by Richard Bird, 2010, page 25 #Haskell
by KC 19 Apr '11

19 Apr '11

Errata for "Pearls of Functional Algorithm Design" by Richard Bird, 2010, page 25 #Haskell smallest :: (Ord a) => Int -> (Array Int a, Array Int a) -> a smallest k (xa,ya) = search k (0,m+1) (0,n+1) where (0,m) = bounds xa (0,n) = bounds ya search k (lx,rx) (ly,ry) | lx == rx = ya ! (k+ly) | ly == ry = xa ! (k+lx) | otherwise = case (xa ! mx < ya ! my, k <= mx-lx+my-ly) of (True, True) -> search k (lx,rx) (ly,my) (True, False) -> search (k-(mx-lx)-1) (mx+1,rx) (ly,ry) (False, True) -> search k (lx,mx) (ly,ry) (False, False)-> search (k-(my-ly)-1) (lx,rx) (my+1,ry) where mx = (lx+rx) `div` 2 my = (ly+ry) `div` 2 -- -- Regards, KC

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ANNOUNCE: floatshow-0.1 faster String representations for Double and Float
by Daniel Fischer 19 Apr '11

19 Apr '11

The Show instances for RealFloat types provided in base are very elegant, as they produce the shortest string which 'read' converts back to the original number. That, however, involves a check after each digit has been determined and arithmetic of usually fairly large Integers, which makes these Show instances rather slow. For cases where having the conversion fast is more important than having it produce elegant output, the floatshow package provides alternative conversions, which avoid the checks and reduce the occurrences of large Integers by just producing a sufficiently long output string. The speed gain can be substantial if the numbers have exponents of large absolute modulus, but for the more common case of numbers whose exponents have small absolute modulus, the difference is (although still significant for Double) too small in my opinion to seriously consider replacing the Show instances. Therefore I've put these functions into their own package. In my benchmarks, the speed gain ranges from 4x to 10x for Double and from 1.25x to 5x for Float, so the package may be a useful alternative for tasks requiring the output of many floating point numbers. Sadly, floatshow is non-portable and requires ghc >= 6.12.

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Re: [Haskell-cafe] [Haskell] select(2) or poll(2)-like function?
by Don Stewart 19 Apr '11

19 Apr '11

Redirecting to haskell-cafe@, where this kind of long discussion belongs. On Mon, Apr 18, 2011 at 9:07 AM, Colin Adams <colinpauladams(a)googlemail.com> wrote: > > > On 18 April 2011 16:54, Ertugrul Soeylemez <es(a)ertes.de> wrote: >> >> > >> > Well, *someone* has to worry about robustness and scalability. Users >> > notice when their two minute system builds start taking four minutes >> > (and will be at my door wanting me to fix it) because something didn't >> > scale fast enough, or have to be run more than once because a failing >> > component build wasn't restarted properly. I'm willing to believe that >> > haskell lets you write more scalable code than C, but C's tools for >> > handling concurrency suck, so that should be true in any language >> > where someone actually thought about dealing with concurrency beyond >> > locks and protected methods. The problem is, the only language I've >> > found where that's true that *also* has reasonable tools to deal with >> > scaling beyond a single system is Eiffel (which apparently abstracts >> > things even further than haskell - details like how concurrency is >> > achieved or how many concurrent operations you can have are configured >> > when you start an application, *not* when writing it). Unfortunately, >> > Eiffel has other problems that make it undesirable. >> >> I can't make a comparison, because I don't know Eiffel. > > I do, and I don't recognize what the OP is referring to - I suspect he meant > Erlang. > > -- > Colin Adams > Preston, Lancashire, ENGLAND > () ascii ribbon campaign - against html e-mail > /\ www.asciiribbon.org - against proprietary attachments > > _______________________________________________ > Haskell mailing list > Haskell(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell > >

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Adjoint Folds and Unfolds Or: Scything through the Thicket of Morphisms
by KC 18 Apr '11

18 Apr '11

Just tripped over this: Adjoint Folds and Unfolds Or: Scything through the Thicket of Morphisms Folds and unfolds are at the heart of the algebra of programming. They allow the cognoscenti to derive and manipulate programs rigorously... Ralf Hinze Lecture Notes in Computer Science, 2010, Volume 6120, Mathematics of Program Construction, Pages 195-228 -- -- Regards, KC

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Adjoint Folds and Unfolds Or: Scything through the Thicket of Morphisms
by KC 18 Apr '11

18 Apr '11

Just tripped over this: Adjoint Folds and Unfolds Or: Scything through the Thicket of Morphisms Folds and unfolds are at the heart of the algebra of programming. They allow the cognoscenti to derive and manipulate programs rigorously... Ralf Hinze Lecture Notes in Computer Science, 2010, Volume 6120, Mathematics of Program Construction, Pages 195-228 -- -- Regards, KC

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Summer school on Applied Functional Programming at Utrecht University; deadline for registration May 15
by S. Doaitse Swierstra 18 Apr '11

18 Apr '11

Again we will teach an "Applied Functional Programming Summer in Haskell" school this year at Utrecht University. In the previous two occasions students were all very happy with the school and we plan to repeat this success this year. The intended audience are prospective master students who have been in contact with Functional Programming, e.g. by taking a general course on programming languages, and want to learn more about Haskell and its typical programming patterns. In the previous two years we have taught an introductory part (advanced bachelor level), an advanced part (beginning master level) and a shared part for both groups. Topics covered are, besides some examples of domain specific languages, also monads, monad transformers, arrows, parser combinators and self-analysing programs, underlying principles, type inferencing, etc. Half of the course time is spent on a larger programming exercise; you can also come with a problem of your own if you want, and get help from the Utrecht University Software Technology group in finding the proper Haskell idioms, tools and libraries, for solving it. Important links: -- our own page where we supply information based on questions asked http://www.cs.uu.nl/wiki/bin/view/USCS2011/WebHome -- the poster you can print and hang somewhere (why not your office door): http://www.cs.uu.nl/wiki/pub/USCS2011/WebHome/USCSpos11.pdf -- the official summerschool site where you can register: http://www.utrechtsummerschool.nl/index.php?type=courses&code=H9 Furthermore we ask for your cooperation to bring this announcement under the attention of potential participants. Best, Doaitse Swierstra PS: apologies if you get this mail more than once

1 0

Re: [Haskell-cafe] How to contact OpenGL package maintainer (where is Sven?)
by Joachim Breitner 18 Apr '11

18 Apr '11

[Moving to haskell-cafe, not really library specific any more] Hi, Am Sonntag, den 17.04.2011, 12:39 -0700 schrieb Jason Dagit: > In this specific case, I'll do what I can to clean things up but your > request makes me pause and think that the debian packaging for cabal > packages is not automated enough. As haskell developers it seems a > little odd to me that we need to consider the cost of creating new > packages for the sake of debian. I like debian, so please don't take > that the wrong way :) the observation is correct, but unfortunately hard to change – Debian is large, slow moving, and unlikely to change policies for a corner-case such as Haskell libraries. Also, the policies that make life a bit more harder for us are also the policies that make Debian a good distribution: E.g. manually creating a copyright file describing who all has copyright on the files, and what license the files are under – .cabal has fields for that, but unfortunately, they very often do not reflect the reality of the .hs files. Or the other one, requiring that an upload of a source package is accompanied by a manually built and signed binary package ensures that the maintainer actually checked that the package builds and hopefully has tested it. If you think more about it, the lack of some of the policies on Hackage, e.g. nobody checking that licenses and copyright are correctly specified and actually compatible with dependencies, make Hackage a dangerous source for serious users. I am not saying that this should change, but the cheapness of Hackage uploads has its downsides. Greetings, Joachim -- Joachim Breitner e-Mail: mail(a)joachim-breitner.de Homepage: http://www.joachim-breitner.de ICQ#: 74513189 Jabber-ID: nomeata(a)joachim-breitner.de

4 4

Killing threads in foreign calls.
by Jason Dusek 18 Apr '11

18 Apr '11

I am building an application that uses Postgres for storage. If a query runs too long, I would like to kill the querying thread, releasing its lock on the connection; if the connection is a in a bad state -- for example, busy -- I would like to clean up the connection. Unfortunately, killing calls in to libpq seems not to work. I have put a minimal example on hpaste: http://hpaste.org/45774/minimal_pg_contention_example If you install libpq with Cabal, you can run it. In the example, the main thread spawns a worker thread that queries Postgres, running "SELECT pg_sleep(10);"; the main thread waits half a second and then tries to kill the worker. Unfortunately, the worker always manages to get as far as printing "complete". In the code, I call `Database.PQ.exec': http://hackage.haskell.org/packages/archive/libpq/0.4.1/doc/html/src/Databa… This in turn calls a `safe' binding, `c_PQexec', to `PQexec' in the C library: http://hackage.haskell.org/packages/archive/libpq/0.4.1/doc/html/src/Databa… There are async interfaces, too; they do not seem to be any more killable then the sync ones. Maybe the problem is that you can't kill a thread while it's in a foreign call? I do not see any documentation to this effect; but I may have missed it. -- Jason Dusek () ascii ribbon campaign - against html e-mail /\ www.asciiribbon.org - against proprietary attachments

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Template Haskell question
by Alexey Karakulov 18 Apr '11

18 Apr '11

I'm interested if it's possible to use functions from some module without explicitly importing it. In ghci it's done on the fly, like this: Prelude> Data.Map.empty Loading package array-0.3.0.2 ... linking ... done. Loading package containers-0.4.0.0 ... linking ... done. fromList [] But without gchi it seems impossible. I have the file Test.hs: > {-# LANGUAGE TemplateHaskell #-} > module Test where > import Language.Haskell.TH > > x :: ExpQ > x = global $ mkName "Data.Map.empty" When I load it in ghci, all works: $ ghci -XTemplateHaskell Test.hs *Test> $x Loading package pretty-1.0.1.2 ... linking ... done. Loading package array-0.3.0.2 ... linking ... done. Loading package containers-0.4.0.0 ... linking ... done. Loading package template-haskell ... linking ... done. fromList [] But when I try to use it from other module, it fails. File Main.hs: > {-# LANGUAGE TemplateHaskell #-} > module Main where > import Test > > main = do > print $x $ runhaskell Main.hs Main.hs:5:9: Not in scope: `Data.Map.empty' In the result of the splice: $x To see what the splice expanded to, use -ddump-splices In the first argument of `print', namely `$x' In the expression: print ($x) -- All the best, Alexey

2 1

"import" functionality in DSLs
by Nikhil A. Patil 18 Apr '11

18 Apr '11

Hi, I am planning a simple monadic DSL (frankly, calling it a DSL is a bit of a stretch; it's just a somewhat glorified state monad), and I wish to implement some kind of "import" functionality for it. The DSL code looks something like this: > doit :: DSL Term > doit = do (+) <- define_function "+" 2 > n0 <- define_constant "0" > k <- define_constant "k" > -- begin beautiful DSL code > let x = k + n0 > return $ x + x The code above adds identifiers "+", "0", "k" to the DSL monad and conveniently exposes haskell identifiers (+), n0, k for the user to use in code that follows. (Note that these define_* functions make state updates in the underlying state monad.) Needless to say, most functions like doit have very similar define_* calls in the beginning. Thus, I want to implement some kind of import functionality. Ideally, the code would look like this: > module DSLPrelude where > > prelude :: DSL () > prelude = do (+) <- define_function "+" 2 > n0 <- define_constant "0" > k <- define_constant "k" > return () > module Main where > import DSLPrelude > > doit :: DSL Term > doit = do prelude > -- begin beautiful DSL code > let x = k + n0 > return $ x + x ...but of course that won't work since (+), n0, k are not in scope. I can think of two solutions, both of which I dislike: Solution 1: > module DSLPrelude where > > prelude :: DSL (Term -> Term -> Term, Term, Term) > prelude = do (+) <- define_function "+" 2 > n0 <- define_constant "0" > k <- define_constant "k" > return ((+), n0, k) > module Main where > import DSLPrelude > > doit :: DSL Term > doit = do ((+), k, n0) <- prelude > -- begin beautiful DSL code > let x = k + n0 > return $ x + x This is quite unsafe: I have mistakenly swapped k and n0 in doit, without failing typecheck. Solution 2: > module DSLPrelude where > > (+) :: DSL (Term -> Term -> Term) > n0 :: DSL Term > k :: DSL Term > (+) = define_function "+" 2 > n0 = define_constant "0" > k = define_constant "k" > module Main where > import DSLPrelude > > doit :: DSL Term > doit = do (+) <- (+) > n0 <- n0 > k <- k > -- begin beautiful DSL code > let x = k + n0 > return $ x + x ...which works, but still has quite a bit of boilerplate crap. I feel this would be a common problem with a lot of DSLs, so I am curious to know how others solve it. Any pointers and suggestions are most welcome and greatly appreciated. Thanks! nikhil

4 5

Oddness with binary-0.5.0.2 user install
by Antoine Latter 18 Apr '11

18 Apr '11

Hello folks, The docs for binary-0.5.0.2 on Hackage claim that their is a Binary instance for lazy ByteStrings, but there does not seem to be such an instance in my local install of binary-0.5.0.2. In addition, the docs on Hackage claim that there is an Applicative instance for the 'Get' type, which my install is also lacking. I've verified this in ghci and by looking at my own locally generated haddocks. The module structure for the 'binary' package is pretty straightforward - so I don't think I'm missing any imports in ghci. Has anyone seen this before? Antoine

1 1

Haskell Platform 2011.2.0.1 now available
by Don Stewart 17 Apr '11

17 Apr '11

We're pleased to announce the 2011.2.0.1 release of the Haskell Platform: a single, standard Haskell distribution for everyone. Download the Haskell Platform 2011.2.0.1: http://haskell.org/platform/ This release adds support for GHC 7.0.3, and significant improvements for Mac OS X users. The specification, along with installers (including Windows, Mac and Unix installers for a full Haskell environment) are available. The Haskell Platform is a single, standard Haskell distribution for every system, in the form of a blessed library and tool suite for Haskell distilled from the thousands of libraries on Hackage, along with installers for a wide variety of systems. It saves developers work picking and choosing the best Haskell libraries and tools to use for a task. When you install the Haskell Platform, you get the latest stable compiler, an expanded set of core libraries, additional development tools, and cabal-install – so you can download anything else you need from Hackage. This release ships with GHC 7.0.3. What you get is specified here: http://hackage.haskell.org/platform/contents.html Thanks! -- The Platform Infrastructure Team P.S. a big thanks to Mark Lentczner and Mikhail Glushenkov who built the Mac and Windows installers!

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Announcing AusHac 2011 - The second annual Australian Haskell Hackathon (July 8-10)
by Alex Mason 17 Apr '11

17 Apr '11

Attention all Australasian Haskellers! After last year’s fantastic turnout for the first Australian Haskell Hackathon - AusHac 2010 - we’ve decided to organise another. As it’s rather boring to have a Hackathon with only two people, we encourage and welcome anyone interested in Haskell in Australia, New Zealand and surrounding areas (or further afield if you want!) to come and join us. Last year we had more than 25 Haskellers come from Brisbane, Canberra, Melbourne, Sydney and even two from as far away as New Zealand. AusHac 2011 will be held from Friday July 8th until Sunday July 10th, again at UNSW’s Computer Science building in Sydney. There’s no need to come for the entire weekend, just for whenever you’re able to. Last year, people worked on several projects including: • Accelerate: a high performance library for array computations using various backends (CUDA, LLVM, etc.) being written at UNSW. • Leksah: the Haskell IDE written in Haskell • DDC: the compiler for the Haskell-like language Disciple, which has strict evaluation by default, region, effect and closure typing and other interesting language features. • Hubris: the Haskell/Ruby bridge for calling Haskell from Ruby and vice-versa. • haskell-mpi: bindings to MPI, the message passing interface used frequently on high performance super-computers to allow programs to run on many nodes in parallel with easy communication. • There was even some work on LLVM to produce HTML output showing register allocations (as an aid to those writing new code generators etc.), which has now been included upstream. This work did not use Haskell, but it did happen at AusHac 2010. • And much more! This is a great chance to collaborate with others and get feedback on your work, while having heaps of fun and meeting other Haskellers, to verify they do actually exist! And don’t worry if you can’t think of a project to work on; just come along and pitch in with whatever strikes your fancy! If you’re interested in coming and have a project you’d like to work on, check out the wikipage http://www.haskell.org/haskellwiki/AusHac2011 where you can add your project ideas, or just take a look and see what others will be working on that you can help out with. People of all skill levels are welcome, from complete beginners to Oleg-level sage masters. We’re more than happy to help you learn Haskell if you’re just starting out or help you solve that little niggly bit in your code. If you’re interested in coming along, fill out our registration form http://axman6.wufoo.com/forms/aushac-2011-sign-up/. Registration is required so we know numbers and for you to get access to the university Wifi. Even if you’re not sure, fill it in to express your interest, obligation free. We’d rather know we have room for too many people than not enough. So come along! And if for some reason you don’t enjoy yourself, registration comes with a money-back guarantee! Hope to see you there, -- Alex Mason and Ivan Miljenovic, AusHac organisers P.S: If you have received this email directly, it is because you have shown previous interest in AusHac by signing up via the previous signup. If you do not wish to receive any more email regarding AusHac, please email me (axman6(a)gmail.com) and I'll be sure to take you off the list of interested people.

1 0

Re: [Haskell-cafe] Cairo and Haskell
by Hans van Thiel 16 Apr '11

16 Apr '11

Hello Paulo, << I guess that would be using Cairo so I can have a 2d canvas to draw in and maybe even preview before exporting to PDF. However, I can't find any documentation on Cairo with Haskell or any code examples related to what I want to do. >> With regard to export to pdf and other formats this might be useful: http://muitovar.com/gtk2hs/app1.html (There's a Spanish translation too.) However, this tutorial is a few years old now, and I don't know if it's still up to date for the later Gtk2Hs versions. Best Regards, Hans van Thiel

1 0

64 bit generic link warning on every compile
by Andrew Pennebaker 16 Apr '11

16 Apr '11

GHC 7 compiles fine, but there's an additional warning during linking. $ system_profiler SPSoftwareDataType | grep "System Version" System Version: Mac OS X 10.6.7 (10J869) $ ghc --version The Glorious Glasgow Haskell Compilation System, version 7.0.2 $ cat hello.hs #!/usr/bin/env runhaskell module Main where main :: IO () main = putStrLn "Hello World" $ ghc --make hello.hs [1 of 1] Compiling Main ( hello.hs, hello.o ) Linking hello ... ld: warning: -read_only_relocs cannot be used with x86_64 $ ./hello Hello World Cheers, Andrew Pennebaker www.yellosoft.us

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ANN: bindings to xfconf-4.8.0.0
by John Obbele 15 Apr '11

15 Apr '11

Hi all, I'm very happy to announce my first hackageDB package: xfconf[0], a Haskell binding to the XFCE xfconfd configuration dæmon. It's mainly a pet project I have done to learn Haskell using the FFI and QuickCheck but if someone can find it useful or instructive I would be happy. This package allows easy access to your favorite XFCE desktop settings (wallpaper, colors, police, etc.) and easier synchronisations between the configuration backend and your own Gtk+2 widget properties (a feature missing in gconf but now part of gsettings). A practical use of this binding can be found in hThemes[1], a quick&dirty front-end to switch between pre-defined desktop themes (eg. switching from a light-white theme for daytime to a dark-black one for your nightly hack sessions). [0.a]: http://hackage.haskell.org/package/xfconf [0.b]: http://patch-tag.com/r/obbele/xfconf/home [1]: http://patch-tag.com/r/obbele/hThemes/home Thanks to Andy Stewart and Axel Simon for their help in integrating xfconf to the gtk2hs stack. Any comments on this first work are welcomed ,-) regards, /John

1 0

Cairo and Haskell
by Paulo J. Matos 15 Apr '11

15 Apr '11

Hi, I would like to use Haskell to generate automatically a poster. I guess that would be using Cairo so I can have a 2d canvas to draw in and maybe even preview before exporting to PDF. However, I can't find any documentation on Cairo with Haskell or any code examples related to what I want to do. Any suggestions? Cheers, -- PMatos

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timsort
by Ben 15 Apr '11

15 Apr '11

hello -- has anyone every implemented (and benchmarked) timsort in haskell? http://bugs.python.org/file4451/timsort.txt it is a stable mergesort-like algorithm, seems like a good fit for haskell. best, ben

2 1

build fails due to sem_close...
by Jiri Skala 15 Apr '11

15 Apr '11

Hi all, I've got error listed below when I've tried to build ghc-7.0.2 on ppc64 arch. I 've tested patch from Debian (-pthread option) but this didn't fix the issue. Any idea what's wrong? Thanks for any tip/help. Jiri used packages: glibc: 2.12.90, 2.13.90 (tested for both) gcc: 4.6.0 I use these options for ppc64 arch in build.mk %ifarch ppc64 GhcUnregisterised=YES GhcWithNativeCodeGen=NO SplitObjs=NO GhcWithInterpreter=NO GhcNotThreaded=YES SRC_HC_OPTS+=-optc-mminimal-toc SRC_CC_OPTS+=-mminimal-toc -Wa,--noexecstack %endif error log: configure: Building in-tree ghc-pwd /usr/bin/ld: /usr/lib64/ghc-7.0.2/unix-2.4.2.0/libHSunix-2.4.2.0.a(Semaphore.o): undefined reference to symbol 'sem_close@@GLIBC_2.3' /usr/bin/ld: note: 'sem_close@@GLIBC_2.3' is defined in DSO /lib64/libpthread.so.0 so try adding it to the linker command line /lib64/libpthread.so.0: could not read symbols: Invalid operation collect2: ld returned 1 exit status configure: error: Building ghc-pwd failed

2 1

Programming Chalenges: The 3n+1 problem
by Dmitri O.Kondratiev 15 Apr '11

15 Apr '11

3n+1 is the first, "warm-up" problem at Programming Chalenges site: http://www.programming-challenges.com/pg.php?page=downloadproblem&probid=11… (This problem illustrates Collatz conjecture: http://en.wikipedia.org/wiki/3n_%2B_1#Program_to_calculate_Collatz_sequences ) As long as the judge on this site takes only C and Java solutions, I submitted in Java some add-hock code (see at the end of this message) where I used recursion and a cache of computed cycles. Judge accepted my code and measured 0.292 sec with best overall submissions of 0.008 sec to solve the problem. *** Question: I wonder how to implement cache for this problem in Haskell? At the moment, I am not so much interested in the speed of the code, as in nice implementation. To illustrate my question I add the problem description and my Java solution at the end of this message. Thanks! *** Problem Consider the following algorithm to generate a sequence of numbers. Start with an integer *n*. If *n* is even, divide by 2. If *n* is odd, multiply by 3 and add 1. Repeat this process with the new value of *n*, terminating when *n* = 1. For example, the following sequence of numbers will be generated for *n* = 22: 22 11 34 17 52 26 13 40 20 10 5 16 8 4 2 1 It is *conjectured* (but not yet proven) that this algorithm will terminate at *n* = 1 for every integer *n*. Still, the conjecture holds for all integers up to at least 1, 000, 000. For an input *n*, the *cycle-length* of *n* is the number of numbers generated up to and *including* the 1. In the example above, the cycle length of 22 is 16. Given any two numbers *i* and *j*, you are to determine the maximum cycle length over all numbers between *i* and *j*, *including* both endpoints. InputThe input will consist of a series of pairs of integers *i* and *j*, one pair of integers per line. All integers will be less than 1,000,000 and greater than 0. OutputFor each pair of input integers *i* and *j*, output *i*, *j* in the same order in which they appeared in the input and then the maximum cycle length for integers between and including *i* and *j*. These three numbers should be separated by one space, with all three numbers on one line and with one line of output for each line of input. Sample Input 1 10 100 200 201 210 900 1000 Sample Output 1 10 20 100 200 125 201 210 89 900 1000 174 *** my Java solution import java.io.BufferedReader; import java.io.InputStreamReader; public class Main { final static BufferedReader reader_ = new BufferedReader(new InputStreamReader(System.in)); /** * @param args */ public static void main(String[] args) { new Problem().run(); } static String[] ReadLn() { String[] tokens = null; try { String line = reader_.readLine(); String REGEX_WHITESPACE = "\\s+"; String cleanLine = line.trim().replaceAll(REGEX_WHITESPACE, " "); tokens = cleanLine.split(REGEX_WHITESPACE); } catch (Exception e) {} return tokens; } } class Problem implements Runnable { long CACHE_SIZE = 65536; private final long[] cache_ = new long[(int) CACHE_SIZE]; /** * Compute cycle length for a single number * * @param n number for which we find cycle length * @return cycle length */ long cycleLen(long n) { long len = 1; if (n != 1) { len = getFromCache(n); if (len == 0) { //not yet in cache // Recursively compute and store all intermediate values of cycle length if ((n & 1) == 0) { len = 1 + cycleLen(n >> 1); } else { len = 1 + cycleLen(n * 3 + 1); } putInCache(n, len); } } return len; } void putInCache(long n, long len) { if(n < CACHE_SIZE) { cache_[(int)n] = len; } } long getFromCache(long n) { long result = 0; if(n < CACHE_SIZE) { result = cache_[(int)n]; } return result; } /** * Find max cycle on interval * * @param from interval start * @param to interval end * @return max cycle */ Long maxCycle(Long from, Long to) { Long result = 0L; Long cycle = 0L; // Get all values of cycle length on the interval and put these values into a sorted set for (long i = from; i <= to; i++) { cycle = cycleLen(i); if (cycle > result) result = cycle; } return result; } public void run() { String[] tokens = null; long from, to, result = 0; long arg1, arg2 = 0; while ((tokens = Main.ReadLn()) != null) { if (tokens.length == 2) { arg1 = new Long(tokens[0]).longValue(); arg2 = new Long(tokens[1]).longValue(); from = (arg1 <= arg2) ? arg1 : arg2; to = (arg2 >= arg1 ) ? arg2 : arg1; result = maxCycle(from, to); out(arg1+" "+arg2+" "+result); } } } static void out(String msg) { System.out.println(msg); } }

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Problem building qtHaskell
by Øystein Kolsrud 15 Apr '11

15 Apr '11

Hi! I am trying to install qtHaskell on a Windows 7 machine, but I am running into some problems while running the build script. I believe I have followed all the steps stated in the installation instructions, but get errors of this type: Creating library file: ..\bin\libqtc_core.a c:\Prog\Qt\2010.05\qt\lib/libQtUiTools.a(quiloader.o):quiloader.cpp:(.text+0x3c3): undefined reference to `_Unwind_Resume' Can anyone help me figure out what is missing? I have the following versions installed which should meet the requirements: C:\Prog\QTHASK~1.4>g++ --version g++ (GCC) 4.4.3 C:\Prog\QTHASK~1.4>gmake --version GNU Make 3.81 This program built for Windows32 C:\Prog\QTHASK~1.4>ghc --version The Glorious Glasgow Haskell Compilation System, version 7.0.2 Thanks! Best regards, Øystein Kolsrud

2 2

ghc7 bug? associated type + implicit parameters
by Michal Konečný 15 Apr '11

15 Apr '11

Hi, I have stumbled across some strange behaviour in ghc7. The following compiles fine with ghc 6.12.3: {-# LANGUAGE ImplicitParams #-} {-# LANGUAGE TypeFamilies #-} {-# LANGUAGE NoMonomorphismRestriction #-} module Test where class C t where type TF t ttt :: TF t -> t b :: (C t, ?x :: TF t) => t b = ttt ?x but ghc7 says: Could not deduce (?x::TF t) arising from a use of implicit parameter `?x' from the context (C t, ?x::TF t) bound by the type signature for b :: (C t, ?x::TF t) => t at Test.hs:13:1-10 In the first argument of `ttt', namely `?x' In the expression: ttt ?x In an equation for `b': b = ttt ?x Moreover, when I comment out the type declaration for b, it compiles and the inferred type for b is identical to the one in the above program: *Test> :t b b :: (C t, ?x::TF t) => t It feels to me like a bug but I am not entirely confident. Any ideas? Michal -- |o| Michal Konecny <mikkonecny(a)gmail.com> |o| http://www-users.aston.ac.uk/~konecnym/ |o| office: (+42) (0)121 204 3462 |o| PGP key http://www-users.aston.ac.uk/~konecnym/ki.aston -- |o| Michal Konecny <mikkonecny(a)gmail.com> |o| http://www-users.aston.ac.uk/~konecnym/ |o| office: (+42) (0)121 204 3462 |o| PGP key http://www-users.aston.ac.uk/~konecnym/ki.aston -- |o| Michal Konecny <mikkonecny(a)gmail.com> |o| http://www-users.aston.ac.uk/~konecnym/ |o| office: (+42) (0)121 204 3462 |o| PGP key http://www-users.aston.ac.uk/~konecnym/ki.aston

2 1

Experimental 64 bit installer fails
by Andrew Pennebaker 15 Apr '11

15 Apr '11

I downloaded the experimental 64 bit installer for from Haskell Platform<http://hackage.haskell.org/platform/mac.html>. It fails with the message "The installation failed." In case Haskell Platform i386 was interfering, I uninstalled it. But the 64 bit installer still yields "The installation failed." Specs - Haskell Platform 2011.2.0.0-x86_64 installer - Haskell Platform 2011.2.0.0-i386 present - Mac OS X 10.6.7 - MacBook Pro 5,1 /var/log/install.log Apr 14 17:58:38 wonko Installer[18657]: Haskell Platform Installation Log Apr 14 17:58:38 wonko Installer[18657]: Opened from: /Users/andrew/Downloads/Haskell Platform 2011.2.0.0-x86_64.pkg Apr 14 17:58:46 wonko runner[18669]: Administrator authorization granted. Apr 14 17:58:46 wonko Installer[18657]: ================================================================================ Apr 14 17:58:46 wonko Installer[18657]: User picked Standard Install Apr 14 17:58:46 wonko Installer[18657]: Choices selected for installation: Apr 14 17:58:46 wonko Installer[18657]: Upgrade: "Haskell Platform" Apr 14 17:58:46 wonko Installer[18657]: Upgrade: "Glasgow Haskell Compiler" Apr 14 17:58:46 wonko Installer[18657]: Haskell Platform 2011.2.0.0-x86_64.pkg#ghc.pkg : org.haskell.HaskellPlatform.GHC.pkg : 1 Apr 14 17:58:46 wonko Installer[18657]: Upgrade: "Haskell Platform Libraries" Apr 14 17:58:46 wonko Installer[18657]: Haskell Platform 2011.2.0.0-x86_64.pkg#haskell.pkg : org.haskell.HaskellPlatform.Libraries.pkg : 1 Apr 14 17:58:46 wonko Installer[18657]: Haskell Platform 2011.2.0.0-x86_64.pkg#haskellPlatformPostflight.pkg : org.haskell.haskellPlatform.postflight.pkg : 1.0 Apr 14 17:58:46 wonko Installer[18657]: ================================================================================ Apr 14 17:58:46 wonko Installer[18657]: It took 0.00 seconds to summarize the package selections. Apr 14 17:58:46 wonko Installer[18657]: -[IFDInstallController(Private) _buildInstallPlan]: location = file://localhost Apr 14 17:58:46 wonko Installer[18657]: -[IFDInstallController(Private) _buildInstallPlan]: file://localhost/Users/andrew/Downloads/Haskell%20Platform%202011.2.0.0-x86_64.pkg#ghc.pkg Apr 14 17:58:46 wonko Installer[18657]: -[IFDInstallController(Private) _buildInstallPlan]: file://localhost/Users/andrew/Downloads/Haskell%20Platform%202011.2.0.0-x86_64.pkg#haskell.pkg Apr 14 17:58:46 wonko Installer[18657]: -[IFDInstallController(Private) _buildInstallPlan]: file://localhost/Users/andrew/Downloads/Haskell%20Platform%202011.2.0.0-x86_64.pkg#haskellPlatformPostflight.pkg Apr 14 17:58:46 wonko Installer[18657]: Will use PK session Apr 14 17:58:46 wonko Installer[18657]: Starting installation: Apr 14 17:58:46 wonko Installer[18657]: Configuring volume "Wonko" Apr 14 17:58:46 wonko Installer[18657]: Preparing disk for local booted install. Apr 14 17:58:46 wonko Installer[18657]: Free space on "Wonko": 52.51 GB (52508946432 bytes). Apr 14 17:58:46 wonko Installer[18657]: Create temporary directory "/var/folders/N-/N-IxaaKuFae5ik1WKxV6wE+++TI/-Tmp-//Install.18657bWOdda" Apr 14 17:58:46 wonko Installer[18657]: IFPKInstallElement (3 packages) Apr 14 17:58:46 wonko installd[18673]: PackageKit: ----- Begin install ----- Apr 14 17:58:46 wonko installd[18673]: PackageKit: request=PKInstallRequest <3 packages, destination=/> Apr 14 17:58:46 wonko installd[18673]: PackageKit: packages=(\n "PKLeopardPackage <file://localhost/Users/andrew/Downloads/Haskell%20Platform%202011.2.0.0-x86_64.pkg#ghc.pkg>",\n "PKLeopardPackage <file://localhost/Users/andrew/Downloads/Haskell%20Platform%202011.2.0.0-x86_64.pkg#haskell.pkg>",\n "PKLeopardPackage <file://localhost/Users/andrew/Downloads/Haskell%20Platform%202011.2.0.0-x86_64.pkg#haskellPlatformPostflight.pkg>"\n) Apr 14 17:58:47 wonko installd[18673]: PackageKit: Extracting file://localhost/Users/andrew/Downloads/Haskell%20Platform%202011.2.0.0-x86_64.pkg#ghc.pkg (destination=/var/folders/zz/zzzivhrRnAmviuee+++++++++++/Cleanup At Startup/PKInstallSandbox-tmp/Root/Library/Frameworks, uid=0) Apr 14 17:59:06 wonko installd[18673]: PackageKit: Extracting file://localhost/Users/andrew/Downloads/Haskell%20Platform%202011.2.0.0-x86_64.pkg#haskell.pkg (destination=/var/folders/zz/zzzivhrRnAmviuee+++++++++++/Cleanup At Startup/PKInstallSandbox-tmp/Root/Library, uid=0) Apr 14 17:59:12 wonko installd[18673]: PackageKit: Extracting file://localhost/Users/andrew/Downloads/Haskell%20Platform%202011.2.0.0-x86_64.pkg#haskellPlatformPostflight.pkg (destination=/var/folders/zz/zzzivhrRnAmviuee+++++++++++/Cleanup At Startup/PKInstallSandbox-tmp/Root, uid=0) Apr 14 17:59:14 wonko installd[18673]: PackageKit: Shoving /var/folders/zz/zzzivhrRnAmviuee+++++++++++/Cleanup At Startup/PKInstallSandbox-tmp/Root (1 items) to / Apr 14 17:59:16 wonko installd[18673]: PackageKit: Executing script "./postinstall" in /private/tmp/PKInstallSandbox.e59JtF/Scripts/org.haskell.HaskellPlatform.GHC.pkg.k6SuIR Apr 14 17:59:16 wonko installd[18673]: PackageKit: Executing script "./postinstall" in /private/tmp/PKInstallSandbox.e59JtF/Scripts/org.haskell.HaskellPlatform.Libraries.pkg.dWwJo8 Apr 14 17:59:17 wonko installd[18673]: ./postinstall: Reading package info from "/Library/Haskell/ghc-7.0.2/lib/registrations/GLUT-2.1.2.1.conf" ... done. Apr 14 17:59:17 wonko installd[18673]: ./postinstall: GLUT-2.1.2.1: dependency "OpenGL-2.2.3.0-f260c686098842b872fcd761a1de8b56" doesn't exist (ignoring) Apr 14 17:59:17 wonko installd[18673]: ./postinstall: Reading package info from "/Library/Haskell/ghc-7.0.2/lib/registrations/HTTP-4000.1.1.conf" ... done. Apr 14 17:59:17 wonko installd[18673]: ./postinstall: HTTP-4000.1.1: dependency "mtl-2.0.1.0-5b7a9cce5565d8cc8721ba4f95becf1b" doesn't exist (ignoring) Apr 14 17:59:17 wonko installd[18673]: ./postinstall: HTTP-4000.1.1: dependency "network-2.3.0.2-980e23d4d2855583b05d6ed337c0eaed" doesn't exist (ignoring) Apr 14 17:59:17 wonko installd[18673]: ./postinstall: HTTP-4000.1.1: dependency "parsec-3.1.1-d6e895e0dfa7aa6990c069c9c9c706bb" doesn't exist (ignoring) Apr 14 17:59:17 wonko installd[18673]: ./postinstall: Reading package info from "/Library/Haskell/ghc-7.0.2/lib/registrations/HUnit-1.2.2.3.conf" ... done. Apr 14 17:59:17 wonko installd[18673]: ./postinstall: Reading package info from "/Library/Haskell/ghc-7.0.2/lib/registrations/OpenGL-2.2.3.0.conf" ... done. Apr 14 17:59:17 wonko installd[18673]: ./postinstall: Reading package info from "/Library/Haskell/ghc-7.0.2/lib/registrations/QuickCheck-2.4.0.1.conf" ... done. Apr 14 17:59:17 wonko installd[18673]: ./postinstall: QuickCheck-2.4.0.1: dependency "mtl-2.0.1.0-5b7a9cce5565d8cc8721ba4f95becf1b" doesn't exist (ignoring) Apr 14 17:59:18 wonko installd[18673]: ./postinstall: Reading package info from "/Library/Haskell/ghc-7.0.2/lib/registrations/cgi-3001.1.7.4.conf" ... done. Apr 14 17:59:18 wonko installd[18673]: ./postinstall: cgi-3001.1.7.4: dependency "mtl-2.0.1.0-5b7a9cce5565d8cc8721ba4f95becf1b" doesn't exist (ignoring) Apr 14 17:59:18 wonko installd[18673]: ./postinstall: cgi-3001.1.7.4: dependency "network-2.3.0.2-980e23d4d2855583b05d6ed337c0eaed" doesn't exist (ignoring) Apr 14 17:59:18 wonko installd[18673]: ./postinstall: cgi-3001.1.7.4: dependency "parsec-3.1.1-d6e895e0dfa7aa6990c069c9c9c706bb" doesn't exist (ignoring) Apr 14 17:59:18 wonko installd[18673]: ./postinstall: cgi-3001.1.7.4: dependency "xhtml-3000.2.0.1-fa3c2d95250480afb9bfb787b25e7398" doesn't exist (ignoring) Apr 14 17:59:18 wonko installd[18673]: ./postinstall: Reading package info from "/Library/Haskell/ghc-7.0.2/lib/registrations/deepseq-1.1.0.2.conf" ... done. Apr 14 17:59:18 wonko installd[18673]: ./postinstall: Reading package info from "/Library/Haskell/ghc-7.0.2/lib/registrations/fgl-5.4.2.3.conf" ... done. Apr 14 17:59:18 wonko installd[18673]: ./postinstall: fgl-5.4.2.3: dependency "mtl-2.0.1.0-5b7a9cce5565d8cc8721ba4f95becf1b" doesn't exist (ignoring) Apr 14 17:59:18 wonko installd[18673]: ./postinstall: Reading package info from "/Library/Haskell/ghc-7.0.2/lib/registrations/haskell-platform-2011.2.0.0.conf" ... done. Apr 14 17:59:18 wonko installd[18673]: ./postinstall: haskell-platform-2011.2.0.0: dependency "haskell-src-1.0.1.4-1aa2c7df30199d039aa020a6edd80791" doesn't exist (ignoring) Apr 14 17:59:18 wonko installd[18673]: ./postinstall: haskell-platform-2011.2.0.0: dependency "html-1.0.1.2-0cdd616c514cf10f5323f121a96991b6" doesn't exist (ignoring) Apr 14 17:59:18 wonko installd[18673]: ./postinstall: haskell-platform-2011.2.0.0: dependency "mtl-2.0.1.0-5b7a9cce5565d8cc8721ba4f95becf1b" doesn't exist (ignoring) Apr 14 17:59:18 wonko installd[18673]: ./postinstall: haskell-platform-2011.2.0.0: dependency "network-2.3.0.2-980e23d4d2855583b05d6ed337c0eaed" doesn't exist (ignoring) Apr 14 17:59:18 wonko installd[18673]: ./postinstall: haskell-platform-2011.2.0.0: dependency "parallel-3.1.0.1-cecd6f84e9c097c6abc356ad52b80887" doesn't exist (ignoring) Apr 14 17:59:18 wonko installd[18673]: ./postinstall: haskell-platform-2011.2.0.0: dependency "parsec-3.1.1-d6e895e0dfa7aa6990c069c9c9c706bb" doesn't exist (ignoring) Apr 14 17:59:18 wonko installd[18673]: ./postinstall: haskell-platform-2011.2.0.0: dependency "regex-base-0.93.2-ea22834f5565d6f16b2cec3e7a63917a" doesn't exist (ignoring) Apr 14 17:59:18 wonko installd[18673]: ./postinstall: haskell-platform-2011.2.0.0: dependency "regex-compat-0.93.1-bfb1854d3b8f7b71bdd9d0efc565110d" doesn't exist (ignoring) Apr 14 17:59:18 wonko installd[18673]: ./postinstall: haskell-platform-2011.2.0.0: dependency "regex-posix-0.94.4-d6abc61e1702c884e7bba4f3e318dda9" doesn't exist (ignoring) Apr 14 17:59:18 wonko installd[18673]: ./postinstall: haskell-platform-2011.2.0.0: dependency "stm-2.2.0.1-88d46817e31bfcb31467cd8ca2c7850b" doesn't exist (ignoring) Apr 14 17:59:18 wonko installd[18673]: ./postinstall: haskell-platform-2011.2.0.0: dependency "syb-0.3-00d8c06f799942b01364e795b2a54d70" doesn't exist (ignoring) Apr 14 17:59:18 wonko installd[18673]: ./postinstall: haskell-platform-2011.2.0.0: dependency "text-0.11.0.5-efa21e4f4b045cc91adb4295f85c7319" doesn't exist (ignoring) Apr 14 17:59:18 wonko installd[18673]: ./postinstall: haskell-platform-2011.2.0.0: dependency "transformers-0.2.2.0-4bbbfde1fb5c4eb17cdb1963dda698f3" doesn't exist (ignoring) Apr 14 17:59:18 wonko installd[18673]: ./postinstall: haskell-platform-2011.2.0.0: dependency "xhtml-3000.2.0.1-fa3c2d95250480afb9bfb787b25e7398" doesn't exist (ignoring) Apr 14 17:59:18 wonko installd[18673]: ./postinstall: haskell-platform-2011.2.0.0: dependency "zlib-0.5.3.1-7e19941cbd00147a79723e25160ffc8b" doesn't exist (ignoring) Apr 14 17:59:18 wonko installd[18673]: ./postinstall: Reading package info from "/Library/Haskell/ghc-7.0.2/lib/registrations/haskell-src-1.0.1.4.conf" ... done. Apr 14 17:59:18 wonko installd[18673]: ./postinstall: haskell-src-1.0.1.4: dependency "syb-0.3-00d8c06f799942b01364e795b2a54d70" doesn't exist (ignoring) Apr 14 17:59:18 wonko installd[18673]: ./postinstall: Reading package info from "/Library/Haskell/ghc-7.0.2/lib/registrations/html-1.0.1.2.conf" ... done. Apr 14 17:59:18 wonko installd[18673]: ./postinstall: Reading package info from "/Library/Haskell/ghc-7.0.2/lib/registrations/mtl-2.0.1.0.conf" ... done. Apr 14 17:59:18 wonko installd[18673]: ./postinstall: mtl-2.0.1.0: dependency "transformers-0.2.2.0-4bbbfde1fb5c4eb17cdb1963dda698f3" doesn't exist (ignoring) Apr 14 17:59:18 wonko installd[18673]: ./postinstall: Reading package info from "/Library/Haskell/ghc-7.0.2/lib/registrations/network-2.3.0.2.conf" ... done. Apr 14 17:59:18 wonko installd[18673]: ./postinstall: network-2.3.0.2: dependency "parsec-3.1.1-d6e895e0dfa7aa6990c069c9c9c706bb" doesn't exist (ignoring) Apr 14 17:59:18 wonko installd[18673]: ./postinstall: Reading package info from "/Library/Haskell/ghc-7.0.2/lib/registrations/parallel-3.1.0.1.conf" ... done. Apr 14 17:59:18 wonko installd[18673]: ./postinstall: Reading package info from "/Library/Haskell/ghc-7.0.2/lib/registrations/parsec-3.1.1.conf" ... done. Apr 14 17:59:18 wonko installd[18673]: ./postinstall: Reading package info from "/Library/Haskell/ghc-7.0.2/lib/registrations/regex-base-0.93.2.conf" ... done. Apr 14 17:59:18 wonko installd[18673]: ./postinstall: Reading package info from "/Library/Haskell/ghc-7.0.2/lib/registrations/regex-compat-0.93.1.conf" ... done. Apr 14 17:59:18 wonko installd[18673]: ./postinstall: regex-compat-0.93.1: dependency "regex-posix-0.94.4-d6abc61e1702c884e7bba4f3e318dda9" doesn't exist (ignoring) Apr 14 17:59:18 wonko installd[18673]: ./postinstall: Reading package info from "/Library/Haskell/ghc-7.0.2/lib/registrations/regex-posix-0.94.4.conf" ... done. Apr 14 17:59:18 wonko installd[18673]: ./postinstall: Reading package info from "/Library/Haskell/ghc-7.0.2/lib/registrations/stm-2.2.0.1.conf" ... done. Apr 14 17:59:18 wonko installd[18673]: ./postinstall: Reading package info from "/Library/Haskell/ghc-7.0.2/lib/registrations/syb-0.3.conf" ... done. Apr 14 17:59:18 wonko installd[18673]: ./postinstall: Reading package info from "/Library/Haskell/ghc-7.0.2/lib/registrations/text-0.11.0.5.conf" ... done. Apr 14 17:59:18 wonko installd[18673]: ./postinstall: Reading package info from "/Library/Haskell/ghc-7.0.2/lib/registrations/transformers-0.2.2.0.conf" ... done. Apr 14 17:59:18 wonko installd[18673]: ./postinstall: Reading package info from "/Library/Haskell/ghc-7.0.2/lib/registrations/xhtml-3000.2.0.1.conf" ... done. Apr 14 17:59:19 wonko installd[18673]: ./postinstall: Reading package info from "/Library/Haskell/ghc-7.0.2/lib/registrations/zlib-0.5.3.1.conf" ... done. Apr 14 17:59:19 wonko installd[18673]: ./postinstall: ln: /Library/Haskell/ghc-7.0.2/doc/ghc-doc: File exists Apr 14 17:59:19 wonko installd[18673]: ./postinstall: ln: /Library/Haskell/ghc-7.0.2/doc/ghc-api: File exists Apr 14 17:59:19 wonko installd[18673]: PackageKit: Install Failed: PKG: post-install scripts for "org.haskell.HaskellPlatform.Libraries.pkg"\nError Domain=PKInstallErrorDomain Code=112 UserInfo=0x102f52820 "An error occurred while running scripts from the package “Haskell Platform 2011.2.0.0-x86_64.pkg”." {\n NSFilePath = "./postinstall";\n NSLocalizedDescription = "An error occurred while running scripts from the package \U201cHaskell Platform 2011.2.0.0-x86_64.pkg\U201d.";\n NSURL = "#haskell.pkg -- file://localhost/Users/andrew/Downloads/Haskell%20Platform%202011.2.0.0-x86_64.pkg";\n PKInstallPackageIdentifier = "org.haskell.HaskellPlatform.Libraries.pkg";\n} Apr 14 17:59:19 wonko Installer[18657]: install:didFailWithError:Error Domain=PKInstallErrorDomain Code=112 UserInfo=0x1002af810 "An error occurred while running scripts from the package “Haskell Platform 2011.2.0.0-x86_64.pkg”." Apr 14 17:59:19 wonko Installer[18657]: Install failed: The Installer encountered an error that caused the installation to fail. Contact the software manufacturer for assistance. Apr 14 17:59:19 wonko Installer[18657]: IFDInstallController 23DC40 state = 7 Apr 14 17:59:19 wonko Installer[18657]: Displaying 'Install Failed' UI. Apr 14 17:59:19 wonko Installer[18657]: 'Install Failed' UI displayed message:'The Installer encountered an error that caused the installation to fail. Contact the software manufacturer for assistance.'. Apr 14 18:02:50 wonko Installer[24606]: @(#)PROGRAM:Install PROJECT:Install-595.1 Apr 14 18:02:50 wonko Installer[24606]: @(#)PROGRAM:Installer PROJECT:Installer-430.1 Apr 14 18:02:50 wonko Installer[24606]: Hardware: MacBookPro5,5 @ 2.53 GHz (x 2), 4096 MB RAM Apr 14 18:02:50 wonko Installer[24606]: Running OS Build: Mac OS X 10.6.7 (10J869) Apr 14 18:02:50 wonko Installer[24606]: Env: PATH=/usr/bin:/bin:/usr/sbin:/sbin Apr 14 18:02:50 wonko Installer[24606]: Env: TMPDIR=/var/folders/N-/N-IxaaKuFae5ik1WKxV6wE+++TI/-Tmp-/ Apr 14 18:02:50 wonko Installer[24606]: Env: SHELL=/bin/bash Apr 14 18:02:50 wonko Installer[24606]: Env: HOME=/Users/andrew Apr 14 18:02:50 wonko Installer[24606]: Env: USER=andrew Apr 14 18:02:50 wonko Installer[24606]: Env: LOGNAME=andrew Apr 14 18:02:50 wonko Installer[24606]: Env: DISPLAY=/tmp/launch-IvsorY/org.x:0 Apr 14 18:02:50 wonko Installer[24606]: Env: SSH_AUTH_SOCK=/tmp/launch-kqVUwk/Listeners Apr 14 18:02:50 wonko Installer[24606]: Env: Apple_PubSub_Socket_Render=/tmp/launch-dpKpq2/Render Apr 14 18:02:50 wonko Installer[24606]: Env: COMMAND_MODE=unix2003 Apr 14 18:02:50 wonko Installer[24606]: Env: __CF_USER_TEXT_ENCODING=0x1F5:0:0 Cheers, Andrew Pennebaker www.yellosoft.us

2 2

Questioning seq
by Andrew Coppin 15 Apr '11

15 Apr '11

A couple of questions: 1. Why is the existence of polymorphic seq bad? 2. Why would the existence of polymorphic rnf be worse? 3. How is pseq different from seq? That is all...

3 2

Type Classes in Haskell - how can I make GHC make a choice of types, when the type chosen doesn't matter?
by Chris Dew 15 Apr '11

15 Apr '11

This is a question about the use of type classes in Haskell. I get an error (below) when trying to compile the code (below and at https://github.com/chrisdew/haskell-sandbox/blob/master/not_working_but_cle… ). As someone just learning Haskell, I have tried following GHC's advice, but I think the cause is different. I believe the problem is that either of the types 'IO String' or plain 'String' could be the type of 'lhello ->> lbracket', but GHC doesn't know which. The problem is that it doesn't matter, either type would work fine. I have posted a working version of the code at https://github.com/chrisdew/haskell-sandbox/blob/master/working_but_ugly.hs . This replaces one of the ->> operators with a new (non type class) operator '->>>' which forces 'lhello ->> lbracket' to be of type 'IO String'. * Is my analysis correct? Or is there something else going on here? * Is there any way of informing GHC what the type of 'lhello ->> lbracket' doen't matter and that is should just chose either of the two possibilities. Or perhaps theres a LANGUAGE option which will let me specify that 'lastest declared matching instance of the class wins' if anything is undecided. Thanks, Chris. Error: chris@chris-linux-desktop:~/nonworkspace/haskell-sandbox$ ghc not_working_but_clean.hs not_working_but_clean.hs:40:16: No instance for (Stream (IO String) (IO String) (IO String) d) arising from a use of `->>' at not_working_but_clean.hs:40:16-34 Possible fix: add an instance declaration for (Stream (IO String) (IO String) (IO String) d) In the first argument of `(->>)', namely `lhello ->> lbracket' In the second argument of `($)', namely `lhello ->> lbracket ->> putStrLn' In a stmt of a 'do' expression: forkIO $ lhello ->> lbracket ->> putStrLn not_working_but_clean.hs:40:16: No instance for (Stream d String (IO ()) (IO ())) arising from a use of `->>' at not_working_but_clean.hs:40:16-47 Possible fix: add an instance declaration for (Stream d String (IO ()) (IO ())) In the second argument of `($)', namely `lhello ->> lbracket ->> putStrLn' In a stmt of a 'do' expression: forkIO $ lhello ->> lbracket ->> putStrLn In the expression: do { forkIO $ (bracket $ hello) ->> putStrLn; forkIO $ lhello ->> lbracket ->> putStrLn; forkIO $ bracket hello ->> putStrLn; forkIO $ lbracket lhello ->> putStrLn; .... } not_working_but_clean.hs: {-# LANGUAGE MultiParamTypeClasses, FlexibleInstances, TypeSynonymInstances, OverlappingInstances #-} {-# OPTIONS_GHC #-} module Main ( main ) where import Control.Concurrent (forkIO, MVar, newEmptyMVar, putMVar, takeMVar, ThreadId, threadDelay) import Control.Monad (forever, liftM) class Stream a b c d where (->>) :: a -> (b -> c) -> d instance Stream (IO d) d (IO c) (IO c) where f ->> g = f >>= g instance Stream d d (IO c) (IO c) where f ->> g = g f instance Stream d d c c where x ->> y = y $ x -- This simply wraps a string in brackets. bracket :: String -> String bracket x = "(" ++ x ++ ")" lbracket :: IO String -> IO String lbracket x = liftM bracket x hello :: String hello = "Hello World!" lhello :: IO String lhello = do return hello main :: IO () main = do forkIO $ (bracket $ hello) ->> putStrLn forkIO $ lhello ->> lbracket ->> putStrLn forkIO $ bracket hello ->> putStrLn forkIO $ lbracket lhello ->> putStrLn threadDelay 10000000 -- Sleep for at least 10 seconds before exiting. working_but_ugly.hs: {-# LANGUAGE MultiParamTypeClasses, FlexibleInstances, TypeSynonymInstances, OverlappingInstances #-} {-# OPTIONS_GHC #-} module Main ( main ) where import Control.Concurrent (forkIO, MVar, newEmptyMVar, putMVar, takeMVar, ThreadId, threadDelay) import Control.Monad (forever, liftM) class Stream a b c d where (->>) :: a -> (b -> c) -> d instance Stream (IO d) d (IO c) (IO c) where f ->> g = f >>= g instance Stream d d (IO c) (IO c) where f ->> g = g f instance Stream d d c c where x ->> y = y $ x x ->>> y = y $ x -- This simply wraps a string in brackets. bracket :: String -> String bracket x = "(" ++ x ++ ")" lbracket :: IO String -> IO String lbracket x = liftM bracket x hello :: String hello = "Hello World!" lhello :: IO String lhello = do return hello main :: IO () main = do forkIO $ (bracket $ hello) ->> putStrLn forkIO $ lhello ->>> lbracket ->> putStrLn forkIO $ bracket hello ->> putStrLn forkIO $ lbracket lhello ->> putStrLn threadDelay 10000000 -- Sleep for at least 10 seconds before exiting.

3 6

Re: [Haskell-cafe] Questioning seq
by Janis Voigtländer 14 Apr '11

14 Apr '11

Am 14.04.2011 18:57:35 schrieb Andrew Coppin<andrewcoppin(a)btinternet.com>: > A couple of questions: > > 1. Why is the existence of polymorphic seq bad? See http://www.iai.uni-bonn.de/~jv/acta.pdf, Sections 1 and 2 and pointers therein. Also, numerous discussions on this list over the years. Best, Janis. > 2. Why would the existence of polymorphic rnf be worse? > > 3. How is pseq different from seq? > > That is all... > -- Jun.-Prof. Dr. Janis Voigtländer http://www.iai.uni-bonn.de/~jv/ mailto:jv@iai.uni-bonn.de

1 0

parallel-haskell mailing list
by Eric Y. Kow 14 Apr '11

14 Apr '11

Hi everybody, I thought I should point out that there is a mailing list dedicated to parallelism and concurrency in Haskell. The group exists to bring together the various groups in the Haskell community that are working on parallelism. It is intended to provide some visibility into the various efforts and to encourage collaboration. So if you want to know what's going on in the parallel Haskell world, this is the place to hang out. You can join the list by sending an email to parallel-haskell+subscribe(a)googlegroups.com You can also visit the group on the web at http://groups.google.com/group/parallel-haskell Or for those of you who prefer GMane http://dir.gmane.org/gmane.comp.lang.haskell.parallel Enjoy! Eric -- Got news for the Parallel Haskell Digest? Send a mail to eric(a)well-typed.com!

1 0

Munich functional programmers' UG
by "Dr. Heinrich Hördegen" 14 Apr '11

14 Apr '11

Dear all, last week, there have been thoughts to create a user group for functional programmers in Munich, Germany. People who are interessted can join the new mailing list: http://www.fs.lmu.de/cgi-bin/mailman/listinfo/high-order-munich The idea is to have meetings every now and then with talks on aspects of functional programming. However, it's not restricted to Haskell. Please spread the word to anyone who could be interested. Thank you, Heinrich -- hoerdegen(a)funktional.info www.funktional.info --

1 0

Haskell on-line judge for Programming Challenges / Contests ?
by Dmitri O.Kondratiev 14 Apr '11

14 Apr '11

Hello, I am looking for a site providing on-line automatic judge for Programming Challenges that can be coded in Haskell. For example this site: http://www.programming-challenges.com provides lots of interesting programming provlems: http://www.programming-challenges.com/pg.php?page=index Yet, unfortunately only C, C++ and Java code can be submitted, judge on that site can't compile and run Haskell code. Is there any similar online judge for Haskell? Thanks, Dmitri

4 5

IO and Cont as monads
by Burak Ekici 14 Apr '11

14 Apr '11

Dear List, I am quite new in Haskell's categorical manner of programming. However I have enough knowledge in Category Theory. I want to ask a question, maybe very well-known one by some of you, about monads of Haskell. For the type constructors like Maybe and [], I managed to prove that together with 2 natural transformations (bind + return), both of these triples construct a monad. But when I try to prove that IO and Cont type constructors with the same natural transformations (bind + return) are monads as well, it was failed. Here my question is: Is there anyone who knows how to prove that IO and Cont are monads with satisfing following properties: join . fmap join = join . joinjoin . fmap return = join . return = idreturn . f = fmap f . returnjoin . fmap (fmap f) = fmap f . join Thanks already now, Burak Ekici.

4 3