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Question about the Monad instance for Iteratee (from the enumerator package)
by Daniel Schüssler 22 Apr '11

22 Apr '11

Hello, for reference, said instance is: > instance Monad m => Monad (Iteratee a m) where > return x = yield x (Chunks []) > > m0 >>= f = ($ m0) $ fix $ > \bind m -> Iteratee $ runIteratee m >>= \r1 -> > case r1 of > Continue k -> return (Continue (bind . k)) > Error err -> return (Error err) > Yield x (Chunks []) -> runIteratee (f x) > Yield x extra -> runIteratee (f x) >>= \r2 -> > case r2 of > Continue k -> runIteratee (k extra) > Error err -> return (Error err) > Yield x' _ -> return (Yield x' extra) The thing I don't understand yet is the last line: Why is it OK to discard the leftover input from the (f x) Iteratee and yield just the leftover input from the first one (m0)? Cheers, Daniel

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QuickCheck, (Ord a)=> [a] -> Property problem
by larry.liuxinyu 22 Apr '11

22 Apr '11

Hi, I found there is similar question as: http://groups.google.com/group/haskell-cafe/browse_thread/thread/7439262e9a… I am still think it's very strange. For example: prop_foo :: (Ord a) => [a] -> Property prop_foo xs = not (null xs) ==> maximum xs == minimum xs This is an extreme case that the property is always wrong. However, QuickCheck produces: *Main> test prop_foo OK, passed 100 tests. Why this happen? If I use verboseCheck, I can find the sample test data are as the following: *Main>verboseCheck prop_foo ... 97: [(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),()] 98: [(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(), (),(),(),(),(),(),()] 99: [(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(),(), (),(),()] OK, passed 100 tests. So Arbitrary () is generated as the instance of Ord a. I have to change the prototype as prop_foo :: (Num a) => [a] -> Property This works at least, However, since 'a'<'b', they are order-able, what if I want to test prop_foo works for char? I am using Haskell Platform (version 6.10.4), with QuickCheck version 1.2.0.0 Thanks -- Larry, LIU Xinyu https://sites.google.com/site/algoxy/home

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ANN: HacPhi 2011
by wagnerdm＠seas.upenn.edu 21 Apr '11

21 Apr '11

Greetings, I am very pleased to officially announce HacPhi 2011, a Haskell hackathon/get-together to be held July 29-31 at the University of Pennsylvania in Philadelphia supported by contributions from Amgen, Jane Street, and Microsoft Research. The hackathon will officially kick off at 2:30 Friday afternoon, and go until 5pm on Sunday (with breaks for sleep, of course). Last year's HacPhi was a lot of fun, drawing an international crowd, and many people have already expressed interest in coming back this year. Come meet your Haskelly brothers-in-arms! I want to stress that everyone is welcome---you do not have to be a Haskell guru. Helping hack on someone else's project could be a great way to increase your Haskell-fu. If you plan on coming, please officially register [1]. Registration, travel, lodging and many other details can be found on the HacPhi wiki [2]. We're also looking for a few people interested in giving short (15-20 min.) talks, probably on Saturday afternoon. Anything of interest to the Haskell community is fair game---a project you've been working on, a paper, a quick tutorial. If you'd like to give a talk, add it on the wiki [3]. Hope to see you in Philadelphia! - The Hac ? team Brent Yorgey (byorgey) Daniel Wagner (dmwit) Chris Casinghino (ccasin) [1] http://haskell.org/haskellwiki/Hac_?/Register [2] http://haskell.org/haskellwiki/Hac_? [3] http://haskell.org/haskellwiki/Hac_?/Talks

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impoosible dependencies
by Kevin Quick 21 Apr '11

21 Apr '11

Hmmm... $ cabal update $ cabal install hakyll Resolving dependencies... cabal: dependencies conflict: ghc-6.12.3 requires unix ==2.4.0.2 however unix-2.4.0.2 was excluded because ghc-6.12.3 requires unix ==2.4.1.0 $ Any advice (other than upgrading to 7.0.3, which is not an option at the moment)? -- -KQ

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Indexed monads and MonoLocalBinds
by Mikhail Vorozhtsov 21 Apr '11

21 Apr '11

Hi Cafe, Here is another example of why 'let' should be sometimes generalised. I've been recently playing with indexed monads (for JSON processing) and found out that the following code fails to typecheck: >{-# LANGUAGE MonoLocalBinds #-} >data M t t' a = M >ipure :: a -> M t t a >ipure a = M >iseq :: M t t' a -> M t' t'' b -> M t t'' b >iseq a b = M >np :: M () Bool () >np = M >test = p `iseq` np `iseq` p > where p = ipure () Test.hs:13:27: Couldn't match expected type `Bool' with actual type `()' Expected type: M Bool t''0 b0 Actual type: M () () () In the second argument of `iseq', namely `p' In the expression: p `iseq` np `iseq` p In practice that means that I need to provide a signature for almost every monadic local binding that is used more than once, which is unbearable, especially when monad transformers and complex indices are used.<http://hackage.haskell.org/trac/ghc/blog/LetGeneralisationInGhc7>

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Hackage haddock not running?
by Chris Smith 21 Apr '11

21 Apr '11

I've noticed a lot of packages recently missing documentation links on Hackage. It looks like the job to build these hasn't been running since perhaps about Saturday. Are appropriate people aware of this? -- Chris Smith

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Haskell Weekly News: Issue 178
by Daniel Santa Cruz 21 Apr '11

21 Apr '11

Welcome to issue 178 of the HWN, a newsletter covering developments in the Haskell community. This release covers the week of April 10 to 16, 2011. You can find the HTML version at: http://bit.ly/feMLgB Announcements Simon Marlow commented (http://goo.gl/XythI) on plans to have a Hackathon at MSR Cambridge this summer. Dr. Heinrich Hordegen informed us (http://goo.gl/lQvqw) of the creation of a Munich funtional programmers' user group. Help us spread the word to anyone interested in functional programming in general. Eric Y. Kow announced (http://goo.gl/PYdKI) the creation of the parallel-haskell mailing list. "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." Don Stewart announced (http://goo.gl/0nWn2) the release of the Haskell Platform 2011.2.0.1. "This release adds support for GHC 7.0.3, and significant improvements for Mac OS X users." Quotes of the Week * Gregory Collins: "Easy to use" and "high performance" don't have to be contradictory, but you shouldn't neglect the latter. * xplat: haskell is good for portability. it can run well on any sort of x86 unix, which is all there is after all. * Spockz: it's just that my first name isn't Simon so I'm not allowed to use unsafePerformIO :P * djahandarie: Category theorists are morphisms for turning cofree theorems into free theorems. * monochrom: lambdabot is partially broken and partially correct when it comes to utf-8 * mm_freak: > case analysis of syntax -> bullshit <lambdabot> True Top Reddit Stories * Simon Peyton-Jones – A Taste Of Haskell Domain: ontwik.com, Score: 57, Comments: On Reddit: http://goo.gl/fCxtx Original: http://goo.gl/4LpJQ * Really, HaskellWiki, really? Domain: i.imgur.com, Score: 43, Comments: 13 On Reddit: http://goo.gl/HTigS Original: http://i.imgur.com/fPpby.png * Haskell Platform 2011.2.0.1 is out -- major improvements for Mac users Domain: hackage.haskell.org, Score: 39, Comments: 9 On Reddit: http://goo.gl/BJWwp Original: http://goo.gl/5vbN8 * Tracing the compilation of Hello Factorial in GHC Domain: blog.ezyang.com, Score: 36, Comments: 1 On Reddit: http://goo.gl/WNA2Z Original: http://goo.gl/fsVtS * acid-state: use regular Haskell data structures without worrying about data loss or durability Domain: haskell.org, Score: 32, Comments: 5 On Reddit: http://goo.gl/2hgGj Original: http://goo.gl/tKpV2 * Persistence of Memory: persistent vs ephemeral data structures Domain: existentialtype.wordpress.com, Score: 28, Comments: 5 On Reddit: http://goo.gl/Ss70R Original: http://goo.gl/XHdJV * Bugs from using the wrong variable Domain: blog.ezyang.com, Score: 26, Comments: 11 On Reddit: http://goo.gl/x7RId Original: http://goo.gl/3HJs2 * Cost of calling Haskell from C? Domain: stackoverflow.com, Score: 20, Comments: 1 On Reddit: http://goo.gl/6OiRH Original: http://goo.gl/XfmRe * Comparing Snap and Yesod Domain: stackoverflow.com, Score: 19, Comments: 4 On Reddit: http://goo.gl/P0Gr6 Original: http://goo.gl/dvbEe * Announcing: Heist v0.5.1 Domain: snapframework.com, Score: 17, Comments: On Reddit: http://goo.gl/cIja8 Original: http://goo.gl/d7FWG Top StackOverflow Questions * Comparing Haskell's Snap and Yesod web frameworks votes: 28, answers: 4 Read on SO: http://goo.gl/AhFH5 * Haskell library for 2D drawing votes: 19, answers: 4 Read on SO: http://goo.gl/61C4e * One REPL to bind them all? votes: 19, answers: 3 Read on SO: http://goo.gl/JUFS2 * Performance considerations of Haskell FFI / C? votes: 16, answers: 4 Read on SO: http://goo.gl/tldoX * What is Haskell's style of polymorphism? votes: 13, answers: 1 Read on SO: http://goo.gl/z3JzT About the Haskell Weekly News To help create new editions of this newsletter, please send stories to dstcruz(a)gmail.com. Until next time, Daniel Santa Cruz

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Data.Binary.IncrementalGet remake
by Sergey Mironov 20 Apr '11

20 Apr '11

Hello cafe. Haskell wiki told me about continuation-based parser Data.Binary.IncrementalGet [1] from binary-strict package. I found the idea very useful and tried to use it. Original library by Lennart Kolmodin raises some questions. The lib's main data structures are: data IResult a = IFailed S String | IFinished S a | IPartial (B.ByteString -> IResult a) newtype Get r a = Get { unGet :: S -> (a -> S -> IResult r) -> IResult r } instance Monad (Get r) where return a = Get (\s -> \k -> k a s) m >>= k = Get (\s -> \cont -> unGet m s (\a -> \s' -> unGet (k a) s' cont)) fail err = Get (\s -> const $ IFailed s err) Here, "S" is parser's state. It works well, but currently doesn't support lookAhead. I tried to add such function and gave up to do it having current implementation, but write simpler one instead. Please see IncrementalGet2.hs (link [2] below). Remake is briefly tested, has no ghc-specific optimizations, but allows user to peek data from stream. What bothering me is the fact that I could actually miss idea (or part of idea) of the original. If it is so, please let me know :) For example, what is the point of using separate result type r in original Get r a? [1] - Original IncrementalGet lib. http://www.haskell.org/haskellwiki/DealingWithBinaryData#Incremental_parsing [2] - Main file of remake https://github.com/ierton/binary-incremental-get2/blob/a9f9afaa0cbc0435a8ac… [3] - whole github project https://github.com/ierton/binary-incremental-get2 -- Sergey sorry for weak English

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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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