April 2011 - Haskell-Cafe

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 20 Apr '11

20 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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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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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 19 Apr '11

19 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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