September 2009 - Haskell-Cafe

NFData question
by Peter Verswyvelen 04 Sep '09

04 Sep '09

When ones makes an ADT with data constructors that has strict (and maybe unpacked) fields, e.g. data Vec2 a = Vec2 {-# UNPACK #-} !a {-# UNPACK #-} !a how does one define an NFData instance? Like this? instance NFData a => NFData (Vec2 a) where rnf (Vec2 x y) = rnf x `seq` rnf y Or is it enough to just do instance NFData a => NFData (Vec2 a) since Vec2 is fully strict anyway, so that default rnf implementation will do? Thanks, Peter

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greencard error message
by mf-hcafe-15c311f0c＠etc-network.de 04 Sep '09

04 Sep '09

greetings, greencard is confusing me: I copied an example from the command line and was hoping to look at some generated Haskell code to understand what's going on, but got this instead: | $ cat M1.gc | module M1 where | %enum PosixError Int [EACCES, ENOENT] | $ greencard M1.gc | greencard: user error (<unknown>, line <unknown>: | Don't know how to unmarshall (int res1) | | <unknown>, line <unknown>: | Don't know how to unmarshall (int res1) | | ) (is there any live code out there that i could use as an example to study? most of the links from haskell.org/greencard are dead. anybody agree to earlier comments that greencard is obsolete? i really like it so far and would be glad to hear otherwise.) thanks, mattias

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How to derive instance for type without exported constructor?
by Grigory Sarnitskiy 04 Sep '09

04 Sep '09

In System.Random StdGen is defined as data StdGen = StdGen Int32 Int32 but its constructor StdGen is not exported. How to make StdGen to be an instance of Binary? The following won't work: instance Data.Binary.Binary StdGen where put (StdGen aa ab) = do Data.Binary.put aa Data.Binary.put ab get = do aa <- get ab <- get return (StdGen aa ab)

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Parallel programming in Haskell : a reading list
by Don Stewart 04 Sep '09

04 Sep '09

Here's a reading list for parallel programming in Haskell (via GHC) I've collected while preparing a tutorial. Might be interesting to those trying to get into multicore Haskell right now, http://donsbot.wordpress.com/2009/09/03/parallel-programming-in-haskell-a-r… Are there any good resources I'm missing? -- Don

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ANN: haskell-src-exts-1.1.4
by Niklas Broberg 03 Sep '09

03 Sep '09

Fellow Haskelleers, I'm pleased to announce the release of haskell-src-exts-1.1.4! * On hackage: http://hackage.haskell.org/package/haskell-src-exts * Via cabal: cabal install haskell-src-exts * Via darcs: darcs get http://code.haskell.org/haskell-src-exts * Report bugs: http://trac.haskell.org/haskell-src-exts haskell-src-exts is a package for Haskell source code manipulation. In particular it defines an abstract syntax tree representation, and a parser and pretty-printer to convert between this representation and String. It handles (almost) all syntactic extensions to the Haskell 98 standard implemented by GHC, and the parsing can be parametrised on what extensions to recognise. haskell-src-exts-1.1.4 follows version 1.1.3 in being a mostly experimental release. From a PVP perspective, the version number indicates that no incompatible changes have been made to the *stable* portion of the package, i.e. anything that doesn't contain Annotated in its module name. The experimental stuff in Language.Haskell.Annotated{.*} has changed quite a lot, but those of you who depend on it already must obviously know what you're doing. ;-) haskell-src-exts-1.1.4: =================== * For the stable part, the only new thing in haskell-src-exts-1.1.4 is a bug fix that allows declaration headers surrounded by parentheses to be parsed, e.g. "data (a :< b) = ...". * For the experimental part, the new and cool feature is a full exact-printer, allowing full round-tripping of parsing plus printing. In other words, using the parser and the exact-printer in conjunction gives the same result back: exactPrint . parse == id . (Well, that's half a lie since you need to use parseWithComments and some uncurrying if you want to so easily compose, but that's the gist of it anyway. :-)) * There have also been a number of further changes to the (annotated) AST to allow it to retain enough source information to allow for said exact-printing. In particular there have been changes to the way contexts and declaration heads are treated, and literals have been embellished to remember just how they were given (e.g. did we use 0xFF or 255? \BEL or \a?). * With a few small caveats, the round-tripping works over the full (alas somewhat small) test suite that comes with (the darcs version of) haskell-src-exts: - Trailing whitespace on a line is not retained. (The test suite uses reverse . dropWhile (isSpace) . reverse on each line to disregard that issue.) - Tab characters in the source are converted to (8) spaces (except when they appear inside comments or literals). (The test suite doesn't contain tabs.) - Exact specification of literals is only remembered for literals used as literals :-). What I mean is, source literals appearing in other positions in the source than as Haskell literals will be converted to a basic form. E.g. 'infixl 0x02 %%' will become 'infixl 2 %%'. Apart from infix declarations (tell me you never considered writing them that way!) this mostly matters for various pragmas. - Parentheses used around the function symbol in a function declaration will not be remembered (I can't figure out how to make a non-outrageous change to the AST to cope with them). E.g. '(foo a) b = ...' will be printed as 'foo a b = ...'. If anyone has an idea for a neat (or at least non-outrageous) AST fix that could handle these cases, drop me a line. - Literate source files are not handled by the exact-printer (can't re-literate them). Please, help me test this on your source code! If you grab a darcs version of the package from the repository, you can put any and all source files you want to test in the "haskell-src-exts/Test/examples" directory and then just run 'cabal test'. Please report any failing cases to my trac! :-) Cheers and Happy Haskelling, /Niklas

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Fwd: [BostonHaskell] Next meeting: September 16th at MIT (32G-882)
by Ravi Nanavati 03 Sep '09

03 Sep '09

I'm pleased to announce the September meeting of the Boston Area Haskell Users' Group. Based on the feedback from the meeting polls and the constraints of our speaker, the September meeting has been scheduled for Wednesday, September 16th from 7pm - 9pm. I've decided to move the meeting time back half an hour since that seems closer to what we've ended up doing in practice. Like the past three meetings, it will be held in the MIT CSAIL Reading Room (32-G882, i.e. a room on the 8th floor of the Gates Tower of the MIT's Stata Center at 32 Vassar St in Cambridge, MA). Based on the topic poll, our featured speaker will be Edward Kmett, who will be presenting the second part of his monoids and parsing presentation: "A Parallel Parsing Trifecta: Iteratees, Parsec, and Monoids". Edward's slides from August are here: http://comonad.com/reader/wp-content/uploads/2009/08/IntroductionToMonoids.… I'd recommend looking at them if you weren't at the August meeting (or want a refresher on the material) since the two parts of his presentation were originally designed to go concurrently. There's also a preview of the September presentation as part of this post on Edward's blog: http://comonad.com/reader/2009/iteratees-parsec-and-monoid/ We don't have any other presenters for September, so there are still openings for Lightning Talks or other relevant announcements / advertisements. If you're interested in one of those openings or in presenting something (large or small) at a future meeting, please let me know as we're still very hungry for content overall. The September attendance poll is at: http://doodle.com/gew97steggwhrnue As always, responding to this poll will help with two things: 1. Getting an idea of what fraction of the Boston-area Haskell community can and can't attend this meeting (to help with future scheduling). 2. Giving me an estimated count of attendees, so my wife can scale her baking of goodies appropriately. Sponsorship of or other assistance with refreshments is still being eagerly solicited. I'm also still having trouble embedding this poll in a page on our Google Group, so if anyone has any ideas about that, please feel free to try (any group member can edit pages) and/or offer other assistance or suggestions. If you have any questions about the meeting please send them to the BostonHaskell mailing list: bostonhaskell(a)googlegroups.com or contact me directly. I look forward to seeing many Boston-area Haskellers at the September meeting! Thank you, - Ravi Nanavati

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ANNOUNCE: lenses -- Simple Functional Lenses
by Job Vranish 03 Sep '09

03 Sep '09

A simple but powerful implementation of function lenses (aka functional references, accessors, etc..). This library provides a convenient way to access and update the elements of a structure. It is very similar to Data.Accessors, but simpler, a bit more generic and has fewer dependencies. I particularly like how cleanly it handles nested structures in state monads. It also contains a couple simple functions that can be used to easily convert a function that fetches data from a structure to a function that modifies data in the structure. Hackage hasn't generated the haddock documentation yet, but there is a mini-tutorial in Data.Lenses that explains it's use. Also for those that are interested it's also hosted on github here: http://github.com/jvranish/Lenses/tree/master Let me know what you think, Criticism is welcome, - Job

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Re: Snow Leopard Breaks GHC
by Christian Maeder 03 Sep '09

03 Sep '09

It might help to add -optc-m32 -opta-m32 -optl-m32 to /usr/bin/ghci, too. (TH uses ghci) Cheers Christian Brian Sniffen wrote: > Having edit the Haskell Platform's /usr/bin/ghc in place, most > packages install fine. I'm still having trouble with Pandoc, even > given the advice: > >> Once cabal works, options --ld-option=-m32 (and also --gcc-option=-m32) >> may be used. These options may also be passed to "./Setup configure" > > The problem appears to come when linking an incompatible zlib version: > > src/Text/Pandoc/ODT.hs:49:26: > Exception when trying to run compile-time code: > user error (Codec.Compression.Zlib: incompatible zlib version) > Code: ($) makeZip "data" </> "odt-styles" > In the first argument of `read', namely > `$(makeZip $ "data" </> "odt-styles")' > In the expression: read ($(makeZip $ "data" </> "odt-styles")) > In the definition of `refArchive': > refArchive = read ($(makeZip $ "data" </> "odt-styles")) > > I do have a universal zlib installed by MacPorts, as well as the > universal zlib that shipped with Snow Leopard and the universal zlib > that came with Cabal. I'm not sure whether this message indicates > that TH code is searching a different library path than non-TH code or > what. Advice is most welcome. >

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WXHaskell problem
by Seb 03 Sep '09

03 Sep '09

I just installed WXHaskell via Cabal and tried one of the examples BouncingBalls.hs. But every time I run it I get this error message in a dialog saying: "assert "m_dynamicEvents" failed in SearchDynamicEventTable(): caller should check that we have dynamic events" It also asks me if I want to stop the program or suppress further warnings. And if I press Cancel in the dialog to suppress further warnings the BouncingBalls-program seems to work just fine. How can i make this warning message go away?

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Can't get my head round monad transformers
by Colin Adams 03 Sep '09

03 Sep '09

I'm trying to add a state monad onto the IO monad for use in a happstack application. I thought this should involve using StateT and Happstack.Server.SimpleHTTP.simpleHTTP', but I can't figure out how to plumb it all together. Any tips will be welcome. -- Colin Adams Preston, Lancashire, ENGLAND

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