- Haskell-Cafe - Haskell.org

ANNOUNCE: netpbm
by Niklas Hambüchen 03 Mar '13

03 Mar '13

I'm happy to announce a new library, "netpbm", a full implementation of the netpbm image formats (PPM, PGM, PBM) in pure Haskell. The P*N formats describe themselves as "as easy as possible"; they mainly consist of dimensions + uncompressed data and are used especially in between graphics programs, e.g. between ImageMagick and its ghostscript part as well as in ffmpeg. There are six formats (bitmap, greyscale, color, each of them coming in binary and ASCII variants). This parses all of them. The main focus of this library is *pedantic* adherence to the spec (http://netpbm.sourceforge.net/doc/pbm.html) - in fact I claim that all implementations out there do not properly implement the spec correctly, not even the netpbm project itself. I have not properly verified this yet, but my library can parse valid PPM files that don't display in my image viewer (which is built on top of the C library in question). To be honest, one cannot really blame anybody for this since netpbm allows pretty insane things (such as comments inside numeric literals) and its claim to be simple is absolutely hilarious. haskell-netpbm is backed by an extensive test suite that checks its compatibility with images created by common programs (GIMP, convert), public data sets, and random pictures I found on the Internet. It is built using attoparsec. You can find it on http://hackage.haskell.org/package/netpbm Any contributions in form of code, tests images or claims that I don't implement the spec right are highly welcome at https://github.com/nh2/haskell-netpbm Please note that I just finished the implementation and that I will clean up (break) the API in the next release, but I found it made sense to announce it as early as possible now that the format implementation is complete. Niklas

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ANN: sandi 0.2 - reasonbly fast data encoding library
by Magnus Therning 02 Mar '13

02 Mar '13

I've just uploaded sandi version 0.2[1] to Hackage. Sandi is a data encoding library. It's aimed to be the successor of dataenc, my old data encoding library. Currently the following encodings are supported: - base16 - base32 - base32 hex - base64 - base64 url - base85 - quoted printable - uu - xx - yenc The main changes since the previous release: - Added benchmarks using criterion. - Modified API to make it better suited for piecemeal decoding by making decoding of base16, qp, and yenc accept correct but incomplete input. - Add a completely new API to allow using sandi with conduit. [1]: http://hackage.haskell.org/package/sandi-0.2 -- Magnus Therning OpenPGP: 0xAB4DFBA4 email: magnus(a)therning.org jabber: magnus(a)therning.org twitter: magthe http://therning.org/magnus

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Re: [Haskell-cafe] Parser left recursion
by Paul Callaghan 02 Mar '13

02 Mar '13

Hi Another alternative is this Haskell library: https://github.com/paulcc/xsaiga This is a combinator library which is suitable for mid-scale NLP work, so handles left recursion and (high amounts of) ambiguity to produce a packed result (which can be decoded to a list of results if required). It uses a technique similar to Danielsson's for termination. The technical details (incl papers) can be found on http://cs.uwindsor.ca/~hafiz/xsaiga/pub.html, particularly in the IWPT paper http://cs.uwindsor.ca/~hafiz/pub/iwpt-07.pdf I'd be interested to see a GLL impl for Haskell, particularly for comparison with the above. Did Roman C. publish some code for this a while back? Paul On Wed, Feb 27, 2013 at 9:50 AM, Paul Callaghan <paulcc.two(a)gmail.com> wrote: > Another alternative is this Haskell library: https://github.com/paulcc/xsaiga > > This is a combinator library which is suitable for mid-scale NLP work, > so handles left recursion and (high amounts of) ambiguity to produce a > packed result (which can be decoded to a list of results if required). > It uses a technique similar to Danielsson's for termination. > > The technical details (incl papers) can be found on > http://cs.uwindsor.ca/~hafiz/xsaiga/pub.html, particularly in the IWPT > paper http://cs.uwindsor.ca/~hafiz/pub/iwpt-07.pdf > > I'd be interested to see a GLL impl for Haskell, particularly for > comparison with the above. Did Roman C. publish some code for this a > while back? > > Paul > > On Tue, Feb 26, 2013 at 7:43 PM, Dominique Devriese > <dominique.devriese(a)cs.kuleuven.be> wrote: >> 2013/2/26 Martin Drautzburg <Martin.Drautzburg(a)web.de>: >>> I wonder if I can enforce the nonNr property somehow, i.e. enforce the rule >>> "will not consider the same nonterminal again without having consumed any >>> input". >> >> You might be interested in this paper: >> >> Danielsson, Nils Anders. "Total parser combinators." ACM Sigplan >> Notices. Vol. 45. No. 9. ACM, 2010. >> >> Regards, >> Dominique >> >> _______________________________________________ >> Haskell-Cafe mailing list >> Haskell-Cafe(a)haskell.org >> http://www.haskell.org/mailman/listinfo/haskell-cafe

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Re: [Haskell-cafe] Announce: Leksah 0.13.2.0 (still a bit experimental)
by Hamish Mackenzie 02 Mar '13

02 Mar '13

12.10 uses webkit 1.10 so the -fwebkit1.8 was probably tripping it up. I have updated webkit so that webkit 1.8 is detected automatically and updated the .travis.yml file. Please try again. On 23 Feb 2013, at 06:20, alejandro.bello67(a)gmail.com wrote: > I compiled Leksah 0.13.2 using Ubuntu 12.04 (I followed the instructions on .travis.yml). It was OK, but when I tried to compile with Ubuntu 12.10, I got the following error: > > Building webkit-0.12.5... > Preprocessing library webkit-0.12.5... > gtk2hsC2hs: Errors during expansion of binding hooks: > > webkit1.8/Graphics/UI/Gtk/WebKit/DOM/File.chs:17: (column 14) [ERROR] > >>> Unknown identifier! > Cannot find a definition for `webkit_dom_file_get_file_name' in the header file. > webkit1.8/Graphics/UI/Gtk/WebKit/DOM/File.chs:23: (column 16) [ERROR] > >>> Unknown identifier! > Cannot find a definition for `webkit_dom_file_get_file_size' in the header file. > > Can you help me, please? > > Alejandro T. Bello Ruiz > > > -- > You received this message because you are subscribed to the Google Groups "leksah" group. > To unsubscribe from this group and stop receiving emails from it, send an email to leksah+unsubscribe(a)googlegroups.com. > For more options, visit https://groups.google.com/groups/opt_out. > >

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Galois is hiring!
by Jason Dagit 01 Mar '13

01 Mar '13

Galois is hiring! We're looking for functional programmers, formal methods practitioners, and project leads, with a variety of positions open at all professional experience levels. For more information on the job openings and Galois in general, see: http://corp.galois.com/careers I've written before about what makes Galois a great organization to be a part of: http://blog.codersbase.com/2012/03/life-at-galois.html Thanks, Jason

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cabal install ghc-mod installs 3 years old version
by Niklas Hambüchen 01 Mar '13

01 Mar '13

Hi, I just did cabal update and cabal install ghc-mod, and for some reason it tries to install version 0.3.0 from 3 years ago: " cabal install ghc-mod -v Reading available packages... Choosing modular solver. Resolving dependencies... Ready to install ghc-mod-0.3.0 Downloading ghc-mod-0.3.0... " cabal --version cabal-install version 1.16.0.2 using version 1.16.0.3 of the Cabal library Does anyone have an idea why that could be? Thanks

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The state of binary (de)serialization
by Nicolas Trangez 01 Mar '13

01 Mar '13

All, In order to implement some network protocol clients recently, I needed binary serialization of commands and deserialization of responses ('Command -> ByteString' and 'ByteString -> Response' functions, preferably for both strict as well as lazy ByteStrings). My go-to packages have always been 'binary' and 'cereal', but I was wondering about the current (and future) state/goals: - cereal supports chunk-based 'partial' parsing (runGetPartial). It looks like support for this is introduced in recent versions of 'binary' as well (runGetIncremental) - cereal can output a strict bytestring (runPut) or a lazy one (runPutLazy), whilst binary only outputs lazy ones (runPut) - Next to binary and cereal, there's bytestring's Builder interface for serialization, and Simon Meier's "blaze-binary" prototype There are some blog posts and comments out there about merging cereal and binary, is this what's the goal/going on (cfr runGetIncremental)? In my use-case I think using Builder instead of binary/cereal's PutM monad shouldn't be a major problem. Is this advisable performance-wise? Overall: what's the advised future-proof strategy of handling binary (de)serialization? Thanks, Nicolas

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Haskell Weekly News: Issue 260
by Daniel Santa Cruz 01 Mar '13

01 Mar '13

Welcome to issue 260 of the HWN, an issue covering crowd-sourced bits of information about Haskell from around the web. This issue covers the week of February 17 to 23, 2013. Quotes of the Week * danharaj: zygohistobfuscatory maintainomorphism * merijn: after a week you can delete 80% of your code and replace it with whatever edwardk wrote that day ;-) * merijn: If Oleg is the unit of type hackery I propose Edward as the unit of haskell productivity. * edwardk: the merijn is the unit for the number of distinct units you've named * elliott: If you encounter an edwardk in the wild: nod reassuringly when he talks about something you understand. Under NO circumstances EVER give your GitHub account name Top Reddit Stories * An awesome Haskell Valentines card, from my non-programmer girlfriend. Domain: i.imgur.com, Score: 127, Comments: 32 On Reddit: [1] http://goo.gl/5Cxwr Original: [2] http://goo.gl/BeksW * Announcing SmallCheck 1.0 Domain: ro-che.info, Score: 61, Comments: 16 On Reddit: [3] http://goo.gl/R0gvO Original: [4] http://goo.gl/7b5Dq * Intro to FRP and purely functional games: live-coding the basics of Mario in ~30 lines in Elm [video] Domain: ustream.tv, Score: 50, Comments: 12 On Reddit: [5] http://goo.gl/E22PW Original: [6] http://goo.gl/q7r8e * Reflections After a Hard Day in Haskell GUI Land Domain: fluffynukeit.com, Score: 49, Comments: 53 On Reddit: [7] http://goo.gl/xQv8i Original: [8] http://goo.gl/0hvT6 * Duncan Coutts on Parallelism and Concurrency with Haskell, Distributed Programming with Cloud Haskell Domain: infoq.com, Score: 44, Comments: 5 On Reddit: [9] http://goo.gl/jU9EH Original: [10] http://goo.gl/UkRDk * A Game of Type-Safe Tic-Tac-Toe Domain: github.com, Score: 42, Comments: 20 On Reddit: [11] http://goo.gl/SrIEx Original: [12] http://goo.gl/dFIyd * The Algebra of Algebraic Data Types, Part 3 Domain: chris-taylor.github.com, Score: 36, Comments: 7 On Reddit: [13] http://goo.gl/NQI8S Original: [14] http://goo.gl/CPkiV * Write Yourself a Haskell... in Lisp Domain: gergo.erdi.hu, Score: 36, Comments: 3 On Reddit: [15] http://goo.gl/YqdSl Original: [16] http://goo.gl/E3yzO * Overloading record fields with Lens Domain: statusfailed.com, Score: 31, Comments: 19 On Reddit: [17] http://goo.gl/oF7zN Original: [18] http://goo.gl/SOEz4 * On partial functions in Idris Domain: edwinb.wordpress.com, Score: 29, Comments: 3 On Reddit: [19] http://goo.gl/0HquS Original: [20] http://goo.gl/OaO2r * Who's using Haskell commercially? New collection of real-world case studies from FP Complete. Domain: fpcomplete.com, Score: 26, Comments: 12 On Reddit: [21] http://goo.gl/IM2rb Original: [22] http://goo.gl/zsf4c * Making 'Super Nario Bros.' in Haskell Domain: youtube.com, Score: 26, Comments: 11 On Reddit: [23] http://goo.gl/2Hg7o Original: [24] http://goo.gl/9Bb5t Top StackOverflow Questions * Why is there no implicit parallelism in Haskell? votes: 29, answers: 4 Read on SO: [25] http://goo.gl/n6VkF * Why is (a,b,c,d) not sugar for (a,(b,(c,(d,()))))? votes: 26, answers: 2 Read on SO: [26] http://goo.gl/aW6K2 * How can I understand “(.) . (.)”? votes: 21, answers: 5 Read on SO: [27] http://goo.gl/kqbcO * Are there any advantages of using Rank2Types in favor of RankNTypes? votes: 18, answers: 2 Read on SO: [28] http://goo.gl/V5naQ * What's the closest thing to Haskell's typeclasses in OCaml? votes: 16, answers: 4 Read on SO: [29] http://goo.gl/AWvgl * Why does ghci give me a type for “1 ++ 2” instead of ragequitting? votes: 16, answers: 2 Read on SO: [30] http://goo.gl/66Pti * Is (>>) supposed to discard all left-side output? votes: 11, answers: 2 Read on SO: [31] http://goo.gl/wvnLJ * Find inferred type for local function votes: 11, answers: 3 Read on SO: [32] http://goo.gl/WFCz1 * STUArray s i e - space leak only when i == Int? votes: 11, answers: 2 Read on SO: [33] http://goo.gl/fbyPE * Connecting to the System GHC votes: 10, answers: 1 Read on SO: [34] http://goo.gl/FGVN5 * Correctness of implicit lifting votes: 10, answers: 2 Read on SO: [35] http://goo.gl/5s8pR Until next time, +Daniel Santa Cruz References 1. http://i.imgur.com/P0d7v7z.jpg 2. http://www.reddit.com/r/haskell/comments/18q510/an_awesome_haskell_valentin… 3. http://ro-che.info/articles/2013-02-19-smallcheck.html 4. http://www.reddit.com/r/haskell/comments/18uhfy/announcing_smallcheck_10/ 5. http://www.ustream.tv/recorded/29330499 6. http://www.reddit.com/r/haskell/comments/190n1t/intro_to_frp_and_purely_fun… 7. http://fluffynukeit.com/2013/02/reflections-after-a-hard-day-in-haskell-gui… 8. http://www.reddit.com/r/haskell/comments/18p0nu/reflections_after_a_hard_da… 9. http://www.infoq.com/interviews/coutts-haskell 10. http://www.reddit.com/r/haskell/comments/18zvk2/duncan_coutts_on_parallelis… 11. https://github.com/leonidas/codeblog/blob/master/2013/2013-02-19-typesafe-t… 12. http://www.reddit.com/r/haskell/comments/18tys6/a_game_of_typesafe_tictacto… 13. http://chris-taylor.github.com/blog/2013/02/13/the-algebra-of-algebraic-dat… 14. http://www.reddit.com/r/haskell/comments/18p3p5/the_algebra_of_algebraic_da… 15. http://gergo.erdi.hu/blog/2013-02-17-write_yourself_a_haskell..._in_lisp/ 16. http://www.reddit.com/r/haskell/comments/18pm7r/write_yourself_a_haskell_in… 17. http://statusfailed.com/blog/2013/02/19/overloading-record-fields-with-lens… 18. http://www.reddit.com/r/haskell/comments/18t2n7/overloading_record_fields_w… 19. http://edwinb.wordpress.com/2013/02/19/on-partial-functions-in-idris/ 20. http://www.reddit.com/r/haskell/comments/18tbkj/on_partial_functions_in_idr… 21. http://fpcomplete.com/announcing-case-studies/ 22. http://www.reddit.com/r/haskell/comments/18p5n4/whos_using_haskell_commerci… 23. http://www.youtube.com/watch?v=gVLFGQGRsDw 24. http://www.reddit.com/r/haskell/comments/18v4ep/making_super_nario_bros_in_… 25. http://stackoverflow.com/questions/15005670/why-is-there-no-implicit-parall… 26. http://stackoverflow.com/questions/14973459/why-is-a-b-c-d-not-sugar-for-a-… 27. http://stackoverflow.com/questions/15029843/how-can-i-understand 28. http://stackoverflow.com/questions/15020428/are-there-any-advantages-of-usi… 29. http://stackoverflow.com/questions/14934242/whats-the-closest-thing-to-hask… 30. http://stackoverflow.com/questions/15041581/why-does-ghci-give-me-a-type-fo… 31. http://stackoverflow.com/questions/14953032/is-supposed-to-discard-all-left… 32. http://stackoverflow.com/questions/15034392/find-inferred-type-for-local-fu… 33. http://stackoverflow.com/questions/15038386/stuarray-s-i-e-space-leak-only-… 34. http://stackoverflow.com/questions/14918453/connecting-to-the-system-ghc 35. http://stackoverflow.com/questions/14928302/correctness-of-implicit-lifting 36. https://plus.google.com/105107667630152149014/about

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Split and substitution using regex-pcre
by Simon Marechal 28 Feb '13

28 Feb '13

I could not find the perl-equivalents of these functions on Hackage, so I quickly wrote mine. Code is there: https://github.com/bartavelle/pcre-utils It should behave like perl (not really tested actually), and do strange things like : > splitCompile "a" "aaaab" Right ["","","","","b"] > splitCompile "a" "aaaabaaa" Right ["","","","","b"] This will go on Hackage as I need it for the language-puppet package, but I am interested in comments, especially concerning the namespaces: * is the name of this package OK ? * is the name of the package OK ? * should I work with the regex-pcre maintainer to try to merge it ?

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How to return a network connection to C
by C K Kashyap 28 Feb '13

28 Feb '13

Hi, I am using Network.Connection to connect to gmail in my Haskell module - that's compiled to DLL and invoked from C. I need a mechanism to return the connection handle to C so that it can pass it in the subsequent calls. How can I achieve this? Regards, Kashyap

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Question about forkIO
by C K Kashyap 28 Feb '13

28 Feb '13

Hi All, Say I have a haskell function 'f' that does a forkIO and starts an action "a". I create a DLL of this haskell code and inovke "f" from C. Can I expect the "a" to continue to run once "f" has returned to C? Regards, Kashyap

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How does one create an input handle bound to a string instead of a file?
by John D. Ramsdell 28 Feb '13

28 Feb '13

How does one create a value of type System.IO.Handle for reading that takes its input from a string instead of a file? I'm looking for the equivalent of java.io.StringReader in Java. Thanks in advance. John

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Implementing name-mangling in GHC?
by Ismael Figueroa Palet 28 Feb '13

28 Feb '13

Hi café, I'm working on a small project, and I need to rename all ocurrences of a data type in a module. For instance, if the program is module Foo where data Bar fun :: Bar -> Bar fun = ... I'd like that during compilation the module looks as if it was written like: module Foo where data _BarXZY fun :: _BarXZY -> _BarXZY fun ... I think the simplest way to do this is using TH to define a special $data splice, which directly mangles the name. But then I'd need another splice every time I want to use the type. I've read other options like implementing a GHC compiler plugin ( http://www.haskell.org/ghc/docs/7.2.1/html/users_guide/compiler-plugins.html ) is it possible to perform the renaming by modifying the ModGuts structure in a compiler pass? Can you please give more pointers on how to perform this task? Thanks -- Ismael

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Problem accessing Hackage
by Roman Cheplyaka 28 Feb '13

28 Feb '13

I have this weird problem accessing hackage, when the first response gets delayed by 30 seconds: http://superuser.com/q/557271/53951 Does anyone else experience it? (No need to reply with "works for me" — I know that it works for most people.) Any clues about what can be the problem? Roman

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Re: [Haskell-cafe] The state of binary (de)serialization
by wren ng thornton 28 Feb '13

28 Feb '13

On 2/27/13 2:17 AM, Vincent Hanquez wrote: > Two major problems of lazy bytestrings is that: > > * you can't pass it to a C bindings easily. > * doing IO with it without rewriting the chunks, can sometimes (depending > how the lazy bytestring has been produced) result in a serious degradation of > performance calling syscalls on arbitrary and small chunks (e.g. socket's 'send'). If you're on a POSIX system, you can always make use of unix-bytestring[1]. In particular, for lazy ByteStrings the function you want is System.Posix.IO.ByteString.Lazy.fdWritev which performs a single syscall to write all the chunks, and without manually concatenating them. [1] http://hackage.haskell.org/package/unix-bytestring -- Live well, ~wren

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ANN: cabal-dev 0.9.2 - sandboxed haskell builds
by Rogan Creswick 28 Feb '13

28 Feb '13

The (ever growing) cabal-dev team is happy to announce v. 0.9.2! Cabal-dev is a tool to test development libraries by creating a sandboxed package and dependency build environment. Executing `cabal-dev install` will create a sandbox, named cabal-dev in the current directory, and populate it with the project dependencies, which are built and installed into a package database within the sandbox. This is a bug-fix/maintenance release, but it's worth noting that it is now possible to build cabal-dev from hackage with a recent GHC. We've also decided to focus our efforts to provide support for the current release of GHC, the current Haskell Platform, and the previous Haskell Platform. This is the long-awaited version that actually builds with recent GHC's (7.6, in particular). A great deal of thanks for this release to go to active members of the community who've patiently directed other Haskell users to the github repo for a working cabal-dev, and in particular to the dozens (!) of you who've submitted bug reports and patches that resulted in this release (in particular: Bob Ippolito for not only fixing a race condition that caused the test suite to fail. Thanks Bob!)). Urls: * Hackage: http://hackage.haskell.org/package/cabal-dev * Issue tracker: http://github.com/creswick/cabal-dev/issues * Github repo: http://github.com/creswick/cabal-dev --Rogan

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Help to write type-level function
by Dmitry Kulagin 27 Feb '13

27 Feb '13

Hi, I try to implement typed C-like structures in my little dsl. I was able to express structures using type-level naturals (type Ty is promoted): > data Ty = TInt | TBool | TStruct Symbol [Ty] That allowed to implement all needed functions, including type-level function: > type family Get (n :: Nat) (xs :: [Ty]) :: Ty But it is not very convenient to identify struct's fields using naturals, and I wanted to change Ty definition to: > data Ty = TInt | TBool | TStruct Symbol [(Symbol, Ty)] It is much closer to how C-struct looks, but I was unable to implement required type function: > type family Find (s :: Symbol) (xs :: [(Symbol,Ty)]) :: Ty Which just finds a type in a associative list. Could someone give me a hint, how to do it? Or perhaps, is it just impossible thing to do? Thanks!

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Conflicting bindings legal?!
by Andreas Abel 27 Feb '13

27 Feb '13

To your amusement, I found the following in the Agda source: abstractToConcreteCtx :: ToConcrete a c => Precedence -> a -> TCM c abstractToConcreteCtx ctx x = do scope <- getScope let scope' = scope { scopePrecedence = ctx } return $ abstractToConcrete (makeEnv scope') x where scope = (currentScope defaultEnv) { scopePrecedence = ctx } I am surprised this is a legal form of shadowing. To understand which definition of 'scope' shadows the other, I have to consult the formal definition of Haskell. But I studied computer science to *not* become a lawyer!! -- Andreas Abel <>< Du bist der geliebte Mensch. Theoretical Computer Science, University of Munich Oettingenstr. 67, D-80538 Munich, GERMANY andreas.abel(a)ifi.lmu.de http://www2.tcs.ifi.lmu.de/~abel/

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Running out of space while concatinating a list of bytestring
by C K Kashyap 27 Feb '13

27 Feb '13

Hi, I have the following code - It looks like things go okay until concatination is attempted. I get the following output There are 2258 ByteStrings *** Exception: <stdout>: hPutBuf: resource exhausted (Not enough space) I am thinking that I should do strict concatination at each point in the support function - how can I go about doing so? (BS is the lazy.char8 bytestring) connectionGetNBytes :: NC.Connection -> Int -> IO ByteString connectionGetNBytes c n = do bs <- connectionGetNBytes' c n putStrLn ("There are " ++ (show (length bs)) ++ " ByteStrings") return (BS.concat bs) connectionGetNBytes' :: NC.Connection -> Int -> IO [ByteString] connectionGetNBytes' _ 0 = return [] connectionGetNBytes' c n = do l <- NC.connectionGet c n let ll = BS.length l remaining <- connectionGetNBytes' c (n - ll) return (l:crlfStr:remaining) Regards, Kashyap

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Help to write type-level function
by oleg＠okmij.org 27 Feb '13

27 Feb '13

Dmitry Kulagin wrote: > I try to implement typed C-like structures in my little dsl. HList essentially had those http://code.haskell.org/HList/ > I was unable to implement required type function: > type family Find (s :: Symbol) (xs :: [(Symbol,Ty)]) :: Ty > Which just finds a type in a associative list. HList also implemented records with named fields. Indeed, you need a type-level lookup in an associative list, and for that you need type equality. (The ordinary List.lookup has the Eq constraint, doesn't it?) Type equality can be implemented with type functions, right now. http://okmij.org/ftp/Haskell/typeEQ.html#TTypeable (That page also defined a type-level list membership function).

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tplot (out of memory)
by Manish Trivedi 27 Feb '13

27 Feb '13

Hi, I am running into a weird out of memory issue. While running timeplot over an input file having ~800 rows. From below provided info, seems like machine has enough ram (1849MB). Please let me know if anyone has pointers. * # free -m * total used free shared buffers cached Mem: 3825 1975 1849 0 13 71 -/+ buffers/cache: 1891 1934 Swap: 4031 111 3920 *#time tplot -o out.png -or 1024x768 -k 'CurrentPerHour' 'lines' -k 'RequiredPerHour' 'lines' -if adgroup_delivery_chart.input -tf 'date %Y-%m-%d %H:%M:%OS' * tplot: user error (out of memory) real 0m0.026s user 0m0.018s sys 0m0.008s -Manish

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GHC API wiki update
by satvik chauhan 27 Feb '13

27 Feb '13

I was playing with the GHC api and found that examples on the GHC wiki don't seem to work with GHC 7.6.1. Does anybody have the updated examples of the same? Regards, Satvik

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derivatives and integrals
by Christopher Howard 27 Feb '13

27 Feb '13

Hi. The scope of this question is likely bigger that Haskell, but this seems like the right crowd to ask. I'm in Calc II right now, and I'm looking for a FOSS desktop application (I use Gnu/Linux) to replace the functionality of my TI-98 in finding derivatives and integrals. (It's very convenient for double checking my own solutions.) I've heard of some software like Octave, but the manuals are usually very large, and I'm not sure what is the best one to start with. I was wondering if there is anyone on this list who uses free software to do that sort of simpler Calc stuff, and what it is that they use. -- frigidcode.com

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package dependencies should distinguish between API and implementation?
by Johannes Waldmann 27 Feb '13

27 Feb '13

Package dependencies are modelled by a relation "A depends-on B". Shouldn't this in fact be two relations: API-depends-on and implementation-depends-on? (meaning that A API-depends-on B iff some type of B is visible to the user of A) So what we currently have is implementation-depends-on and API-depends-on is a sub-relation of that. The point being the following: assume * A implementation-depends-on B.1 * not (A API-depends-on B.1), * U implementation-depends-on A * U implementation-depends-on B >= 2 Then U (indirectly) implementation-depends on two versions of B but it should still be safe? (e.g., I can install B.3, re-compile U, but keep A) Example: A = template-haskell, B = containers (at least I don't see any mention of Data.Map/Set in th's API, I think the only dependency is in the implementation of PprM http://hackage.haskell.org/packages/archive/template-haskell/2.8.0.0/doc/ht… ) or in general, A (and B.1) are baked into ghc, but there is some B.2/B.3 out there which U wants to use. Or is this what already happens? (ghc would notice that B.1.foo is different from B.2.foo. cabal-install would warn, but proceed? Then the effect of the proposal would just be to switch off these warnings in some cases?) - J.W.

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Thunks and GHC pessimisation
by oleg＠okmij.org 26 Feb '13

26 Feb '13

Tom Ellis wrote: > To avoid retaining a large lazy data structure in memory it is useful to > hide it behind a function call. Below, "many" is used twice. It is hidden > behind a function call so it can be garbage collected between uses. As you discovered, it is quite challenging to ``go against the grain'' and force recomputation. GHC is quite good at avoiding recomputation. This is a trade-off, of time vs space. For large search tree, it is space that is a premium, and laziness and similar strategies are exactly the wrong trade-off. The solution (which I've seen in some of the internal library code) is to confuse GHC with extra functions: http://okmij.org/ftp/Haskell/misc.html#memo-off So, eventually it is possible to force recomputation. But the solution leaves a poor taste -- fighting a compiler is never a good idea. So, this is a bug of sort -- not the bug of GHC, but of lazy evaluation. Lazy evaluation is not the best evaluation strategy. It is a trade-off, which suits a large class of problems and punishes another large class of problems.

2 1

Parser left recursion
by Martin Drautzburg 26 Feb '13

26 Feb '13

Hello all, this was previously asked on haskell-beginners, but only partially answered. As an exercise I am writing a parser roughly following the expamples in Graham Hutton's book. The language contains things like: data Exp = Lit Int -- literal integer | Plus Exp Exp My naive parser enters an infinite recursion, when I try to parse "1+2". I do understand why: "hmm, this expression could be a plus, but then it must start with an expression, lets check". and it tries to parse expression again and again considers Plus. Twan van Laarhoven told me that: > Left-recursion is always a problem for recursive-descend parsers. and suggested to do something like: > parseExp = do > lit <- parseLit > pluses <- many (parsePlusToken *> parseLit) > return (combinePlusesWithLit lit pluses) > > combinePlusesWithLit = foldr Plus -- or foldl This indeed does the trick, but only when the first token is a Lit (literal integer). I then added the possibility to optionally put things in parentheses. But then I cannot parse "(1+2)+3". The original code fails, because "(1+2)" is not a Lit and when I allow an expression as the first argument to "+" I get infinite recursion again. I am generally confused, that saying "a plus expression is an integer followed by many "plus somethings" is not what the language says. So this requires a lot of "paying attention" to get right. I'd much rather say "a plus expression is two expressions with a '+' in between". I do know for sure, that it is possible to parse "(1+2)+3" (ghci does it just fine). But I seem to be missing a trick. Can anyone shed some light on this? -- Martin

9 24

Status of GHC type-nats branch
by Takayuki Muranushi 26 Feb '13

26 Feb '13

Hi, everyone, In order to merge two type-level key-value pairs (More specifically, to use TypeNats branch of unittyped: https://bitbucket.org/xnyhps/haskell-unittyped/commits/all/tip/branch%28%22… ) I'd like to use overlapping type families and the type level Naturals at the same time. However, I couldn't build the type-nats branch of ghc, as I've been trying this week. I could build that in late 2012, so maybe this is due to updates in the git submodules. Do you reproduce this? Is there anyone else trying to build the type-nats branch? Any advice is helpful, including negative ones, if it's temporally broken or no more supported. And, I'm particularly glad if anyone has a x86-64 linux tarball of type-nats branch, and share it with me :) Best regards, -- Takayuki MURANUSHI The Hakubi Center for Advanced Research, Kyoto University http://www.hakubi.kyoto-u.ac.jp/02_mem/h22/muranushi.html

2 2

ifdef based on which OS you're on
by Andrew Cowie 26 Feb '13

26 Feb '13

I've got a piece of code that looks like this: baselineContextSSL :: IO SSLContext baselineContextSSL = do ctx <- SSL.context SSL.contextSetDefaultCiphers ctx #if defined __MACOSX__ SSL.contextSetVerificationMode ctx SSL.VerifyNone #elif defined __WIN32__ SSL.contextSetVerificationMode ctx SSL.VerifyNone #else SSL.contextSetCADirectory ctx "/etc/ssl/certs" SSL.contextSetVerificationMode ctx $ SSL.VerifyPeer True True Nothing #endif return ctx all very nice (this being necessary because apparently the non-free operating systems don't store their certs in a reliably discoverable place; bummer). That, however, is not the problem. After all, this sort of thing is what #ifdefs are for. The problem is needing to get an appropriate symbol based on what OS you're using defined. I naively assumed there would be __LINUX__ and __MACOSX__ and __WIN32__ defined by GHC because, well, that's just the sort of wishful thinking that powers the universe. So my question is: what's an appropriate Haskell mechanism for building code that is OS / arch / distro specific? It's not like I have autoconf running generating me a config.h I could #include, right? This feels simple and an appropriate use of CPP; even the symbol names look just about like what I would have expected; stackoverflow said so, must be true. Just need to get the right symbol defined at build time. Any suggestions? AfC Sydney

8 14

ANN: lazy-csv - the fastest and most space-efficient parser for CSV
by Malcolm Wallace 26 Feb '13

26 Feb '13

There are lots of Haskell CSV parsers out there. Most have poor error-reporting, and do not scale to large inputs. I am pleased to announce an industrial-strength library that is robust, fast, space-efficient, lazy, and scales to gigantic inputs with no loss of performance. http://code.haskell.org/lazy-csv/ Downloads from Hackage: http://hackage.haskell.org/package/lazy-csv This library has been in industrial use for several years now, but this is the first public release. No doubt the API is not as general as it could be, but it already serves many purposes very well. I'm happy to receive bug reports and suggestions for improvements. Regards, Malcolm

7 7

ANN: conduit-network-stream, A base layer for network protocols with Conduits
by Nils 25 Feb '13

25 Feb '13

Hello everyone, I just uploaded my "conduit-network-stream" package to hackage. It's a base layer for network protocols based on the "Conduit" package by Michael Snoyman which makes it possible to send multiple messages over a continuous network connection in a convenient way. I wrote more about it at: https://github.com/mcmaniac/conduit-network-stream/blob/master/README.md It is available on hackage and github: http://hackage.haskell.org/package/conduit-network-stream https://github.com/mcmaniac/conduit-network-stream Until the documentation on hackage is generated, I also host the haddock documentation at: http://hs.nils.cc/conduit-network-stream/index.html For any questions/problems/requests, please ask! - Nils

3 5

Thunks and GHC pessimisation
by Tom Ellis 25 Feb '13

25 Feb '13

To avoid retaining a large lazy data structure in memory it is useful to hide it behind a function call. Below, "many" is used twice. It is hidden behind a function call so it can be garbage collected between uses. That's good. When compiling with "-O" it seems that GHC 7.4.1 decides to keep it in memory anyway. That's bad. (I can't read core so I don't know exactly what's going on). Replacing one of the "many" in "twice" with "different_many" makes everything fine again. Is this considered a bug in GHC? Is it a known bug? It is incredibly concerning that GHC would perform this kind of pessimisation. Tom % cat thunkfail.hs {-# OPTIONS_GHC -fno-warn-unused-binds #-} import Data.List million :: Int million = 10 ^ (6 :: Int) many :: () -> [Int] many () = [1..million] different_many :: () -> [Int] different_many () = [1..million] twice :: (Int, Int) twice = (foldl' (+) 0 (many ()), foldl' (+) 0 (many ())) main :: IO () main = print twice % ghc -fforce-recomp -Wall -Werror -rtsopts thunkfail.hs && ./thunkfail +RTS -M5M [1 of 1] Compiling Main ( thunkfail.hs, thunkfail.o ) Linking thunkfail ... (1784293664,1784293664) % ghc -O -fforce-recomp -Wall -Werror -rtsopts thunkfail.hs && ./thunkfail +RTS -M5M [1 of 1] Compiling Main ( thunkfail.hs, thunkfail.o ) Linking thunkfail ... Heap exhausted; Current maximum heap size is 5242880 bytes (5 MB); use `+RTS -M<size>' to increase it.

3 5

Tournament knock out
by Xinyu LIU 25 Feb '13

25 Feb '13

Hi, Tournament knock out is explained in [1] by Knuth. The limitation is that it can only handle the sequence of the length 2^m for some integer m. When the champion element is removed, it was replaced with negative infinity. Typically negative infinity is represented by a predefined big negative number. Although heap sort solves all these limitation, I found tournament knock out itself can work out. Here is the Haskell code: data Infinite a = NegInf | Only a | Inf deriving (Eq, Show, Ord) only (Only x) = x data Tr a = Empty | Br (Tr a) a (Tr a) deriving Show key (Br _ k _ ) = k wrap x = Br Empty (Only x) Empty branch t1 t2 = Br t1 (min (key t1) (key t2)) t2 fromList :: (Ord a) => [a] -> Tr (Infinite a) fromList = build . (map wrap) where build [] = Empty build [t] = t build ts = build $ pair ts pair (t1:t2:ts) = (branch t1 t2):pair ts pair ts = ts pop (Br Empty _ Empty) = Br Empty Inf Empty pop (Br l k r) | k == key l = let l' = pop l in Br l' (min (key l') (key r)) r | k == key r = let r' = pop r in Br l (min (key l) (key r')) r' top = only . key tsort :: (Ord a) => [a] -> [a] tsort = sort' . fromList where sort' Empty = [] sort' (Br _ Inf _) = [] sort' t = (top t) : (sort' $ pop t) The detailed explanation can be found at: https://github.com/liuxinyu95/AlgoXY/blob/algoxy/preview/ssort-en.pdf?raw=t… [1]. Donald E. Knuth. The art of computer programming, Volume 3, sorting and searching. Larry, LIU Xinyu https://sites.google.com/site/algoxy/ https://github.com/liuxinyu95/AlgoXY

1 0

Re: [Haskell-cafe] Parser left recursion
by wren ng thornton 24 Feb '13

24 Feb '13

On 2/24/13 7:56 AM, Kim-Ee Yeoh wrote: > On Sun, Feb 24, 2013 at 7:47 PM, Roman Cheplyaka <roma(a)ro-che.info> wrote: > >> Or perhaps you meant that the production itself, when interpreted as a >> definition, is corecursive? > > I was merely thrown off by your mention of "well-founded" and the assertion > that you're left with a "strictly smaller" input. I don't see any of this. But the problem here really is an issue of well-foundedness rather than productivity. Consider the grammar, A ::= epsilon | A B B ::= ...whatever... In order to use this grammar as-is, when processing a string we must be able to magically determine how many times to recurse in order to get our epsilon. This is magic because the correct number of times to recurse is defined by the rest of the string--- which we haven't looked at! Proof theoretically speaking, this is the same as magically knowing when to use the inductive hypothesis vs when to bottom out at a base case. Using this grammar as-is, the recursive descent parser always decides to use the inductive hypothesis. Hence, infinite loop. It should be apparent that this isn't an issue with left-recursion (when reading the string from right to left), because on left-recursion we're always consuming some of the string, and therefore the input string is getting smaller on each recursive call, and therefore it is guaranteed that we will eventually run out of string, at which point we can backtrack as necessary. The problem with right-recursion is that we never know when to stop recurring. If we stop at some arbitrary point, well maybe the parse will end up failing when it could've succeeded if only we had recursed a bit further. But recursive descent doesn't allow the kind of backtracking that would be required to exhaust the search space for right-recursion. -- Live well, ~wren

1 0

presentation on Diffusion of Innovation mentioning several times Haskell
by Luc TAESCH 24 Feb '13

24 Feb '13

Just found an Interesting presentation mixing Diffusion of Innovation with Social Patterns, mentioning several times Haskell. Good introduction to a mix of techniques for whoever interested in Haskell proselytism, and go to market strategy, http://www.taesch.com/haskell/go-to-market/introduction-to-diffusion-of-inn…

1 0

Install qtHaskell 1.1.4 using Visual Studio 2010 and GHC 7.4.2
by leonardo davinci 24 Feb '13

24 Feb '13

I am trying to install qtHaskell in Windows. I have Qt 4.8.4 (the Visual Studio 2010 version). I am following the section <<2.3.2 Windows Manual Installation>> from the userGuide of qtHaskell. I set the path as described I added the line to qt.cabal, I changed dir to qws I gave qmake -recursive and then instead of mingw32-make I gave nmake which is the equivalent for VS2010. After the compilation process finished with success I switched back to root directory of qtHaskell and I gave "runhaskell Setup.hs configure --ghc" and I took the message: "Setup.hs:1:12: Warning: -fglasgow-exts is deprecated: Use individual extensions instead Configuring qt-1.1.4... " After that I gave the command: "runhaskell Setup.hs makefile" and I got the message: "Setup.hs:1:12: Warning: -fglasgow-exts is deprecated: Use individual extensions instead unrecognised command: makefile (try --help)" Any help in order to continue? Thank in advance

1 0

Munich Haskell Meeting
by Heinrich Hördegen 24 Feb '13

24 Feb '13

Dear all, please note that our monthly Haskell get-together is scheduled on Wed, 27th Feb 2013 at 19h30 at Cafe Puck in Munich, Germany. If you plan to join, please go to http://www.haskell-munich.de/dates and click the button. Everyone interested in functional programming is welcome! I wish everyone of you, joining or not, a nice week, Heinrich

1 0

Fixing undeduceable instance ==> overlapping instance
by Michael Orlitzky 24 Feb '13

24 Feb '13

I'm trying to write a determinant function that works on matrices parameterized by their dimensions (Peano naturals). If I declare the following, -- Define a class so that we get a different determinant function -- on the base/recursive cases. class (Eq a, Ring.C a) => Determined m a where determinant :: (m a) -> a -- Base case, 1x1 matrices instance (Eq a, Ring.C a) => Determined (Mat (S Z) (S Z)) a where determinant m = m !!! (0,0) -- Recursive case, (n+2) x (n+2) matrices. instance (Eq a, Ring.C a, Arity n) => Determined (Mat (S (S n)) (S (S n))) a where determinant m = ... -- Recursive algorithm, the i,jth minor has dimension -- (n+1) x (n+1). foo bar (determinant (minor m i j)) I get an error stating that I'm missing an instance: Could not deduce (Determined (Mat (S n) (S n)) a) ... Clearly, I *have* an instance for that case: if n == Z, then it's the base case. If not, it's the recursive case. But GHC can't figure that out. So maybe if I define a dummy instance to make it happy, it won't notice that they overlap? instance (Eq a, Ring.C a) => Determined (Mat m m) a where determinant _ = undefined No such luck: >>> let m = fromList [[1,2],[3,4]] :: Mat2 Int >>> determinant m Overlapping instances for Determined (Mat N2 N2) Int arising from a use of `determinant' Matching instances: instance (Eq a, Ring.C a) => Determined (Mat m m) a -- Defined at Linear/Matrix2.hs:353:10 instance (Eq a, Ring.C a, Arity n) => Determined (Mat (S (S n)) (S (S n))) a ... I even tried generalizing the (Mat m m) instance definition so that OverlappingInstances would pick the one I want, but I can't get that to work either. Is there some way to massage this?

2 2

Need urgent help with Network.tls
by C K Kashyap 24 Feb '13

24 Feb '13

Hi, I am trying to use Haskell to download email/attachments from gmail. For which I am exploring Network.TLS. I got this sample from the net that connects to gmail smtp and works just fine - http://hpaste.org/82890 However, when I modify it a bit to try to connect to imap, it simply does not work. To debug, I tried to connect to a webserver, even that seems to not work. I modified the function as follows. emailGmail2 = do let host = "localhost" let port = 443 let params = defaultParamsClient{pCiphers = ciphers} g <- RNG.makeSystem h <- connectTo host (PortNumber (fromIntegral port)) hSetBuffering h LineBuffering cWrite h "GET / HTTP/1.0" cWrite h "" cWaitFor h "ABCD" con <- contextNewOnHandle h params g handshake con bye con And I get a "BAD request" response from the server - Can some please give me a sample code that uses Network.TLS to connect to a imap.google.com (or even a webserver for that matter)? Regards, Kashyap

5 11

Re: [Haskell-cafe] Type checking the content of a string
by Corentin Dupont 23 Feb '13

23 Feb '13

Finally, I solved the problem using typeOf instead of interpret: cr :: QuasiQuoter cr = QuasiQuoter { quoteExp = quoteRuleFunc} quoteRuleFunc :: String -> Q TH.Exp quoteRuleFunc s = do res <- runIO $ runInterpreter $ do setImports ["Prelude", "Language.Nomyx.Expression"] typeOf s case res of Right "RuleFunc" -> [| s |] Right _ -> fail $ "Rule doesn't typecheck" Left e -> fail $ show e I don't know really why, but now it's working very well! On Sat, Feb 23, 2013 at 7:56 PM, Daniel Gorín <jcpetruzza(a)gmail.com> wrote: > Hi Corentin, > > I've never used TH, but from what I understand, trying to combine hint and > TH would be redundant (even if it worked): whatever String you can evaluate > using hint, you can evaluate it directly in TH. Is this not the case? > > Cheers, > Daniel > > On Feb 23, 2013, at 6:53 PM, Corentin Dupont wrote: > > > Hi Daniel, > > Did you already tried to use Hint in a QuasiQuote? This would come handy > to check at compile time the validity of some strings... > > However I have the error hereunder. > > The duplicate symbol found in the object file is in fact the first > symbol in this file. So I guess GHCi tries to load it twice... > > Best, > > Corentin > > > > > > On Sat, Feb 23, 2013 at 12:03 AM, Corentin Dupont < > corentin.dupont(a)gmail.com> wrote: > > I'm trying to load my interpreter in the Q monad: > > > > cr :: QuasiQuoter > > cr = QuasiQuoter { quoteExp = quoteRuleFunc} > > > > quoteRuleFunc :: String -> Q TH.Exp > > quoteRuleFunc s = do > > res <- runIO $ runInterpreter $ do > > setImports ["Prelude", "Language.Nomyx.Rule", > "Language.Nomyx.Expression", "Language.Nomyx.Test", > > "Language.Nomyx.Examples", "GHC.Base", "Data.Maybe"] > > interpret s (as :: RuleFunc) > > case res of > > Right _ -> [| s |] > > Left e -> fail $ show e > > > > > > However, I always obtain an error during compilation: > > > > ... > > Loading package XXX ... linking ... done. > > > > > > GHCi runtime linker: fatal error: I found a duplicate definition for > symbol > > __stginit_ghczm7zi4zi1_DsMeta > > whilst processing object file > > /usr/lib/ghc/ghc-7.4.1/libHSghc-7.4.1.a > > This could be caused by: > > * Loading two different object files which export the same symbol > > * Specifying the same object file twice on the GHCi command line > > * An incorrect `package.conf' entry, causing some object to be > > loaded twice. > > GHCi cannot safely continue in this situation. Exiting now. Sorry. > > > > > > I vaguely understand that the interpreted modules are conflicting with > the compiled ones... > > > > > > > > On Fri, Feb 22, 2013 at 11:51 PM, Corentin Dupont < > corentin.dupont(a)gmail.com> wrote: > > Great! That seems very powerful. So you can do what you want during > compilation, readin files, send data over the network? > > Other question, in my example how can I halt the compilation if a test > program is wrong? > > > > > > On Fri, Feb 22, 2013 at 8:30 PM, Francesco Mazzoli <f(a)mazzo.li> wrote: > > At Fri, 22 Feb 2013 19:43:51 +0100, > > Corentin Dupont wrote: > > > Hi Adam, > > > that looks interresting. I'm totally new to TH and QuasiQuotes, though. > > > Can I run IO in a QuasiQuoter? I can run my own interpreter. > > > > Yes, you can: > > < > http://hackage.haskell.org/packages/archive/template-haskell/2.8.0.0/doc/ht… > >. > > > > Francesco > > > > > > > >

1 0

Type checking the content of a string
by Corentin Dupont 23 Feb '13

23 Feb '13

Hi all, I have a program able to read another program as a string, and interpret it (using Hint). I'd like to make unit tests, so I have a file "Test.hs" containing a serie of test programs as strings. However, how could I be sure that these test program are syntactically valid, at compile time? Those programs should have the type "RuleFunc". I tried some TH: printProg :: Q Exp -> String printProg p = unsafePerformIO $ do expr <- runQ p return $ pprint expr myTest = printProg [| <my test program> :: RuleFunc |] But it's not very satisfatory yet. When pretty printing TH changes the program quite a bit and my interpreter cannot compile it due to scoping problems. I'd like to have my test program copied back as is. Is it possible? Any other solutions? Thanks a lot! Corentin

4 7

ANN: data-fix-cse -- Common subexpression elimination for EDSLs
by Anton Kholomiov 23 Feb '13

23 Feb '13

I'm glad to announce the package for Common subexpression elimination [1]. It's an implementation of the hashconsig algorithm as described in the paper 'Implementing Explicit and Finding Implicit Sharing in EDSLs' by Oleg Kiselyov. Main point of the library is to define this algorithm in the most generic way. You can define the AST for your DSL as fixpoint type[2]. And then all you need to use the library is to define the instance for type class `Traversable`. This idea is inspired by `data-reify` [3] package which you can use to transform your ASTs to DAGs too. But it relies on inspection of the references for values when `data-fix-cse` doesn't sacrifices the purity. A short example: Let's define a tiny DSL for signals import Data.Fix type Name = String type E = Fix Exp data Exp a = Const Double | ReadPort Name | Tfm Name [a] | Mix a a deriving (Show, Eq, Ord) We can make constant signals, read them from some ports and transform them (apply some named function to the list of signals) and mix two signals. Let's define an instance of the Traversable (hence for the Functor and Foldable) import Control.Applicative import Data.Monoid import Data.Traversable import Data.Foldable instance Functor Exp where fmap f x = case x of Const d -> Const d ReadPort n -> ReadPort n Mix a b -> Mix (f a) (f b) Tfm n as -> Tfm n $ fmap f as instance Foldable Exp where foldMap f x = case x of Mix a b -> f a <> f b Tfm n as -> mconcat $ fmap f as _ -> mempty instance Traversable Exp where traverse f x = case x of Mix a b -> Mix <$> f a <*> f b Tfm n as -> Tfm n <$> traverse f as a -> pure a Now we can use the functio `cse` cse :: (Eq<http://hackage.haskell.org/packages/archive/base/4.6.0.1/doc/html/Data-Eq.h…>(f Int<http://hackage.haskell.org/packages/archive/base/4.6.0.1/doc/html/Data-Int.…>), Ord<http://hackage.haskell.org/packages/archive/base/4.6.0.1/doc/html/Data-Ord.…>(f Int<http://hackage.haskell.org/packages/archive/base/4.6.0.1/doc/html/Data-Int.…>), Traversable<http://hackage.haskell.org/packages/archive/base/4.6.0.1/doc/html/Data-Trav…>f) => Fix<http://hackage.haskell.org/packages/archive/data-fix/0.0.1/doc/html/Data-Fi…>f -> Dag<http://hackage.haskell.org/packages/archive/data-fix-cse/0.0.1/doc/html/Dat…>f to transform our AST to DAG. DAG is already sorted. Later we can define a handy wrapper to hide the details from the client newtype Sig = Sig { unSig :: E } You can find examples in the package archive Extra-Source-Files: test/Exp.hs test/Impl.hs test/Expl.hs If you want to see a real world example of usage you can find it in the csound-expression[4]. An edsl for the Csound language. One side-note form my experience: Fixpoint types can be very flexible. It's easy to compose them. If suddenly we need to add some extra data to all cases from the example above we can easily do it with just another Functor: Imagine that we want to use a SampleRate value with all signals. Then we can do it like this: type E = Fix SampledExp data SampledExp a = SampledExp SampleRate (Exp a) then we should define an instance of the type class Traversable for our new type SampleRate. The Exp doesn't change. [1] http://hackage.haskell.org/package/data-fix-cse-0.0.1 [2] http://hackage.haskell.org/package/data-fix-0.0.1 [3] http://hackage.haskell.org/package/data-reify [4] http://hackage.haskell.org/package/csound-expression Anton

3 8

Parsec without data declarations/AST
by Sean Cormican 22 Feb '13

22 Feb '13

I have been trying to create a parser for a functional programming language, but there is no need to create an AST but merely check that the code is valid according to the grammar. In the following tutorial I have been trying to take some pointers from, data declarations are used to create an AST for the language, There is, as I understand a way to parse the language without an AST. http://www.haskell.org/haskellwiki/Parsing_a_simple_imperative_language My question is what should the type signatures for example parseFile function instead of "Stmt" accept as input if the parser is to accept Strings and numerical expressions alike ? Thanks for any help, Seán

2 4

question about: --hyperlink-source
by Doaitse Swierstra 22 Feb '13

22 Feb '13

I ran into the problem that for the packages which I install using cabal install The generated html does not contain links to the sources. This issue was raised before in: http://stackoverflow.com/questions/1587635/haddock-for-cabal-installed-modu… I have been looking into the documentation available, but could not find a way to set this e.g. as a default in some .haddock file. Am I overlooking something? Doaitse

3 2

How to input Unicode string in Haskell program?
by Semyon Kholodnov 22 Feb '13

22 Feb '13

Imagine we have this simple program: module Main(main) where main = do x <- getLine putStrLn x Now I want to run it somehow, enter "résumé 履歴書 резюме" and see this string printed back as "résumé 履歴書 резюме". Now, the first problem is that my computer runs Windows, which means that I can't use ghci ":main" or result of "ghc main.hs" to enter such an outrageous string — Windows console is locked to one specific local code page, and no codepage contains Latin-1, Cyrillic and Kanji symbols at the same time. But there is also WinGHCi. So I do ":main", copy-paste this string into the window (It works! Because Windows has Unicode for 20 years now), but the output is all messed up. In a rather curious way, actually: the input string is converted to UTF-8 byte string, and its bytes are treated as being characters from my local code page. So, it appears that I have no way to enter Unicode strings into my Haskell programs by hands, I should read them from files. That's sad, and I refuse to think I am the first one with such a problem, so I assume there is a solution/workaround. Now would someone please tell me this solution? Except from "Just stick to 127 letters of ASCII", of course.

6 6

ANNOUNCE: smallcheck-1.0
by Roman Cheplyaka 22 Feb '13

22 Feb '13

I am glad to announce a new major release of SmallCheck, a property-based testing library for Haskell. The major changes in this release are documented here: http://ro-che.info/articles/2013-02-19-smallcheck.html Roman

2 2

Is the haksell-pkg-janitors group on github alive?
by Jan Stolarek 22 Feb '13

22 Feb '13

Does anyone know if haksell-pkg-janitors group on github is alive? I've submitted a pull request a week ago but no response so far. Janek

2 2

GHC panicking when using -fext-core -O2
by Christian Gosch 21 Feb '13

21 Feb '13

Hi Cafe, when I try to create core files with the above options, ghc panics with this message: ghc: panic! (the 'impossible' happened) (GHC version 7.6.2 for x86_64-unknown-linux): make_exp (App _ (Coercion _)) Please report this as a GHC bug: http://www.haskell.org/ghc/reportabug I tried this with 7.4.1 and with 7.6.2. Does anyone have an idea why this could happen, without looking at the code I am trying to compile? Thanks for any hint, Christian

1 0

websockets client
by Stephen Olsen 21 Feb '13

21 Feb '13

Are there any good websockets client libraries for haskell. I've been searching for one but can only come up with server implementations.

3 2

Re: [Haskell-cafe] generalized, tail-recursive left fold that can finish tne computation prematurely
by oleg＠okmij.org 21 Feb '13

21 Feb '13

As others have pointed out, _in principle_, foldr is not at all deficient. We can, for example, express foldl via foldr. Moreover, we can express head, tail, take, drop and even zipWith through foldr. That is, the entire list processing library can be written in terms of foldr: http://okmij.org/ftp/Algorithms.html#zip-folds That said, to express foldl via foldr, we need a higher-order fold. There are various problems with higher-order folds, related to the cost of building closures. The problems are especially severe in strict languages or strict contexts. Indeed, foldl_via_foldr f z l = foldr (\e a z -> a (f z e)) id l z first constructs the closure and then applies it to z. The closure has the same structure as the list -- it is isomorphic to the list. However, the closure representation of a list takes typically quite more space than the list. So, in strict languages, expressing foldl via foldr is a really bad idea. It won't work for big lists. BTW, this is why foldM is _left_ fold. The arguments against higher-order folds as a `big hammer' were made back in 1998 by Gibbons and Jones http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.42.1735 So, the left-fold with the early termination has a good justification. In fact, this is how Iteratees were first presented, at the DEFUN08 tutorial (part of the ICFP2008 conference). The idea of left fold with early termination is much older though. For example, Takusen (a database access framework) has been using it since 2003 or so. For a bit of history, see http://okmij.org/ftp/Streams.html#fold-stream

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ANNOUNCE: mcpi-0.0.0.2 (The MineCraft-Pi API in Haskell)
by Doug Burke 21 Feb '13

21 Feb '13

I have released versions 0.0.0.1 and 0.0.0.2 (the latter being documentation improvements but it hasn't yet been built) of mcpi: http://hackage.haskell.org/package/mcpi https://github.com/DougBurke/hmcpi It is a very simple, and very incomplete, interface to MineCraft: Pi edition http://pi.minecraft.net/ Now I just need to find out how to get ghc 7.4.2 on my Raspberry Pi (oops, I mean the Raspberry Pi I told my wife I bought as a Christmas gift for the kids) so that I can use this from ghci. Any comments and improvements are more than welcome, Doug

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