May 2010 - Haskell-Cafe

Haskell, Queries and Monad Comprehension
by Günther Schmidt 25 May '10

25 May '10

Hi all, is there anybody currently using Haskell to construct or implement a query language? I have read a couple of papers on "Monad Comprehension Calculus" and similar but none using Haskell nor any other existing programming language to build an actual implementation. Most papers give some sort of "Pseudo code", but I couldn't find any meat. Günther

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Re: [Haskell-cafe] Haskell, Queries and Monad Comprehension
by Günther Schmidt 25 May '10

25 May '10

Hello Torsten, well thank you for taking the time and answering that. It seems that the one thing I am good for, as far as contributions to this list go, is coaxing answers out of our Functional Pros. :) That is good news then, I was getting frustrated reading fantastic papers which eventually were nothing more than teasers. So, will Ferry then be usable from within Haskell? Best regards Günther Am 25.05.10 21:28, schrieb Torsten Grust: > Günther, > > we are currently underway building the second version (as in: > done right this time) of Ferry, a query compiler that > > (1) accepts queries over ordered, nested lists, > (2) compiles these queries into an intermediate algebraic form, then > (3) emits (small bundles of) SQL queries that evaluate the input > query over your off-the-shelf RDBMS. > > We've used Ferry's first version to build new LINQ to SQL providers > for Microsoft's LINQ as well as a new code generator for Phil Wadler's > Links. > > The Ferry compiler itself is built in Haskell. Surf to www.ferry-lang.org > for more information, screencasts, papers, talks, and contact us for > more details. We will be happy to share Ferry's Haskell code once in > digestable shape (soon). > > Cheers, > --Torsten > > On May 24, 2010, at 03:20 , Günther Schmidt wrote: >> Hi all, >> >> is there anybody currently using Haskell to construct or implement a query language? >> >> I have read a couple of papers on "Monad Comprehension Calculus" and similar but none using Haskell nor any other existing programming language to build an actual implementation. >> >> Most papers give some sort of "Pseudo code", but I couldn't find any meat. >> >> >> >> Günther >> >> _______________________________________________ >> Haskell-Cafe mailing list >> Haskell-Cafe(a)haskell.org >> http://www.haskell.org/mailman/listinfo/haskell-cafe >> > >

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Exception: : changeWorkingDirectory: does not exist (No such file or directory)
by Anatoly Yakovenko 25 May '10

25 May '10

anyone else seeing this behavior? anatolyy@anatolyy-linux ~ $ ghci GHCi, version 6.12.1: http://www.haskell.org/ghc/ :? for help Loading package ghc-prim ... linking ... done. Loading package integer-gmp ... linking ... done. Loading package base ... linking ... done. Loading package ffi-1.0 ... linking ... done. > 1/2 0.5 *** Exception: : changeWorkingDirectory: does not exist (No such file or directory) > 1/2 *** Exception: : changeWorkingDirectory: does not exist (No such file or directory) >

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TDD in Haskell
by Ionut G. Stan 25 May '10

25 May '10

Hi, I'm doing TDD in pretty much all of the languages that I know, and I want to introduce it early in my Haskell learning process. I wonder though, if there's some established process regarding TDD, not unit testing. I've heard of QuickCheck and HUnit, but I've also read that QuickCheck is used mostly for testing pure code, while HUnit for impure code? What framework lends itself better for writing tests before the actual production code? Can you point out to some resources regarding this? Oh, and a small off-topic question? Is it considered a good practice to use implicit imports in Haskell? I'm trying to learn from existing packages, but all those "import all" statements drive me crazy. Thanks, -- Ionuț G. Stan | http://igstan.ro

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Data instance for a GADT
by Ozgur Akgun 25 May '10

25 May '10

Cafe, How can I provide a Data instance for a GADT? I am trying to TH on it, and Uniplate requires Data. I tried StandaloneDeriving, but it seems not to work. Best, -- Ozgur Akgun

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Re: [Haskell-cafe] Writing a web service client
by Ionut G. Stan 25 May '10

25 May '10

Hello, Thanks for all the links. It seems that I have a lot to study before I produce a decent library :) The Network.HTTP package looks nice too. It seems to be at the right level of abstraction compared to Network.Browser. I have to get used with searching the Hackage repository... :) On 5/25/10 3:12 PM, Christopher Done wrote: > Hello Ionut, > > On 25 May 2010 13:26, Ionut G. Stan<ionut.g.stan(a)gmail.com> wrote: >> The first step that I want to undertake is to write a client library for the >> GitHub API, so my questions would be: >> >> 1. Is there any such library? > > I don't know of nor have seen such a library on Hackage or Github. > There was some brief talk on StackOverflow[1], but nothing about a > full library. I would be interested in using such a library as I am an > enthusiastic user of Github. > > [1]: http://stackoverflow.com/questions/1816092/using-the-github-gist-api-from-h… > >> 2. What other client library should I consider as an API model? Or maybe >> there's some paper/blog post on how do design a Haskell API? > > There are some good examples of Haskell code that uses REST-ish > APIs.[1][2][3][4] > > Regarding designing a Haskell API, Real World Haskell has some > chapters about that.[5] > > My personal advice would be at least to ensure you make it > cabal-install-able, so that entails having a proper cabal file, > documented with Haddock format, with QuickCheck test cases if > appropriate, and provide one or two examples in the documentation, see > Don's guide about writing a Haskell program[8]. > > [1]: http://hackage.haskell.org/package/flickr > [2]: http://hackage.haskell.org/package/twitter > [3]: http://hackage.haskell.org/package/feed-cli > [4]: http://hackage.haskell.org/package/Bitly > [5]: http://book.realworldhaskell.org/read/writing-a-library-working-with-json-d… > [6]: http://book.realworldhaskell.org/read/io-case-study-a-library-for-searching… > [7]: http://book.realworldhaskell.org/read/advanced-library-design-building-a-bl… > [8]: http://www.haskell.org/haskellwiki/How_to_write_a_Haskell_program > >> 3. What network package do people use for accessing simple, rest-like web >> services? Network.Curl? If not, which other? > > Network.Curl is OK but it relies on a C library which is another > dependency and it's maybe preferable to use the HTTP library or > similar library written in pure Haskell, that you can find on Hackage. > -- Ionuț G. Stan | http://igstan.ro

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[ANNOUNCE] SyntaxMacros-0.1
by Marcos Viera 25 May '10

25 May '10

We are pleased to announce the first release of SyntaxMacros [1]. SyntaxMacros is an EDSL for constructing compilers out of a collection of pre-compiled, statically type-checked, possibly mutually dependent ``language-definition fragments''. This way of constructing a compiler brings syntax macros for free. This approach is based on a collection of techniques we have previously developed: - transformation of typed abstract syntax trees [2] makes it possible to construct parsers on the fly in a type-safe way - parser combinators [3] make it possible to construct parsers dynamically - first-class attribute grammars [4] make it possible to define semantics in a compositional way More information can be found at: http://www.cs.uu.nl/wiki/Center/SyntaxMacrosForFree Regards, Marcos Viera, Doaitse Swierstra, Atze Dijkstra and Arthur Baars [1]- http://hackage.haskell.org/package/SyntaxMacros [2]- http://www.cs.uu.nl/wiki/Center/TTTAS [3]- http://www.cs.uu.nl/wiki/bin/view/HUT/ParserCombinators [4]- http://www.cs.uu.nl/wiki/Center/AspectAG

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Writing a web service client
by Ionut G. Stan 25 May '10

25 May '10

Hi, café I've begun learning Haskell for some time already, and although I don't really feel confident writing real world Haskell, I have a little idea that I want to materialize in Haskell code so I'm pretty eager to do something instead of just reading books and blog posts. The first step that I want to undertake is to write a client library for the GitHub API, so my questions would be: 1. Is there any such library? 2. What other client library should I consider as an API model? Or maybe there's some paper/blog post on how do design a Haskell API? 3. What network package do people use for accessing simple, rest-like web services? Network.Curl? If not, which other? Thanks, -- Ionuț G. Stan | http://igstan.ro

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Re: [Haskell-cafe] TDD in Haskell
by Liam O'Connor 25 May '10

25 May '10

QuickCheck is great for TDD. I have used it for such purposes. You literally encode the contract of the function as quickcheck properties. It's very lovely. Cheers. ~Liam > > > > On 25 May 2010 21:36, Ionut G. Stan <ionut.g.stan(a)gmail.com> wrote: >> Hi, >> >> I'm doing TDD in pretty much all of the languages that I know, and I want to >> introduce it early in my Haskell learning process. I wonder though, if >> there's some established process regarding TDD, not unit testing. >> >> I've heard of QuickCheck and HUnit, but I've also read that QuickCheck is >> used mostly for testing pure code, while HUnit for impure code? >> >> What framework lends itself better for writing tests before the actual >> production code? Can you point out to some resources regarding this? >> >> >> Oh, and a small off-topic question? Is it considered a good practice to use >> implicit imports in Haskell? I'm trying to learn from existing packages, but >> all those "import all" statements drive me crazy. >> >> Thanks, >> -- >> Ionuț G. Stan | http://igstan.ro >> _______________________________________________ >> Haskell-Cafe mailing list >> Haskell-Cafe(a)haskell.org >> http://www.haskell.org/mailman/listinfo/haskell-cafe >> >

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How to deal with huge text file?
by Magicloud Magiclouds 25 May '10

25 May '10

Hi, I have a file including some operation logs, in the format of the following. And I have some code to analyze it. Well, it ate all my memories. --- log: Log for item A =========== 09:10 read accountA 09:20 read accountB Log for item B .... --- code: file <- U.readFile filename mapM_ (\text -> let ope = parseOPE text in if findMissingOperation ope then U.putStrLn $ show $ fst ope else return () ) $ seperateOutput file --This function separates the input text file into paragraph by "Log for". -- 竹密岂妨流水过山高哪阻野云飞

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