August 2012 - Haskell-Cafe

Working with remote HTTP file as ZIP archive
by Dennis Yurichev 16 Aug '12

16 Aug '12

Hi. I want %subj%. Is it possible with lazy evaluation and any existing Haskell libraries do: 1) open remote file via HTTP, read random pieces of it but not to download the whole file? HTTP standard supporting partial reads... 2) open remote file via HTTP as ZIP archive, get list of files, get specific file (whole) or get part of specific file, but again, using lazy eval, not to fetch all stuff. So is it possible? Thanks in advance!

1 0

Parsing pragmas in a Haskell-like language
by Björn Peemöller 16 Aug '12

16 Aug '12

Dear cafe, I'm experimenting with extending the parser for a Haskell-like language by module pragmas. The parser is written using parser combinators. Currently, I adapted the lexer to ignore whitespace and comments, but create lexemes for both the pragma start and end (and the pragma's content, of course). While these lexemes are necessary for parsing the pragmas before the module header, they somehow should be ignored (treated like comments) afterwards. Could anyone give me a hint me how this behaviour (treat pragmas like comments) is achieved in GHC or in haskell-src-exts? Thanks in advance, Björn

2 1

Can pipes solve this problem? How?
by oleg＠okmij.org 16 Aug '12

16 Aug '12

> Consider code, that takes input from handle until special substring matched: > > > matchInf a res s | a `isPrefixOf` s = reverse res > > matchInf a res (c:cs) = matchInf a (c:res) cs > > hTakeWhileNotFound str hdl = hGetContents hdl >>= return.matchInf str [] > > It is simple, but the handle is closed after running. That is not good, > because I want to reuse this function. This example is part of one of Iteratee demonstrations http://okmij.org/ftp/Haskell/Iteratee/IterDemo1.hs Please search for -- Early termination: -- Counting the occurrences of the word ``the'' and the white space -- up to the occurrence of the terminating string ``the end'' The iteratee solution is a bit more general because it creates an inner stream with the part of the outer stream until the match is found. Here is a sample application: run_bterm2I fname = print =<< run =<< enum_file fname .| take_until_match "the end" (countWS_iter `en_pair` countTHE_iter) It reads the file until "the end" is found, and counts white space and occurrences of a specific word, in parallel. All this processing happens in constant space and we never need to accumulate anything into string. If you do need to accumulate into string, there is an iteratee stream2list that does that. The enumeratee take_until_match, as take and take_while, stops when the terminating condition is satisfied or when EOF is detected. In the former case, the stream may contain more data and remains usable. A part of IterDemo1 is explained in the paper http://okmij.org/ftp/Haskell/Iteratee/describe.pdf I am not sure though if I answered your question since you were looking for pipes. I wouldn't call Iteratee pipes.

2 1

Fwd: 'let' keyword optional in do notation?
by David Feuer 16 Aug '12

16 Aug '12

---------- Forwarded message ---------- From: David Feuer <david.feuer(a)gmail.com> Date: Wed, Aug 8, 2012 at 12:22 PM Subject: Re: [Haskell-cafe] 'let' keyword optional in do notation? To: Martijn Schrage <martijn(a)oblomov.com> Changing scoping rules based on whether things are right next to each other? No thanks. On Wed, Aug 8, 2012 at 11:44 AM, Martijn Schrage <martijn(a)oblomov.com> wrote: > On 08-08-12 17:27, Ertugrul Söylemez wrote: > > Vo Minh Thu <noteed(a)gmail.com> wrote: > > This is not a parsing problem, but a scoping one: try to run this > program: > > main = do > let x = y > y = 5 > let a = b > let b = 6 > print (x, y, a, b) > > Cheers, > Thu > > Martijn has actually covered this question: > > Where each sequence of let-less bindings is put in a separate > binding group. I'm no parsing wizard, but I couldn't come up with > any situations in which this would cause ambiguity. To me, the > let-less version is easier on the eyes, more consistent with <- > bindings, and also makes it less of a hassle to move stuff around. > > > To make it more clear, this is the transformation I propose: > > do ... -- not a let-less binding > x1 = exp1 -- \ > .. -- only let-less bindings > xn = expn -- / > ... -- not a let-less binding > > becomes > > do ... > let x1 = exp1 > .. > xn = expn > ... > > So > > main = do > > x = y > y = 5 > a = b > > b = 6 > print (x, y, a, b) > > > would put everything in the same binding group and compile successfully. To > get Thu's example, you would write > > main = do > > x = y > y = 5 > let a = b > let b = 6 > print (x, y, a, b) > > The last let could even be left out. > > Cheers, Martijn > > The suggestion seems sound to me, and the additional 'let' can really be > annoying in cases where you have a lot of 'let' bindings among very few > monadic actions. My current way to deal with this is to move the stuff > to separate computations, but it's certainly not a nice solution: > > myComp = c >>= f > where > f x = ... > where > a = ... > b = ... > > > Greets > Ertugrul > > > > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe > > > > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe >

5 6

[Pipes] Can pipes solve this problem? How?
by Daniel Hlynskyi 16 Aug '12

16 Aug '12

Hello Cafe. Consider code, that takes input from handle until special substring matched: > matchInf a res s | a `isPrefixOf` s = reverse res > matchInf a res (c:cs) = matchInf a (c:res) cs > hTakeWhileNotFound str hdl = hGetContents hdl >>= return.matchInf str [] It is simple, but the handle is closed after running. That is not good, because I want to reuse this function. Code can be rewritten without hGetContent, but it is much less comprehensible: hTakeWhileNotFound str hdl = fmap reverse$ findStr str hdl [0] [] where findStr str hdl indeces acc = do c <- hGetChar hdl let newIndeces = [ i+1 | i <- indeces, i < length str, str!!i == c] if length str `elem` newIndeces then return (c : acc) else findStr str hdl (0 : newIndeces) (c : acc) So, the question is - can pipes (any package of them) be the Holy Grail in this situation, to both keep simple code and better deal with handles (do not close them specifically)? How?

3 2

linking Haskell app with Curl on Windows
by Eugene Dzhurinsky 16 Aug '12

16 Aug '12

Hi! I'm facing strange issue with linking my application against Curl (using Haskell Curl binding curl-1.3.7): the application compiles well, but fails on linking stage: ===================================================================================== C:\haskell\bin\ghc.exe --make -o dist\build\imgpaste\imgpaste.exe -hide-all-packages -fbuilding-cabal-package -package-conf dist\package.conf.inplace -i -idist\build\imgpaste\imgpaste-tmp -i. -idist\build\autogen -Idist\build\autogen -Idist\build\imgpaste\imgpaste-tmp -optP-include -optPdist\build\autogen\cabal_macros.h -odir dist\build\imgpaste\imgpaste-tmp -hidir dist\build\imgpaste\imgpaste-tmp -stubdir dist\build\imgpaste\imgpaste-tmp -package-id base-4.5.0.0-597748f6f53a7442bcae283373264bb6 -package-id bytestring-0.9.2.1-df82064cddbf74693df4e042927e015e -package-id curl-1.3.7-ed08f87bd8c487f1e11a8c3b67bf4e51 -package-id directory-1.1.0.2-0270278088d4b2588b52cbec49af4cb7 -package-id hxt-9.2.2-e687550fbbb6ff367ee9c95584c3f0a0 -package-id hxt-xpath-9.1.2-4a15d34a0b66fa21832bb4bb0f68477f -package-id regex-pcre-0.94.4-f2f06ed579a684904354d97b04a74d9e -O -XHaskell98 .\Main.hs -llibcrypto -lssh2 -lssl -lz -lidn -LC:\curl\lib Linking dist\build\imgpaste\imgpaste.exe ... C:\Program Files\Haskell\curl-1.3.7\ghc-7.4.1/libHScurl-1.3.7.a(curlc.o):curlc.c:(.text+0xd2): undefined reference to `_imp__curl_easy_getinfo' C:\Program Files\Haskell\curl-1.3.7\ghc-7.4.1/libHScurl-1.3.7.a(curlc.o):curlc.c:(.text+0xee): undefined reference to `_imp__curl_easy_getinfo' C:\Program Files\Haskell\curl-1.3.7\ghc-7.4.1/libHScurl-1.3.7.a(curlc.o):curlc.c:(.text+0x10a): undefined reference to `_imp__curl_easy_getinfo' [ lots of error messages skipped ] C:\curl\lib/libcurl.a(md5.o):(.text+0x3b): undefined reference to `MD5_Update' C:\curl\lib/libcurl.a(md5.o):(.text+0x4e): undefined reference to `MD5_Final' C:\curl\lib/libcurl.a(md5.o):(.rdata+0x0): undefined reference to `MD5_Init' C:\curl\lib/libcurl.a(md5.o):(.rdata+0x4): undefined reference to `MD5_Update' C:\curl\lib/libcurl.a(md5.o):(.rdata+0x8): undefined reference to `MD5_Final' C:\curl\lib/libcurl.a(md5.o):(.rdata+0x14): undefined reference to `MD5_Init' C:\curl\lib/libcurl.a(md5.o):(.rdata+0x18): undefined reference to `MD5_Update' C:\curl\lib/libcurl.a(md5.o):(.rdata+0x1c): undefined reference to `MD5_Final' cabal.EXE: Error: some packages failed to install: imgpaste-0.2 failed during the building phase. The exception was: ExitFailure 1 ===================================================================================== What may be wrong here? Curl itself was installed without errors. Looking for symbols 'MD5_Final' in directory C:\curl\lib results in 'libcrypto.a', 'libcrypto.dll.a' and 'libcurl.a' -- Eugene N Dzhurinsky

1 0

Custom QuickCheck Gens without custom shrinks
by Niklas Hambüchen 15 Aug '12

15 Aug '12

I wrote this Gen to generate lines of texts without \NUL and \n: fullLinesProp = forAll linesGen ... linesGen = listOf . listOf $ arbitrary `suchThat` (`notElem` ['\NUL', '\n']) -- alternatively: linesGen = arbitrary `suchThat` (all (all (`notElem` ['\NUL', '\n']))) However, I just realized that QuickCheck disables shrinking when I use this instead of (linesGen = arbitrary :: Gen [String]). Why is that? Is there a way to get what I want + shrinking without defining a custom shrink (e.g. forAllShrink)? Thanks

1 1

Haskell Weekly News: Issue 240
by Daniel Santa Cruz 15 Aug '12

15 Aug '12

Welcome to issue 240 of the HWN, an issue covering crowd-sourced bits of information about Haskell from around the web. This issue covers the week of August 05 to 11, 2012. Quotes of the Week * johnw: Monads are an interface, all bets are off until you see what the interface does for a given Type * Evbn: I heard that Untyped and Well Typed tried to set up a corporate softball league, but they couldn't agree on a ruleset to use. * wuttf: i think i have to learn this language, the type systems i know just dont feel right Top Reddit Stories * Studying protein structures in Haskell Domain: haskellforall.com, Score: 53, Comments: 6 On Reddit: [1] http://goo.gl/94PA4 Original: [2] http://goo.gl/VJJD0 * Haskell Live Episode 2: Time To Refactor is up! Domain: youtube.com, Score: 52, Comments: 14 On Reddit: [3] http://goo.gl/BOmK7 Original: [4] http://goo.gl/5wTVu * Announcing Raskell - user ratings for Hackage Domain: ocharles.org.uk, Score: 43, Comments: 29 On Reddit: [5] http://goo.gl/wgK4E Original: [6] http://goo.gl/7q4RP * Yesod 1.1 Released Domain: yesodweb.com, Score: 38, Comments: 14 On Reddit: [7] http://goo.gl/jiwLx Original: [8] http://goo.gl/B5czq * Running Haskell on Raspberry Pi Domain: alenribic.com, Score: 37, Comments: 5 On Reddit: [9] http://goo.gl/zPcDA Original: [10] http://goo.gl/FsESS * Koka - a "function"-oriented language with side-effect inteference Domain: rise4fun.com, Score: 36, Comments: 18 On Reddit: [11] http://goo.gl/lyKhA Original: [12] http://goo.gl/n35n1 * Is Haskell liberal or conservative? (Response post to Yegge) Domain: blog.ezyang.com, Score: 34, Comments: 51 On Reddit: [13] http://goo.gl/AJ5Wp Original: [14] http://goo.gl/Juis3 * ANNOUNCE: basic-prelude Domain: haskell.org, Score: 34, Comments: 8 On Reddit: [15] http://goo.gl/iiDLq Original: [16] http://goo.gl/4Taz0 * ANNOUNCE: tardis Domain: haskell.org, Score: 31, Comments: 3 On Reddit: [17] http://goo.gl/e6yi7 Original: [18] http://goo.gl/590QR * Any Hackage mirrors? How do you deal with Hackage outages? Domain: self.haskell, Score: 30, Comments: 27 On Reddit: [19] http://goo.gl/1BOMF Original: [20] http://goo.gl/1BOMF * Is there a standard Haskell development setup for Vim? Domain: self.haskell, Score: 29, Comments: 44 On Reddit: [21] http://goo.gl/gSRWp Original: [22] http://goo.gl/gSRWp * O'Reilly Webinar this Thursday: Designing Type-Safe Haskell APIs Domain: yesodweb.com, Score: 28, Comments: 8 On Reddit: [23] http://goo.gl/GNogN Original: [24] http://goo.gl/ZRkrH * acid-state with Yesod Domain: meadowstalk.com, Score: 25, Comments: 13 On Reddit: [25] http://goo.gl/NM4Bc Original: [26] http://goo.gl/8MDXo * Philip Wadler: Faith, Evolution, and Programming Languages [InfoQ] Domain: infoq.com, Score: 25, Comments: 6 On Reddit: [27] http://goo.gl/WFxLq Original: [28] http://goo.gl/fBqx7 * Hiring Haskell Developers (Singapore) Domain: cufp.org, Score: 22, Comments: 13 On Reddit: [29] http://goo.gl/bhqrb Original: [30] http://goo.gl/umhh4 * Verifying Software Transactional Memory: bisimulation in Agda [pdf] Domain: cs.nott.ac.uk, Score: 22, Comments: On Reddit: [31] http://goo.gl/X1zvj Original: [32] http://goo.gl/vJ1gA * iterate (*2) 1 Domain: postimage.org, Score: 17, Comments: 13 On Reddit: [33] http://goo.gl/lwLv1 Original: [34] http://goo.gl/lfIab * Error Handling: How would you solve this? Domain: self.haskell, Score: 16, Comments: 11 On Reddit: [35] http://goo.gl/jHLZ8 Original: [36] http://goo.gl/jHLZ8 Top StackOverflow Questions * Advice defining a data structure in Haskell votes: 13, answers: 4 Read on SO: [37] http://goo.gl/xVxlx * What is the preferred way to combine two sinks? votes: 13, answers: 2 Read on SO: [38] http://goo.gl/QUczW * Why is numCapabilities a pure function? votes: 12, answers: 1 Read on SO: [39] http://goo.gl/4HU27 * Using generics to convert between types of the same “shape” votes: 10, answers: 1 Read on SO: [40] http://goo.gl/gx8ZL * Controlling parallel execution votes: 9, answers: 2 Read on SO: [41] http://goo.gl/zzvtU * Lists of lists of lists votes: 9, answers: 5 Read on SO: [42] http://goo.gl/HEicW * Positive integer type votes: 8, answers: 5 Read on SO: [43] http://goo.gl/xXY49 * Haskell bit arrays votes: 7, answers: 1 Read on SO: [44] http://goo.gl/g9TDQ * How to keep track of number of guesses in a simple guessing game (Haskell) votes: 6, answers: 2 Read on SO: [45] http://goo.gl/VQaWn * A haskell program that controls the mouse votes: 5, answers: 1 Read on SO: [46] http://goo.gl/rtveO Until next time, Daniel Santa Cruz References 1. http://www.haskellforall.com/2012/08/code-example-1.html 2. http://www.reddit.com/r/haskell/comments/y0wlw/studying_protein_structures_… 3. http://www.youtube.com/watch?v=6KkF5-_erns 4. http://www.reddit.com/r/haskell/comments/xy6gt/haskell_live_episode_2_time_… 5. http://ocharles.org.uk/blog/posts/2012-08-05-announcing-raskell.html 6. http://www.reddit.com/r/haskell/comments/xq5ap/announcing_raskell_user_rati… 7. http://www.yesodweb.com/blog/2012/08/announcing-yesod-1-1 8. http://www.reddit.com/r/haskell/comments/xujq6/yesod_11_released/ 9. http://alenribic.com/writings/post/running-haskell-on-raspberry-pi 10. http://www.reddit.com/r/haskell/comments/xs5hn/running_haskell_on_raspberry… 11. http://www.rise4fun.com/koka/tutorial 12. http://www.reddit.com/r/haskell/comments/xwe10/koka_a_functionoriented_lang… 13. http://blog.ezyang.com/2012/08/is-haskell-liberal-or-conservative/ 14. http://www.reddit.com/r/haskell/comments/xzukd/is_haskell_liberal_or_conser… 15. http://www.haskell.org/pipermail/haskell/2012-August/023461.html 16. http://www.reddit.com/r/haskell/comments/y0kld/announce_basicprelude/ 17. http://www.haskell.org/pipermail/haskell/2012-August/023455.html 18. http://www.reddit.com/r/haskell/comments/xt7yg/announce_tardis/ 19. http://www.reddit.com/r/haskell/comments/y1tgc/any_hackage_mirrors_how_do_y… 20. http://www.reddit.com/r/haskell/comments/y1tgc/any_hackage_mirrors_how_do_y… 21. http://www.reddit.com/r/haskell/comments/xxxyi/is_there_a_standard_haskell_… 22. http://www.reddit.com/r/haskell/comments/xxxyi/is_there_a_standard_haskell_… 23. http://www.yesodweb.com/blog/2012/08/webinar-oreilly 24. http://www.reddit.com/r/haskell/comments/xrebo/oreilly_webinar_this_thursda… 25. http://meadowstalk.com/post/migration-to-acid-state 26. http://www.reddit.com/r/haskell/comments/xs0t2/acidstate_with_yesod/ 27. http://www.infoq.com/presentations/Faith-Evolution-Programming-Languages 28. http://www.reddit.com/r/haskell/comments/xtusf/philip_wadler_faith_evolutio… 29. http://cufp.org/jobs/haskell-development-role-within-quant-development-team… 30. http://www.reddit.com/r/haskell/comments/xrpl4/hiring_haskell_developers_si… 31. http://www.cs.nott.ac.uk/~gmh/hu-thesis.pdf 32. http://www.reddit.com/r/haskell/comments/xw1p5/verifying_software_transacti… 33. http://postimage.org/image/ucqiynz17/ 34. http://www.reddit.com/r/haskell/comments/xwfbm/iterate_2_1/ 35. http://www.reddit.com/r/haskell/comments/y1gg0/error_handling_how_would_you… 36. http://www.reddit.com/r/haskell/comments/y1gg0/error_handling_how_would_you… 37. http://stackoverflow.com/questions/11833920/advice-defining-a-data-structur… 38. http://stackoverflow.com/questions/11844523/what-is-the-preferred-way-to-co… 39. http://stackoverflow.com/questions/11851005/why-is-numcapabilities-a-pure-f… 40. http://stackoverflow.com/questions/11887666/using-generics-to-convert-betwe… 41. http://stackoverflow.com/questions/11828654/controlling-parallel-execution 42. http://stackoverflow.com/questions/11850821/lists-of-lists-of-lists 43. http://stackoverflow.com/questions/11910143/positive-integer-type 44. http://stackoverflow.com/questions/11853306/haskell-bit-arrays 45. http://stackoverflow.com/questions/11856135/how-to-keep-track-of-number-of-… 46. http://stackoverflow.com/questions/11896870/a-haskell-program-that-controls…

1 0

Improvement suggestions
by José Lopes 15 Aug '12

15 Aug '12

Hello everyone, I am quite new to monadic code so I would like to ask for improvement suggestions on the last line of the code below. I know I could do something like "do strs <- mapM ...; intercalate ..." etc, but I would like to avoid the use of "<-". Thank you, José data XmlState = XmlState Int type XmlM a = State XmlState a loop :: Document -> XmlM String someFn :: [Document] -> XmlM String someFn docs = return concat `ap` (sequence $ intersperse (return "\n") (map loop docs)) <--- improve this line -- José António Branquinho de Oliveira Lopes 58612 - MEIC-A Instituto Superior Técnico (IST), Universidade Técnica de Lisboa (UTL) jose.lopes(a)ist.utl.pt

3 3

Upcoming resourcet 0.4
by Michael Snoyman 15 Aug '12

15 Aug '12

Hi all, I've been working with Aristid on an enhancement to resourcet[1]. Please see the issue for more details, this email isn't about that change. Instead, now that we're looking at a new breaking release, I was wondering if anyone had ideas of something they thought should be changed in resourcet. Thanks, Michael [1] https://github.com/snoyberg/conduit/issues/61

1 0