March 2012 - Haskell-Cafe

Re: [Haskell-cafe] Theoretical question: are side effects necessary?
by Christopher Svanefalk 17 Mar '12

17 Mar '12

Many thanks for the replies and references all of you! I will continue to read up on this from here, and you have all boosted my interest in investigating how complex systems could be developed using functional languages. On Fri, Mar 16, 2012 at 1:46 PM, Dimitri Scheftelowitsch < d.scheftelowitsch(a)googlemail.com> wrote: > Hi, > > it depends on what your definition of „problem“ is. If a problem is just > something that is defined by „given input A, generate output B such that > some constraints are met“, then you can go with anything Turing-complete. > > If you think of problems in the sense of software engineering and disallow > side effects, then you cannot do anything — any sensible software requires > I/O. > > If you want to compare the programming models of lambda calculus vs. > random access machines, then you have nothing worse than a polynomial > slowdown in terms of runtime. Strong Church-Turing thesis and so on. > > HTH > Dimitri > > On 16.03.2012, at 13:23, Christopher Svanefalk wrote: > > > Dear all, > > > > there is a question I have been thinking about a bit. In short, we could > simply formulate it like this: > > > > Are there any problems which cannot be solved a side effect-free > language (such as Haskell)? In other words, are there problems that would > explicitly demand semantics that can only be provided by a language > allowing direct modification of external state variables, such as Java and > C++? > > > > If not, are there problems which are simply infeasible to solve with a > side effect-free language? > > > > I realize this question is very broad, and I am not expecting an exact > answer by any means. I am asking since I am curious about the relation > between functional and imperative/procedural languages in general. I > primarily come from a Java background, but I can program Haskell and > Erlang, and have recently started exploring Scala, so this would be > interesting to know. > > > > -- > > Best, > > > > Christopher Svanefalk > > > > Student, > > Department of Computer Science and Engineering > > University of Gothenburg / Chalmers University of Technology > > -- Best, Christopher Svanefalk

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is the evaluation order deterministic when using applicative with IO
by Rouan van Dalen 17 Mar '12

17 Mar '12

Hi everyone. I was wondering if I can make assumptions about the evaluation order of the following code: isTrue :: Int -> IO Bool isTrue val = pure (||) <*> boolTest1 val <*> boolTest2 val {- boolTest1 is an inexpensive, quick check -} boolTest1 :: Int -> IO Bool boolTest1 val = undefined {- boolTest2 is a very expensive check -} boolTest2 :: Int -> IO Bool boolTest2 val = undefined When using Applicative in the isTrue function, I would like to make use of the short-circuit behaviour of || and rely on the fact that the boolTest1 will be executed first. The reason I am asking is because the boolTest functions are in the IO monad, instead of just returning pure Bool values. Regards Rouan.

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Re: [Haskell-cafe] HXT: how to get sibling element
by Никитин Лев 17 Mar '12

17 Mar '12

Oh, yes! In this situation with so poor structured source I can try to use tagsoup. (or I'll take a look at xml-conduit). Nevertheless for better undestanding HXT it will be interesting to solve this problem in HXT. Or is it impossible? 15.03.2012, 20:08, "Asten, W.G.G. van (Wilfried, Student B-TI)" <w.g.g.vanasten(a)student.utwente.nl>: > You might want to check out the xml-conduit package. It has preceding > and following sibling Axis. I am not sure how the package works > exactly, but it seems to be a good starting point. > > 2012/3/15 Никитин Лев <leon.v.nikitin(a)pravmail.ru>: > >> I absolutly agree with you but unfortunetly, it is not my xml file. >> It is extraction from html page of public web server. I cannot to change >> format of this html page. >> Sorry. I had to explain it in first letter. >> >> But than what about to get sibling text (geting sibling is an separate >> interesting tasks with no matter for my contrete case). >> >> _______________________________________________ >> Haskell-Cafe mailing list >> Haskell-Cafe(a)haskell.org >> http://www.haskell.org/mailman/listinfo/haskell-cafe

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Too much inlining on text package
by Felipe Almeida Lessa 17 Mar '12

17 Mar '12

Hello! tl;dr: text package's pack function is creating huge chunks of code everywhere. Michael Snoyman and I have been trying to nail the performance problems of persistent's Template Haskell code -- GHC was taking a lot of memory and CPU time to compile these. What we found out is that the code size was getting increased 20-fold by the simplifier on phase 0 on GHC 7.0 (c.f. http://groups.google.com/group/yesodweb/msg/9f625fcf85575263) So, what was increasing in size? Consider this extremely simple module (attached as Bug.hs): module Bug (text) where import qualified Data.Text as T text :: T.Text text = T.pack "text" Until simplifier phase 0, the code size floats but tops at 12. Here's the core: Bug.text :: Data.Text.Internal.Text [LclIdX, Unf=Unf{Src=<vanilla>, TopLvl=True, Arity=0, Value=False, ConLike=False, Cheap=False, Expandable=False, Guidance=IF_ARGS [] 11 0}] Bug.text = Data.Text.Fusion.unstream (Data.Text.Fusion.Common.streamList @ GHC.Types.Char (GHC.Base.map @ GHC.Types.Char @ GHC.Types.Char Data.Text.Internal.safe (GHC.Base.unpackCString# "text"))) Which is straightforward. However, on simplifier phase 0 the code size jumps to 391 (!!), a 32-fold increase. I've attached the core (after.hs) since it's too large to copy here on the body. So it seems that the (unstream . streamList) pair above is getting inlined to a HUGE chunk of code (at least Data.Text.Array.new is getting inlined). Worse yet, this happens for every single pack that you use, even those packs hidden by OverloadedStrings! Does anyone have any ideas why GHC is inlining so much code everywhere? While everything I said here was tested on GHC 7.0, we have evidence that GHC 7.4 suffers from the same problem. We don't know about GHC 6.12, though. This seems to be a problem for everyone who uses text, which we hope is everyone using Haskell ;-). Cheers, -- Felipe.

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Haskell Weekly News: Issue 218
by Daniel Santa Cruz 16 Mar '12

16 Mar '12

Welcome to issue 218 of the HWN, an issue covering crowd-sourced bits of information about Haskell from around the web. This issue cover the weeks of February 26 to March 10, 2012. You can find the HTML version at: http://contemplatecode.blogspot.com/2012/03/haskell-weekly-news-issue-218.h… Quotes of the Week * monochrom: shapr is not "normal". he got a decent job, then went to school. * quicksilver: typeclasses are absolutely not sugar for a record of functions quicksilver: they're unsugar. quicksilver: they're a benighted bitter poison. * edwardk: i'm not willing to give up orders of magnitude worth of performance to get a bad hack edwardk: if you want that, go code in scheme * (on pointless black magic) mgsloan: welcome to excessively pointless plumbing operators :) byorgey: mgsloan: that's... terrifying DanBurton: you should put it on hackage * cheater_: maybe you should try /part since we're talking about perfect-world situations * edwardk: i used to be a huge proponent of unicode syntax everywhere, then i used agda ;) * cmccann: free advice is, of course, any advice which satisfies the properties expected given the definition of "advice", but nothing further ;] * shachaf: "magic" is just another word for "primitive" * dylukes: I'm getting way more acquainted with the GHC build system than I'd like to be. * JoeyA: Inform7 is a programming language. A programming language tells a computer what to do. The computer is in a room. You are facing West. * mauke: sorry, the current time is not a constant lukish: It's a pity * dfletcher: > ((++" Batman!") . take 48 . cycle . show) (0/0) Top Reddit Stories * New unordered-containers release improves performance by as much as 2x, using HAMTs Domain: blog.johantibell.com, Score: 64, Comments: 46 On Reddit: [1] http://goo.gl/D6yTb Original: [2] http://goo.gl/5YUph * A humble suggestion for visual pure functional programming Domain: i.imgur.com, Score: 59, Comments: 59 On Reddit: [3] http://goo.gl/eUMrk Original: [4] http://goo.gl/DSFnj * Spike: experimental web browser in Haskell Domain: self.haskell, Score: 53, Comments: 22 On Reddit: [5] http://goo.gl/ZTR1j Original: [6] http://goo.gl/ZTR1j * GHC 7.4.1 speeds up arbtt by a factor of 22 Domain: joachim-breitner.de, Score: 51, Comments: 18 On Reddit: [7] http://goo.gl/XVBDt Original: [8] http://goo.gl/IL6EL * Eff 3.0 is out, a functional language with first-class computational effects and handlers (an alternative to monads) Domain: math.andrej.com, Score: 49, Comments: 12 On Reddit: [9] http://goo.gl/1Idw0 Original: [10] http://goo.gl/iwHlG * What's the most clever piece of Haskell code you know? Domain: self.haskell, Score: 47, Comments: 64 On Reddit: [11] http://goo.gl/T1u9t Original: [12] http://goo.gl/T1u9t * Diagrams 0.5 released: powerful, flexible embedded domain-specific language for creating vector graphics Domain: byorgey.wordpress.com, Score: 40, Comments: 7 On Reddit: [13] http://goo.gl/e75Yx Original: [14] http://goo.gl/565c1 * Faster Javascript Through Category Theory Domain: johnbender.us, Score: 39, Comments: 5 On Reddit: [15] http://goo.gl/UgvTr Original: [16] http://goo.gl/TLK4k * I'm writing a Haskell book. Almost finished the 3rd chapter. Would like feedback. Domain: bit.ly, Score: 36, Comments: 48 On Reddit: [17] http://goo.gl/AzwhR Original: [18] http://goo.gl/AzwhR * Level 0 -- a Snake clone using SDL, with a nice interactive map editor Domain: quasimal.com, Score: 28, Comments: 5 On Reddit: [19] http://goo.gl/x9gZk Original: [20] http://goo.gl/meSPo * Parallel Haskell Digest 8: MVar, summer school, Simon PJ @ YOW, and more Domain: well-typed.com, Score: 27, Comments: 1 On Reddit: [21] http://goo.gl/MDzrI Original: [22] http://goo.gl/OC6O6 * Avoid cabal hell: find nirvana Domain: yesodweb.com, Score: 27, Comments: 30 On Reddit: [23] http://goo.gl/ryd5e Original: [24] http://goo.gl/Yyzqd * Improvements to HashMap and HashSet creation Domain: blog.johantibell.com, Score: 26, Comments: 1 On Reddit: [25] http://goo.gl/yh2zs Original: [26] http://goo.gl/u9uhg * The Day Python Embarassed Imperative Programming » What, then, Shall We Say? Domain: the-27th-comrade.appspot.com, Score: 25, Comments: 20 On Reddit: [27] http://goo.gl/5KIOx Original: [28] http://goo.gl/Wi47v * Improbable uses of unsafeCoerce Domain: gist.github.com, Score: 25, Comments: 25 On Reddit: [29] http://goo.gl/cN0LE Original: [30] http://goo.gl/cxfSc Top StackOverflow Questions * Haskell: Lists, Arrays, Vectors, Sequences votes: 32, answers: 1 Read on SO: [31] http://goo.gl/YHVpC * Why type classes in Haskell were difficult to implement? votes: 20, answers: 1 Read on SO: [32] http://goo.gl/LoLHe * runST and function composition votes: 18, answers: 3 Read on SO: [33] http://goo.gl/6NSvR * How do you compute the difference between successive elements of a list of unknown size, functionally? votes: 16, answers: 7 Read on SO: [34] http://goo.gl/rnc8l * Do we care about the 'past' in FRP? votes: 16, answers: 1 Read on SO: [35] http://goo.gl/oR2rd * “evaluate” function votes: 16, answers: 1 Read on SO: [36] http://goo.gl/ShLMT * What's preferred among liftM, lilftA, etc votes: 15, answers: 1 Read on SO: [37] http://goo.gl/y8apA * Name of type pattern: R a b = Q (a -> (R a b,b)) votes: 14, answers: 3 Read on SO: [38] http://goo.gl/kRMgn * Haskell offline documentation? votes: 12, answers: 2 Read on SO: [39] http://goo.gl/Hih3z * Program design in Haskell: how to do simulation without mutability votes: 12, answers: 3 Read on SO: [40] http://goo.gl/ODJjT Until next time, Daniel Santa Cruz References 1. http://blog.johantibell.com/2012/03/announcing-unordered-containers-02.html 2. http://www.reddit.com/r/haskell/comments/ql9m5/new_unorderedcontainers_rele… 3. http://i.imgur.com/TOZZZ.png 4. http://www.reddit.com/r/haskell/comments/q7m8i/a_humble_suggestion_for_visu… 5. http://www.reddit.com/r/haskell/comments/qhce6/spike_experimental_web_brows… 6. http://www.reddit.com/r/haskell/comments/qhce6/spike_experimental_web_brows… 7. http://www.joachim-breitner.de/blog/archives/546-GHC-7.4.1-speeds-up-arbtt-… 8. http://www.reddit.com/r/haskell/comments/q86iw/ghc_741_speeds_up_arbtt_by_a… 9. http://math.andrej.com/eff/ 10. http://www.reddit.com/r/haskell/comments/qosr7/eff_30_is_out_a_functional_l… 11. http://www.reddit.com/r/haskell/comments/qn7nk/whats_the_most_clever_piece_… 12. http://www.reddit.com/r/haskell/comments/qn7nk/whats_the_most_clever_piece_… 13. http://byorgey.wordpress.com/2012/03/09/announcing-diagrams-0-5/ 14. http://www.reddit.com/r/haskell/comments/qpkxb/diagrams_05_released_powerfu… 15. http://johnbender.us/2012/02/29/faster-javascript-through-category-theory/ 16. http://www.reddit.com/r/haskell/comments/qbuk8/faster_javascript_through_ca… 17. http://bit.ly/xDsDSj 18. http://www.reddit.com/r/haskell/comments/qquc5/im_writing_a_haskell_book_al… 19. http://quasimal.com/projects/level_0.html 20. http://www.reddit.com/r/haskell/comments/qcrhj/level_0_a_snake_clone_using_… 21. http://www.well-typed.com/blog/64 22. http://www.reddit.com/r/haskell/comments/qefzt/parallel_haskell_digest_8_mv… 23. http://www.yesodweb.com/blog/2012/03/cabal-nirvana 24. http://www.reddit.com/r/haskell/comments/qkqdj/avoid_cabal_hell_find_nirvan… 25. http://blog.johantibell.com/2012/03/improvements-to-hashmap-and-hashset.html 26. http://www.reddit.com/r/haskell/comments/qp7w1/improvements_to_hashmap_and_… 27. http://the-27th-comrade.appspot.com/blog/ahJzfnRoZS0yN3RoLWNvbXJhZGVyDAsSBU… 28. http://www.reddit.com/r/haskell/comments/q7ykt/the_day_python_embarassed_im… 29. https://gist.github.com/1934179 30. http://www.reddit.com/r/haskell/comments/qadwm/improbable_uses_of_unsafecoe… 31. http://stackoverflow.com/questions/9611904/haskell-lists-arrays-vectors-seq… 32. http://stackoverflow.com/questions/9602176/why-type-classes-in-haskell-were… 33. http://stackoverflow.com/questions/9468963/runst-and-function-composition 34. http://stackoverflow.com/questions/9512513/how-do-you-compute-the-differenc… 35. http://stackoverflow.com/questions/9526836/do-we-care-about-the-past-in-frp 36. http://stackoverflow.com/questions/9553943/evaluate-function 37. http://stackoverflow.com/questions/9509350/whats-preferred-among-liftm-lilf… 38. http://stackoverflow.com/questions/9487760/name-of-type-pattern-r-a-b-q-a-r… 39. http://stackoverflow.com/questions/9454793/haskell-offline-documentation 40. http://stackoverflow.com/questions/9548913/program-design-in-haskell-how-to…

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puzzling polymorphism behavior (7.0.3 windows)
by gladstein＠gladstein.com 16 Mar '12

16 Mar '12

Why does the following program compile and produce the results it does? It seems like 3 and "x" got interpreted as the same type, (). Thanks in advance for your help. import Data.IORef import System.IO.Unsafe cell = unsafePerformIO $ newIORef [] push i = modifyIORef cell (++ [i]) main = do push 3 push "x" readIORef cell >>= return *Main> :browse cell :: GHC.IORef.IORef [a] push :: a -> IO () main :: IO [a] *Main> main [(),()]

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Trying to use more than one array in runSTUArray
by Juan Miguel Vilar 16 Mar '12

16 Mar '12

Hello, café: I am trying to use more than one array with runSTUArray but I don't seem to be able to understand how it works. My first try is this: test1 n = runSTUArray $ do a <- newArray (1, n) (2::Int) b <- newArray (1, n) (3::Int) forM_ [1..n] $ \i -> do v <- readArray a i writeArray b i (v+1) return b but it does not work. However, when I write test2 n = runSTUArray $ do let createArray v n = newArray (1, n) (v::Int) a <- createArray 2 n b <- createArray 0 n forM_ [1..n] $ \i -> do v <- readArray a i writeArray b i (v+1) return b everything is fine although I expected the two versions to be equivalent. To further complicate matters, the following createArray v n = newArray (1, n) (v::Int) test3 n = runSTUArray $ do a <- createArray 2 n b <- createArray 3 n forM_ [1..n] $ \i -> do v <- readArray a i writeArray b i (v+1) return b does not work either. Where can I find an explanation for this behaviour? Furthermore, what I am after is to use two arrays with different types (Int and Bool), is it possible? Thanks in advance, Juan Miguel -- Juan Miguel Vilar Torres Profesor titular de universidad Vicedirector de la ESTCE para ITIG e ITIS Departamento de Lenguajes y Sistemas Informáticos Escuela Superior de Tecnología y Ciencias Experimentales Universitat Jaume I Av. de Vicent Sos Baynat s/n 12071 Castelló de la Plana (Spain) Tel: +34 964 72 8365 Fax: +34 964 72 8435 jvilar(a)lsi.uji.es

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HXT: how to get sibling element
by Никитин Лев 15 Mar '12

15 Mar '12

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Functional programming podcast
by Christopher Done 15 Mar '12

15 Mar '12

Show of hands, who would be interested in working on a podcast weekly or biweekly and what would you like to provide? Light banter is an acceptable answer. Some points that might be covered on such a podcast might be: * Latest FP conferences/hackathons/etc * Competitions * Interesting papers (new, or old and dug up) * Interesting blog posts or stackoverflow questions * New and interesting libraries/tech released * Developments in the communities, funding, business developments * Interviews with prominent FP chappies This bleeds over a little with Haskell Weekly News, but it's more like FP weekly "interesting stuff", and therefore there would be a lot more news to pack in. It would be something to listen to while you're walking to work with your ipod. On 21 February 2012 15:55, Mats Rauhala <mats.rauhala(a)gmail.com> wrote: > On 15:15 Tue 21 Feb , Christopher Done wrote: >> I recently thought it would be Pretty Cool to make an FP podcast, sort >> of a spoken Haskell Weekly News but covering all FP, blogs, packages, >> conferences, papers, standards, mailing lists, even stackoverflow, >> whatever's interesting in FP. We could use Gtalk or Mumble (both quite >> high quality audio) to conduct it and cut it up in audacity. > > I for one would be an interested listener

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HXT: how to get sibling element
by Никитин Лев 15 Mar '12

15 Mar '12

Hello, haskellers. Suppose we have this xml doc (maybe, little stupid): <div> <span>Some story</span> <span>Description</span>: This story about... <span>Author</span>: Tom Smith </div> In the end I whant to get list: [("Title", "Some story"), ("Description","This story about..."), ("Author", "Tom Smith")], or, maybe this: Book "Some story" [("description","This story about..."), ("Author", "Tom Smith")] (Book = Book String [(String, String)]. First span is a special case then others and I undestand how to process it: =============== import Text.XML.HXT.Core import Text.XML.HXT.Curl import Text.XML.HXT.HTTP pageURL = "http://localhost/test.xml" main = do r <- runX (configSysVars [withCanonicalize no, withValidate no, withTrace 0, withParseHTML no] >>> readDocument [withErrors no, withWarnings no, withHTTP []] pageURL >>> getChildren >>> isElem >>> hasName "div" >>> listA (getChildren >>> hasName "span") >>> getTitle <+> getSections) putStrLn "Статьи:" putStr "<" mapM_ putStr $ map (\i -> (fst i) ++ ": " ++ (snd i) ++ "| ") r putStrLn ">" getTitle = arr head >>> getChildren >>> getText >>> arr trim >>> arr ("Title",) getSections = arr tail >>> unlistA >>> ((getChildren >>> getText >>> arr trim) &&& (getChildren >>> getText >>> arr trim)) ltrim [] = [] ltrim (' ':x) = ltrim x ltrim ('\n':x) = ltrim x ltrim ('\r':x) = ltrim x ltrim ('\t':x) = ltrim x ltrim x = x rtrim = reverse . ltrim . reverse trim = ltrim . rtrim =================== And I' get list: [("Title", "Some story"), ("Description","Description"), ("Author", "Author")] (Maybe, there is a better way to get this list?) But I cannot find a way to get text that followes some span. I suppose that I have to use function from Data.Tree.NavigatableTree.XPathAxis, but I don't "puzzle out" how to do it. Please, help me.

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