April 2012 - Beginners

Is it only one data structure per ST monad?
by KC 24 Apr '12

24 Apr '12

Is it only one data structure per ST monad? -- -- Regards, KC

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Fwd: Implementing a spellchecker - problem with Data.HashTable performance
by Radosław Szymczyszyn 24 Apr '12

24 Apr '12

Hello fellow Haskellers, It's my first post to this list, so at first: Greetings to everyone! I've wanted to pick up Haskell for some time and I've run in to a perfect (at least it seemed so) oocasion at my Natural Language Processing course. At first I tried using Python, but it turned out that some extra performance wouldn't hurt and I hoped Haskell would give me some boost without forcing me to sacrifice high-level expressiveness at the same time. The first lesson was to use ByteStrings instead of ordinary Strings for IO. Then I needed to build a dictionary of words found in the source text. That's the problematic part - I've tried using a few structures, namely Data.Map and Data.HashTable from base package and Data.HashMap from unordered-containers. In all cases I couldn't reach acceptable performance. As of that, I'd like to ask about some advice and comments about the following examples. First example [1] just builds a HashTable of ~1 500 000 ByteStrings. The program takes about 11 seconds to execute. Second example [2] does roughly the same, but taking the HashTable data from a file (just a dictionary of valid Polish forms). Reading data itself is not a problem, as I've measured the execution time of a program which just reads the file (and prints its last line to actually force the read, hence "handle" Haskell's laziness) and it takes no more than a second. Coupled together reading and building, however, takes unbearably long. I didn't wait till the program stopped and just terminated it, but it had been running for ~10 minutes already. The most disappointing fact is that a roughly equivalent (and rather effortless to write in contrast to the Haskell example - I'm just learning though) Python test which just reads everything into a dict takes about 10 seconds to execute (reading and updating the dict of course). Writing the spellchecker in Python seemed pretty straightforward for me (but its certainly not the only language I know; I've had some Java (who didn't nowadays), system level C and am daily working in Erlang). I understand of course, that the most convenient tool is generally the one you know the best, but my experience writing the Haskell equivalent reached both extremes - effortless expression of the logic, but frustration because of the unexplainable (for me at least) performance behaviour. The question is why is it so slow? (or AM I RILY DAT DUMB? ;) The link to examples: [1] https://gist.github.com/2411699 TestHashTableArtificial [2] https://gist.github.com/2411699 TestReadIntoHashTable I'm looking forward to somebody shedding some light on this. Thanks in advance! Best regards, Radek Szymczyszyn

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Two questions for a production project...
by Mike Meyer 24 Apr '12

24 Apr '12

I've got a project coming up that I could use haskell for, providing I can convince myself that it's appropriate. The critical question is: Anyone using Haskell in a long-running production application? I'm talking about something where the program would run for weeks or months non-stop, with either multiple threads or multiple copies. <mike -- Mike Meyer <mwm(a)mired.org> http://www.mired.org/ Independent Software developer/SCM consultant, email for more information. O< ascii ribbon campaign - stop html mail - www.asciiribbon.org

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state monad
by Paul Higham 22 Apr '12

22 Apr '12

I am trying to come to grips with the state monad and as a starting point I copied the some code directly from the book Learn You a Haskell for Great Good then added the type synonym for Stack import Control.Monad.State type Stack = [Int] pop :: State Stack Int pop = State $ \(x:xs) -> (x,xs) push :: Int -> State Stack () push a = State $ \xs -> ((),a:xs) However, attempting to load this into ghci produces the errors stackPlay.hs:6:7: Not in scope: data constructor `State' stackPlay.hs:9:10: Not in scope: data constructor `State' Please excuse the cognitive deficit, but I cannot figure out why this should not work as advertised. ::paul

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Embed haskell
by Rosario Borda 22 Apr '12

22 Apr '12

Hi All, There is a simple method for embed haskell in other languages, like latex? An example in quasiquotation notation: \documentclass{article} \title{Hello World} \begin{document} \maketitle Hello world! and reversed: [haskell: putStrLn reverse "Hello, World!"] \end{document} Many thanks, :) Rosario

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Re: [Haskell-beginners] Training tasks
by umptious 21 Apr '12

21 Apr '12

On 20 April 2012 19:47, Michael Litchard <michael(a)schmong.org> wrote: > A copy would be most helpful. Thanks :) > The most useful part of the Elite/Traveller trading engine problem is that it's extremely easy to come up with changes to the requirements once you've finished the thing. We all have some idea how international trade works, so you can think about the domain and say - "Hey - I'm going to add tariffs! And I'm going to add alliances - most worlds will belong to one, and if you're registered with Alliance A then you don't pay tariffs at their ports, but you do pay extra at B's, and you can't dock at C's at all. But those relationships may change at an any moment." Politics and trade are complex enough so that you can always come up with some tweak to the model that you never thought of before. Which means you get to see how easy it is to refactor for completely unanticipated requirements - it's not a hard test that gives you numbers, but it does give you a feel for how the language being tested copes with the sort of unanticipated change you can get in real world requirements. Jay

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Re: [Haskell-beginners] Training tasks
by umptious 21 Apr '12

21 Apr '12

On 19 April 2012 20:20, Michael Litchard <michael(a)schmong.org> wrote: > Hi, I noticed one of your suggestions > > > - A version of the classic Traveller/Elite trading system - a really nice > > little toy business rules system that can be code either minimally or to > > show off the funkier features of a language to get more flexibility and > more > > interesting pricing rules. Minimal version would be a few tens of lines > and > > the config file declaring item > > I couldn't find anything on Elite, could you give me a reference? > I found a reference concerning Traveller, but if you had anything in > mind, it may be helpful. > > Thanks :) > If you download Oolite, available from most Linux repositories, it should have the same trading rules as the original Elite (which it stole from the Traveller pen and paper game.) There's a space trading game for the Palm Pilot that has improved more interesting rules - maybe you can find a Palm emulator if you don't have one? - http://en.wikipedia.org/wiki/Space_Trader_%28Palm_OS%29 Basically, you like goods that can be traded and you have worlds to sell them on. Worlds have government types and economic types - and in more complex variants ecologies and events - that affect the price and supply of goods. Eg Fish will be cheap on a Farming world with an Ocean environment, but expensive on an Industrial Desert world especially if it is Rich; Guns will sell for more if a War is going on; Survival Equipment and any Food Item will sell for more in the aftermath of a disaster. You can also model legality based on a Government type. The challenge is to get a system that allows trade rules to be concise and expressive while being written in a form that can be edited by a non-programmer - the config can be an .hs file, but it should be one that even a non-coder can understand and edit. It's a nice problem because it's scalable (you increase the complexity of the rules) and it has a lot of characteristics of real business systems. I knocked out the core of a solution last night - together with the recursive Shape Editor it's one of the best language test programs I know - and should have something nice around Monday if you want a copy. - Jay

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cmath Install troubles
by Tom Murphy 20 Apr '12

20 Apr '12

Any advice? If you can't even install a package with no dependencies except base, I think there's trouble: $ cabal install cmath Resolving dependencies... Configuring cmath-0.3... Preprocessing library cmath-0.3... Building cmath-0.3... [1 of 1] Compiling Foreign.C.Math.Double ( dist/build/Foreign/C/Math/Double.hs, dist/build/Foreign/C/Math/Double.o ) /var/folders/+p/+pUVycVKFDWsYP8YiYnM3k+++TM/-Tmp-/ghc70799_0/ghc70799_0.hc: In function ‘s2sB_ret’: /var/folders/+p/+pUVycVKFDWsYP8YiYnM3k+++TM/-Tmp-/ghc70799_0/ghc70799_0.hc:2850:0: error: ‘gamma’ undeclared (first use in this function) /var/folders/+p/+pUVycVKFDWsYP8YiYnM3k+++TM/-Tmp-/ghc70799_0/ghc70799_0.hc:2850:0: error: (Each undeclared identifier is reported only once /var/folders/+p/+pUVycVKFDWsYP8YiYnM3k+++TM/-Tmp-/ghc70799_0/ghc70799_0.hc:2850:0: error: for each function it appears in.) /var/folders/+p/+pUVycVKFDWsYP8YiYnM3k+++TM/-Tmp-/ghc70799_0/ghc70799_0.hc: In function ‘s2t0_ret’: /var/folders/+p/+pUVycVKFDWsYP8YiYnM3k+++TM/-Tmp-/ghc70799_0/ghc70799_0.hc:2896:0: error: ‘gamma’ undeclared (first use in this function) /var/folders/+p/+pUVycVKFDWsYP8YiYnM3k+++TM/-Tmp-/ghc70799_0/ghc70799_0.hc: In function ‘s2uH_ret’: /var/folders/+p/+pUVycVKFDWsYP8YiYnM3k+++TM/-Tmp-/ghc70799_0/ghc70799_0.hc:3088:0: error: ‘isinf’ undeclared (first use in this function) /var/folders/+p/+pUVycVKFDWsYP8YiYnM3k+++TM/-Tmp-/ghc70799_0/ghc70799_0.hc: In function ‘s2v6_ret’: /var/folders/+p/+pUVycVKFDWsYP8YiYnM3k+++TM/-Tmp-/ghc70799_0/ghc70799_0.hc:3134:0: error: ‘isinf’ undeclared (first use in this function) /var/folders/+p/+pUVycVKFDWsYP8YiYnM3k+++TM/-Tmp-/ghc70799_0/ghc70799_0.hc: In function ‘s2vv_ret’: /var/folders/+p/+pUVycVKFDWsYP8YiYnM3k+++TM/-Tmp-/ghc70799_0/ghc70799_0.hc:3180:0: error: ‘isnan’ undeclared (first use in this function) /var/folders/+p/+pUVycVKFDWsYP8YiYnM3k+++TM/-Tmp-/ghc70799_0/ghc70799_0.hc: In function ‘s2vU_ret’: /var/folders/+p/+pUVycVKFDWsYP8YiYnM3k+++TM/-Tmp-/ghc70799_0/ghc70799_0.hc:3226:0: error: ‘isnan’ undeclared (first use in this function) /var/folders/+p/+pUVycVKFDWsYP8YiYnM3k+++TM/-Tmp-/ghc70799_0/ghc70799_0.hc: In function ‘s2wj_ret’: /var/folders/+p/+pUVycVKFDWsYP8YiYnM3k+++TM/-Tmp-/ghc70799_0/ghc70799_0.hc:3272:0: error: ‘finite’ undeclared (first use in this function) /var/folders/+p/+pUVycVKFDWsYP8YiYnM3k+++TM/-Tmp-/ghc70799_0/ghc70799_0.hc: In function ‘s2wI_ret’: /var/folders/+p/+pUVycVKFDWsYP8YiYnM3k+++TM/-Tmp-/ghc70799_0/ghc70799_0.hc:3318:0: error: ‘finite’ undeclared (first use in this function) /var/folders/+p/+pUVycVKFDWsYP8YiYnM3k+++TM/-Tmp-/ghc70799_0/ghc70799_0.hc: In function ‘s2Kh_ret’: /var/folders/+p/+pUVycVKFDWsYP8YiYnM3k+++TM/-Tmp-/ghc70799_0/ghc70799_0.hc:4856:0: error: ‘significand’ undeclared (first use in this function) /var/folders/+p/+pUVycVKFDWsYP8YiYnM3k+++TM/-Tmp-/ghc70799_0/ghc70799_0.hc: In function ‘s2KG_ret’: /var/folders/+p/+pUVycVKFDWsYP8YiYnM3k+++TM/-Tmp-/ghc70799_0/ghc70799_0.hc:4902:0: error: ‘significand’ undeclared (first use in this function) cabal: Error: some packages failed to install: cmath-0.3 failed during the building phase. The exception was: ExitFailure 1 Thanks for any help! amindfv / Tom

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Seq question
by Lee Short 20 Apr '12

20 Apr '12

I've got the following code, and I'm not sure why the seq seems to be doing nothing. The code should never terminate if the seq were forcing the full evaluation of y. Instead, it runs just fine (though stack overflows if I ask for too many elements of primes). I didn't see anything in the tutorial section of haskell.org that seems to explain this -- pointers to a useful source would be welcome. sieve :: [Int] -> [Int] sieve xs = let ys = filter (\z -> 0 /= z `mod` head xs) xs in seq ys $ head xs:sieve ys primes = sieve [2..] thanks Lee

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Cross-platform .hs files on Linux and Windows
by Vinay Sajip 20 Apr '12

20 Apr '12

I've been given a set of .hs files which contain the shebang line #!/usr/bin/env runhaskell and I would like them to work on Windows, but none of the Windows binaries seem to be able to process them without a <script.hs>:1:1: parse error on input `#!' These files came from a Linux machine, where they run without trouble. Is there any way I can get the Windows executables to run these files? I asked on IRC and it was suggested that I change the files to literate Haskell, but I'd rather have some way of having cross-platform operation which does not involve making changes to the scripts themselves. Is there something that can be done e.g. by using particular command line options or configuration settings? Thanks & regards, Vinay Sajip

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