- Beginners - Haskell.org

Re: [Haskell-beginners] [Haskell-cafe] Tutorial: Haskell for the Evil Genius
by KC 14 Oct '12

14 Oct '12

The latest Haskell Platform is 2012.2.0.0 You are apparently running a much older version. >> #!"c:/Program Files/Haskell Platform/2010.2.0.0/bin/" runhaskell -- -- Regards, KC

2 1

import checking and filtering
by Christopher Howard 13 Oct '12

13 Oct '12

Maybe I'm losing it, but I seem to remember, when I first started out with Haskell, using some program which would check the import list in each of my source files, and then it would output a new import list that only imported exactly those functions that my program actually used. At the time I didn't think that was important, but now I really want it, yet I can't remember which utility it was that did this. Does anyone happen to know which utility it is I'm thinking of? -- frigidcode.com indicium.us

2 1

parsec problem
by Tobias 13 Oct '12

13 Oct '12

Hello, I would like to parse the input "word1 word2 word3 ." into ["word1","word2","word3"] using Parsec. My code below fails with: /> unexpected "."// //> expecting letter or digit /I guess the problem is that the blank before the dot is considered as belonging to the "sepBy word blank" parsing and therefore a next word is expected, and it is missing. I would like the "sepBy word blank" parsing to stop after "word3". How can I do this? / //sentence :: Parser [String]// //sentence = do words <- word `sepBy` blank// // blank// // oneOf ".?!"// // return words// // //word :: Parser String// //word = many1 (letter <|> digit) // // //blank :: Parser String// //blank = string " " /Regards, Tobias

2 2

Re: [Haskell-beginners] Beginners Digest, Vol 52, Issue 16
by Gregory Guthrie 12 Oct '12

12 Oct '12

I am trying to install yesod, But it fails and one of the failures is to install http-conduit. So I tried to install that, and it fails, with this message: Building http-conduit-1.6.1.1... Preprocessing library http-conduit-1.6.1.1... <command line>: cannot satisfy -package-id asn1-data-0.7.1-25c9f2b3799c26151af97c0d9c79754c: asn1-data-0.7.1-25c9f2b3799c26151af97c0d9c79754c is unusable due to missing or recursive dependencies: base-4.5.0.0-597748f6f53a7442bcae283373264bb6 bytestring-0.9.2.1-df82064cddbf74693df4e042927e015e cereal-0.3.5.2-b01d76d961ef4facbe3e52a7b6ccef63 mtl-2.1.1-87121149c5eb4fc03f06f5894ea4ac0c text-0.11.2.0-b2986f5478f0efa626f8ba0494ed9670 (use -v for more information) Failed to install http-conduit-1.6.1.1 cabal.exe: Error: some packages failed to install: http-conduit-1.6.1.1 failed during the building phase. The exception was: ExitFailure 1 Any hints? I find that with each usage of cabal-install, my success rate and appreciation for it falls! :-) Each time it fails, and says to -force-reinstalls, that breaks more, etc... Not sure how to break out of this maze of "twisty turny passages that all look alike".. Haskell-platform 2.0.0, GHCi, version 7.4.1 cabal-install version 0.14.0 using version 1.14.0 of the Cabal library -------------------------------------------

1 0

conduit and happstack dependence problem
by Никитин Лев 12 Oct '12

12 Oct '12

3 2

calling inpure functions from pure code
by Emmanuel Touzery 12 Oct '12

12 Oct '12

Hello, I'm trying to write my first real program in Haskell, a web page scraper; at the top level, which is impure anyway, i fetch the top-level pages (which is IO), then call the pure functions parsing the structure. So far so good. That was the first part of the program and up to this point I'm pretty sure I did it right (for the structure anyway). Most of the demo programs and Haskell books cover this: you do the IO at the top level, then you process and you come back to the top level to print your results for instance. But here comes the problem: these pure functions that parse the structure, sometimes they find links and must open another page on the site... And opening that new page, well it's IO and can't be pure.. Now if I call from those pure methods IO methods, then they're not pure anymore, and in fact since those are leaf calls basically my entire program becomes impure... I had this idea, that I would make some sort of input data structure, which would be like a lazy String reading from a file: doing IO while the caller doesn't even realize and the caller can be pure. So some sort of fake webserver or website or htmlpage data structure... which is lazy. And then I give this data structure to my pure methods which parse the data, they call functions on that structure, and they can stay pure. But it sounds really contrived and probably completely the wrong solution. I can see that this is a very basic question which was probably answered hundreds of time, but I could not find the answer so far... Thank you! Emmanuel

6 12

TextMate with Haskell GHC
by Patrick Lynch 12 Oct '12

12 Oct '12

Good morning, Can anyone tell me how to configure TextMate with the :edit command in ghci when used with a Mac [I'm new to the Mac]? [BTW: TextMate has a "bundle" that works well with Haskell.] Also, can anyone recommend an online course on Category Theory? Good weekend

2 1

Errors and the compiler
by Christopher Howard 11 Oct '12

11 Oct '12

Point of curiosity: Does an error in a Haskell program have a negative impact on the compilers reasoning about or optimization of the code, even if said error is never reached in the control flow? For example, both of the following would have the same result: code: -------- case mExp of Nothing -> expr Just x -> f x -- or... if isNothing mExp then expr else f (fromJust mExp) -------- In my naive reasoning, I would think that in the latter case the use of fromJust introduces, from the compilers perspective, an additional possible outcome (an error being thrown). This presumably would complicate the compilers reasoning about the code, unless of course the compiler is smart enough to figure out that the error will never be reached. -- frigidcode.com indicium.us

3 2

Memory leak after going from type Integer to Int
by Andreas Lemke 11 Oct '12

11 Oct '12

Hi everyone, I wrote a simple program that returns the n-th prime number when given n as its command-line argument: import System.Environment isPrime n | n < 2 = False | n == 2 = True | otherwise = not $ any (`divides` n) dividerList where a `divides` b = b `mod` a == 0 dividerList = 2:[3, 5 .. floor $ sqrt $ fromIntegral n] primes = filter isPrime [1 ..] main = do [arg] <- getArgs print $ primes !! (read arg) I compiled it with "ghc -O2". As expected, it runs in constant space, i. e. space complexity O(1). Then, I added the declaration: "primes :: [Int]", expecting a performance improvement. Runtime was indeed reduced by about 50%. However, it no longer runs in constant space. Memory usage rises significantly for arguments > 1e5. I tried all sorts of strictness annotations, which did not help. Turning on profiling with "ghc -O2 -prof -auto-all" made the problem disappear, so I could not analyze the memory leak using the profiler. Only if I remove the cases "n < 2" and "n == 2" from the definition of isPrime, does the program run in constant space with type Int. If I replace the guards by an if statement or pattern matching, the problem remains. I have no idea how to continue from here. Can anyone explain why going from type Integer to Int causes a different space complexity? How would I have to change the program to get it to run in constant space with type Int? I used ghc 7.4.1. Regards, Andreas

1 0

What is a closer to functional (haskell) ideology
by Никитин Лев 10 Oct '12

10 Oct '12

3 3