August 2007 - Haskell-Cafe

runInteractiveCommand behaves differently on linux and windows
by Thomas Hartman 29 Aug '07

29 Aug '07

Maybe this is by design, but I just thought I would point this behavior out and ask for comment. test1 merely shows that runInteractiveCommand reacts differently to perl warnings than perl errors. Okay, maybe the inconsistency in that case is due to perl and not haskell. test2 behaves the same on win and nix. This is "pipe like" in that the ouptut of a command (which could be the result of a shell call, but just as easily be the return of a haskell function) gets fed into a shell command. In this case, if the shell command is simply "tail" the behavior is consistent from win to nix. test3 shows that the behavior stops being consistent if ssh enters the picture. (piping to tail via ssh). again, maybe this is due to ssh and not haskell. however... note however that on windows ghc -e 'mapM_ ( putStrLn . show ) [1..1000] ' | ssh `whoami`@localhost 'tail -n2' works fine. so it's not *just* ssh, but ssh in conjuction with runInteractiveCommand which seems to cause problems FWIW, using 10 lines instead of 1000 still hangs on windows. Is there a way to code up shell pipelike behavior in a more portable way? curious what the cafe thinks... thomas. import Test.HUnit import Misc ( (>>=^) ) import System.Process import System.IO import System.Exit -- works on linux, error on windows test1 = do res1 <- test_shellrunStderrOk runTestTT $ TestCase ( assertEqual "test1" "made it" res1 ) where test_shellrunStderrOk = do runprocessStdErrAllowed' "" cmdPerlwarn return "made it" cmdPerldie = " perl -e 'die \"error\"' " cmdPerlwarn = " perl -e 'warn \"blee\"' " -- works on linux, windows test2 = pipeTo "tail -n2" -- works on linux, hangs on windows test3 = pipeTo "ssh `whoami`@localhost 'tail -n2'" pipeTo cmd = do res2 <- test_shellrunPipeinLike runTestTT $ TestCase ( assertEqual ( "pipe to, cmd: " ++ cmd) (show l) res2 ) where test_shellrunPipeinLike = do runprocessStdErrAllowed' (unlines $ map show [1..l]) ( cmd ) >>=^ filter (not . ( == '\n') ) l = 1000 runprocessStdErrAllowed' inp s = do (ih,oh,eh,pid) <- runInteractiveCommand s so <- hGetContents oh se <- hGetContents eh hPutStrLn ih inp hClose ih ex <- waitForProcess pid case ex of ExitFailure e -> fail $ "shell command " ++ s ++ "\nFailed with status: " ++ show e _ | otherwise -> return so --- This e-mail may contain confidential and/or privileged information. If you are not the intended recipient (or have received this e-mail in error) please notify the sender immediately and destroy this e-mail. Any unauthorized copying, disclosure or distribution of the material in this e-mail is strictly forbidden.

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Re: [Haskell-cafe] Norvig's Sudoku Solver in Haskell
by manu 29 Aug '07

29 Aug '07

Daniel Fischer's modifications to my original program lead to a 400 % speed boost !!! (It now runs in 22 seconds on my machine) He avoided unecessary calls to 'length', uses Array instead of Map, refactored 'search' function (details below) I've put up his version on hpaste : http://hpaste.org/2452#a1 Manu On Aug 26, 2007, at 10:56 PM, Daniel Fischer wrote: > Without much thinking I can spped it up by a factor of 4 (from 280s > to 60s). > The most important things are: > - don't use length unless you need it > instead of > newV2 <- case length newCell of > 0 -> Nothing > ... > and > case length dPlaces of > 0 -> ... > use > case newCell of > [] -> Nothing > [d'] -> ... > and > case dPlaces of > [] -> Nothing > [s'] -> ... > > - let dPlaces = [ s' | u <- lookup s units, s' <- u, elem d (lookup > s' newV2)] > is bad > let dPlaces = [s' | s' <- lookup s peers, elem d (lookup s' newV2)] > scans each peer only once > > - search is really bad, you lookup all squares several times, > potentially > compute all lengths multiple times... > much better is > > search :: Grid -> Maybe Grid > search g = case [(l,a) | a@(_,xs) <- M.assocs g, let l = length xs, > l /= 1] of > [] -> return g > ls -> do let (_,(s,ds)) = minimum ls > msum [assign g (s,d) >>= search | d <- ds] > > (I also changed the type, and instead of foldl' you should use > foldr, since > "some" is lazy in the second argument, further, since Maybe is a > MonadPlus, > it's "mplus" and 'foldr mplus Nothing' is msum) > > - Maps aren't good here, too slow lookup and you know the keys, so > use arrays >

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Searching for a subsequence in Data.Sequence structure?
by Justin Bailey 28 Aug '07

28 Aug '07

Does anyone have code for finding a subsequence within a Data.Sequence.Seq value, or is there a way to do it with the already defined instances that I am missing? A quick search didn't turn up much. Thanks in advance! Justin

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GHC 6.6.1 and SELinux issues
by Alexander Vodomerov 28 Aug '07

28 Aug '07

Hello! Does anybody here uses GHC on Linux with SELinux turned on? I've just installed SELinux and run into GHC/SELinux incompatibility. It seems that the similar problem was reported some time ago and was fixed in 6.4.3. However, I use 6.6.1 and the problem is still here. $ ghc ghc-6.6.1: internal error: getMBlock: mmap: Permission denied (GHC version 6.6.1 for i386_unknown_linux) Please report this as a GHC bug: http://www.haskell.org/ghc/reportabug Exactly the same bug is appearing on x86_64 architecture. Much more details is available at http://hackage.haskell.org/trac/ghc/ticket/738 Any ideas? With best regards, Alexander. PS. Is it possible to turn on global allow_execmem boolean in SELinux policy, thus enabling executable memory mapping. This solves the problem. However, this is major drawback for system security and should be avoided at any cost.

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Wildly off-topic
by Andrew Coppin 28 Aug '07

28 Aug '07

http://importantshock.wordpress.com/2007/08/21/haskell-curry-yes-i-dated-hi… (Actually, you hardly need to click that. The URL says it all...)

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[Off Topic Amusement] Lylle Lovett lyrics
by David Pollak 28 Aug '07

28 Aug '07

http://www.lyricsdepot.com/lyle-lovett/west-texas-highway.html -- lift, the fast, powerful, easy web framework http://liftweb.net

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[ANN] An efficient lazy suffix tree library
by Bryan O'Sullivan 28 Aug '07

28 Aug '07

I just posted a library named suffixtree to Hackage. http://www.serpentine.com/software/suffixtree/ It implements Giegerich and Kurtz's lazy construction algorithm, with a few tweaks for better performance and resource usage. API docs: http://darcs.serpentine.com/suffixtree/dist/doc/html/Data-SuffixTree.html I've tested it on multi-megabyte input strings. <b

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"quoting" in Haskell
by Peter Verswyvelen 28 Aug '07

28 Aug '07

In Scheme, on can "quote" code, so that it becomes data. Microsoft's F# and C# 3.0 also have something similar that turns code into "expression trees". The latter is used extensively in LINQ which translates plain C# code into SQL code or any other code at runtime (this idea came from FP I heared) I can't find something similar for Haskell? Maybe I am looking at the wrong places? In Haskell, I know one can use a data constructor as a function (as in (map Just [1..3])), but a function cannot be turned into a data constructor (= "quoting"), can it? Now this is all really fuzzy for a newbie like me, because aren't all functions initially just data constructors waiting to be evaluated in a lazy language? I'm actually looking for something like (loose terminilogy follows) "context-based-semi-quoting". The idea is to only quote a set of functions, while evaluating all the others. For example, in the code 1 `add` 2 `mul` 3 where add = (+) mul = (*) I want to write something like selectiveQuote [add] (1 `add` 2 `mul` 3) which would result in an expression tree like add / \ 1 6 So the `mul` is not quoted because it is not part of the "context" = [add] Maybe this is just impossible, I did not dig deep into this. Thanks, Peter

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No Enum nor Ix instance on Graphics.UI.GLUT.Key
by Peter Verswyvelen 27 Aug '07

27 Aug '07

I want to make an array using GLUT.Key as an index, but it is not an instance of Enum nor Ix Of course, I can make it an instance myself, but I guess it would be easier to do so in the library? Cheers, Peter

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Geometry
by Tony Morris 27 Aug '07

27 Aug '07

-----BEGIN PGP SIGNED MESSAGE----- Hash: SHA1 I went camping on the weekend and a friend of mine who is a builder asked me many questions on geometry as they apply to his every day work - - most of which I could answer. However, there was one that I couldn't and I am having trouble googling a solution (for lack of keywords?). I'm hoping a fellow Haskeller could help me out (in Haskell of course). The problem is finding the unknown x from the two knowns a and b in the given image below (excuse my Microsoft Paintbrush skills). I may have misunderstood his problem (we were drawing in dirt) and actually, it is the straight line between the two points on the circumference that are known and not the specified 'b', but I figure I could derive one solution from another if I have misunderstood him. Here is my image: http://tinyurl.com/2kgsjy Thanks for any tips or keywords with which to google! - -- Tony Morris http://tmorris.net/ -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.4.6 (GNU/Linux) Comment: Using GnuPG with Mozilla - http://enigmail.mozdev.org iD8DBQFG0iM6mnpgrYe6r60RAqfDAJ4gFAdr7zP1ehLl8H2MaCzCNfAvhQCgmL8D 4nrxrK13O9EBNv/ojPIMJXI= =eaxX -----END PGP SIGNATURE-----

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