June 2007 - Haskell-Cafe

tail recursion ?
by H. 12 Jun '07

12 Jun '07

Hello @ all, Sometimes one has an imperative algorithm, and wants to write a program in Haskell which do has the same effect... So the question is - how a construct as the following can efficiently be written? -- Pseudo code: n[1..10000] = false for (1..10000) |i| for (i,2*i..10000) |j| n[j] = not n[j] -- Certainly it is in this special case equivalent to (True where the index is): map (^2) [1..100] But I mean the destructive updates in the imperative code in Haskell without filling the (many times more than in imperative languages) memory with recursively called functions... The idea, I thought, is tail recursion, so perhaps I just have a big bug in my code, caused by the fact, that it needs even for 5000 approximately 100MB memory: -- import Data.Array main :: IO () main = putStr $! unlines $! map show $! filter snd $! zip [1..] $! elems $! calc $! la 5000 where la x = array (1,x) [(y,False)|y<-[1..x]] calc :: Array Int Bool -> Array Int Bool calc x = f 1 x where k :: Int k = snd $ bounds x f :: Int -> Array Int Bool -> Array Int Bool f !a !x | a < k = f (a+1) $! g a x | otherwise = g a x g !a !x = x//[(j,not (x!j))|j<-[a,a*2..k]] -- -- Thanks for you answers in advance H.

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killThread on a terminated thread.
by Bit Connor 12 Jun '07

12 Jun '07

Hello, Is it safe to use killThread to terminate a thread that has already terminated(it's IO action has run to completion)? Or are ThreadIds reused, potentially causing an unwanted thread to be terminated? I'm using ghc 6.6 Thanks, bit

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Thought for today
by Andrew Coppin 11 Jun '07

11 Jun '07

"Data in Haskell is like Schrodinger's famous undead cat - it doesn't 'exist' until you 'obverse' it." I just thought I'd share this useful (?) metaphore for describing to people what lazy evaluation is all about. ;-)

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Head and tail matching question
by Olivier Boudry 11 Jun '07

11 Jun '07

Hi all, I'm trying to write a untab function that would split a string on tabs and return a list. Code is here. import Data.List (break, unfoldr) import Data.Char (String) untab :: String -> [String] untab s = unfoldr untab' s untab' :: String -> Maybe (String, String) untab' s | s == "" = Nothing | otherwise = Just (h, ts) where (h, t:ts) = break (== '\t') s This code raises an exception when handling the last portion of the string. Break returns a ("something", "") and t:ts cannot match on "". I was wondering if there way a clean way of handling this last case without adding tons of code. Some kind of "idiomatic expression" ;-) Thanks, Olivier.

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More on the random idea
by Andrew Coppin 11 Jun '07

11 Jun '07

Since the online lambdabot still doesn't seem to want to talk to me, I've been thinking about how I might rectify the situation... Apparently GHC has a flag that makes it execute a Haskell expression directly. For example, C:\> ghc -e "map (2*) [1,2,3]" [2,4,6] Now, if I could just figure out how to make a web server call GHC... I'm laughing! Oh, but there is the *minor* detail that I am literally allowing unauthenticated users to perform arbitrary code execution. For example, C:\> ghc -e "writeFile \"Test.txt\" \"Hi mum!\"" (Generates a file on my harddrive "Test.txt" containing the text "Hi mum!".) AFAIK, Lambdabot dissalows any expression that performs IO. In Haskell, this is beautifully easy: reject any expression having an IO type. And it seems that GHC accepts not just an *expression*, but a *command*. In particular, this works: C:\> ghc -e ":t writeFile \"Test.txt\" \"Hi mum!\"" writeFile "Test.txt" "Hi mum!" :: IO () However, when you consider that the result type could be "IO ()" or "IO String" or "IO [Either (Maybe Int, (String, Bool)) [Either (Int -> String) (Complex Integer)]]", and the expression itself may well contain the "::" sequence... you see we have a nontrivial parsing task here! (Get the parsing wrong and somebody might be able to do Evil Things to the box.) The other possibility is to somehow run GHC under a user context that doesn't *have* write access to anything on the filesystem. That way there is no margin for error. This leaves only the problem of how to make a web server call GHC. I can think of a number of possibilities. - Write my own HTTP server from scratch. (Not keen...) - Configure Apache to do it. (Is that physically possible?) - Use Apache and some bizzare Perl scripting to do the actual call. (Assuming *that* is possible.) - Use Apache and some Perl scripts to write the data to a text file, and write a small Haskell program to poll the filesystem waiting for request files to appear, run then though GHC, and put the result back into a file. Write another Perl script to slurp up the result and send it back to the caller. - Doesn't Java have a free HTTP server implementation? Maybe I could use that... - ...others? (I'm not sure why this should be, but it seems that Don has made several replies to my emails that didn't show up in my inbox, and only show up on the list archives. Oh well, anyway...) I lurk on the POV-Ray NNTP server, and we recently had quite a discussion about Haskell. I'd *love* to be able to say to people "hey, you don't even need to bother working out how to install GHC, just CLICK THIS LINK and you can play with Haskell instantly!" But at the moment I'm not entirely sure how to set this up. Ideas?

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Examples of OpenAL and ALUT?
by Dan Piponi 11 Jun '07

11 Jun '07

Does anyone have any sample Haskell code they'd like to share for doing things like creating a waveform from a list of samples or a mathematical function and playing them using these libraries (or indeed any easy to install on MacOS X Haskell library)? -- Dan

2 1

LaTeX
by Andrew Coppin 11 Jun '07

11 Jun '07

Does anybody know what the magical LaTeX command is to turn (say) "++" into two overprinted pluses? (As seems to be fashionable...)

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OSCON 2007, who's going?
by Christopher Milton 11 Jun '07

11 Jun '07

Are a lot of Haskellers going to be at OSCON, or just Simon Peyton Jones and myself? http://conferences.oreillynet.com/os2007/ I've never been to Portland, Oregon, before. Chris Milton AIM: cmiltonperl

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Which regular syntax does Text.Regex use?
by L.Guo 11 Jun '07

11 Jun '07

Hi All: I wrote this func : match = matchRegex . mkRegex And when using it, I found that I have not even know the syntax of Regex. Eager for your hint. Regards -------------- L.Guo 2007-06-11

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Shim missing Distribution.Verbosity
by Johan Grönqvist 11 Jun '07

11 Jun '07

Hi, I am trying to install shim (http://shim.haskellco.de/trac/wiki) on windows XP (GHC 6.6.1). When I run "./Setup.lhs build", ghc tells me that the module Distribution.Verbosity can not be found. The module is imported by Shim.GhcCompat. ghcii also tells me that Distribution.Verbosity can not be found. When I look at http://www.haskell.org/ghc/docs/latest/html/libraries/index.html, I do not see Distribution.Verbosity there either. I have no idea how to proceed. Thanks for any help, Johan

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