March 2008 - Haskell-Cafe

[Newbie] Problem with Data.Map (or something else ?)
by Bruno Carnazzi 01 Apr '08

01 Apr '08

Dears Haskellers, As an Haskell newbie, I'm learning Haskell by trying to resolve Euler Project problems (http://projecteuler.net/ ). I'm hanging on problem 14 (Collatz problem). I've written the following program... Which does not end in a reasonable time :( My algorithm seems ok to me but I see that memory consumption is gigantic... Is this a memory problem with Data.Map ? Or an infinite loop ? (Where ?) In a more general way, how can I troubleshoot these kind of problem ? Here's the code : import qualified Data.List as List import qualified Data.Map as Map f n | even n = n `div` 2 | otherwise = 3 * n + 1 chain m n = let chain' cn cm | Map.member cn m = Map.map (+ (m Map.! cn)) cm | otherwise = chain' (f cn) $! Map.insert cn 1 (Map.map (+1) cm) in chain' n Map.empty chains n = List.foldl' (\m i -> Map.union m (chain m i)) (Map.singleton 1 1) [2..n] maxCollatz c1@(_,l1) c2@(_,l2) | l1 < l2 = c2 | otherwise = c1 maxChain = List.foldl' maxCollatz (0,0) . Map.toList . chains main = let n = 1000000 in putStrLn $ show $ maxChain n Hope someone can help me, I really don't see what is th problem... Best regards, Bruno.

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How can I represent 4x4 map in haskell
by iliali16 01 Apr '08

01 Apr '08

Hi can you please tell me how can I represent 4x4 map in haskell.Should I show it as a list of lines and each line to be a list of char or I am mistaken? Thanks in advance! -- View this message in context: http://www.nabble.com/How-can-I-represent-4x4-map-in-haskell-tp16396273p163… Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

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[GSoC] Student applications deadline extended one week
by Don Stewart 01 Apr '08

01 Apr '08

Hey all, As you might know, the Google Summer of Code application submission period is open. Google has decided to extend the deadline for initial student applications by one week, http://groups.google.com/group/google-summer-of-code-announce/browse_thread… If you've thought about submitting, and are interested in hacking on Haskell for 3 months this summer, and bootstrapping into the Haskell developer community, check our project wiki, http://hackage.haskell.org/trac/summer-of-code/wiki/SoC2008 and get your applications into google this week! -- Don

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all threads are blocked by recvFrom
by Vitaliy Akimov 01 Apr '08

01 Apr '08

Hello, I have a problem with building multithreaded UDP server. If main thread is waiting for new request in recvFrom all other threads are blocked too. I've checked every variant with forkIO,forkOS,-threaded etc, nothing's helped. After reading GHC docs I've understood this is happened becouse foreign function call from recvFrom (network library) is marked to be unsefe, so it's execution blocks every other thread. How can I resolve it? Thank you. Vitaliy.

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Unescaping with HaXmL (or anything else!)
by Anton van Straaten 01 Apr '08

01 Apr '08

I want to unescape an encoded XML or HTML string, e.g. converting " to the quote character, etc. Since I'm using HaXml anyway, I tried using xmlUnEscapeContent with no luck, e.g. with HaXml 1.19.1: let (CString _ s _) = head $ xmlUnEscapeContent stdXmlEscaper $ [CString False "This is a "quoted string"" ()] in s The result is unchanged, i.e. "This is a "quoted string"". Am I doing something wrong, or are my expectations wrong, or is this a bug? Or, is there any other library that includes a simple unescape function for XML or HTML? (The Network.URI module includes an unescape function, but that's specific to URIs, naturally.) Anton

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[GSoC] X Haskell bindings
by Roman Cheplyaka 31 Mar '08

31 Mar '08

Hi! I've written a draft proposal for GSoC on XCB Haskell Bindings[1]. I'd appreciate any feedback. Especially I need help with "Milestones" section. I'm not quite sure how it should look like for such project. 1. http://ro-che.info/docs/xhsb.txt -- Roman I. Cheplyaka :: http://ro-che.info/ ...being in love is totally punk rock...

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Seeking Daan Leijen
by John Goerzen 31 Mar '08

31 Mar '08

I tried to email this to Daan, but his mail is bouncing... Subject: http://legacy.cs.uu.nl/daan/parsec.html Hi Daan, I noticed Parsec 3.0.0 on Hackage, and went to your homepage to read about the new package. But it looks like your homepage still has the old version put out in 2003. This might cause confusion for some that are looking for new releases at your website. Thanks, -- John PS... The HaXml website has the same problem, and has an incompatible API with the version in Hackage... Some Linux distros are using one, some the other.

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HTTP client libraries
by John Goerzen 31 Mar '08

31 Mar '08

Hi folks, I was recently looking for HTTP client libraries for Haskell. I investigated several, and it seems that there aren't any really ready for primetime. Am I missing anything? Here's what I found: * Bjorn's String-based HTTP It eats RAM. Does not appear to read data lazily, returns a String, and may have a memory leak as well. Does not appear to be suited for anything except very small file downloads. * nominolo's ByteString port of Bjorn's HTTP Appears to exist only in a darcs repo linked from his personal blog. Makes me hesitant to use it in production code, for fear that it won't be around long-term. * tagsoup Broken in many ways. Does not support chunked encoding that is mandated by HTTP RFC. Will be incompatible with numerous HTTP servers, and the manual states as much. Port 80 hardcoded. * network-minihttp Doesn't appear to actually be very useful as a client. Also, as far as I have been able to deduce, none of these have built-in support for https (SSL) URLs. There is also a Curl binding (or two?) floating around, which I haven't investigated. It would be nicer to have a native Haskell solution. Or should I just go write one? -- John

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lexicographic order
by Simeon Mattes 31 Mar '08

31 Mar '08

Hello everyone, I would like to ask something that I found in the ebook "a Gentle Introduction to Haskell". http://haskell.org/tutorial/stdclasses.html#sect8.4 data Tree a = Leaf a | Branch (Tree a) (Tree a) instance (Ord a) => Ord (Tree a) where (Leaf _) <= (Branch _) = True (Leaf x) <= (Leaf y) = x <= y (Branch _) <= (Leaf _) = False (Branch l r) <= (Branch l' r') = l == l' && r <= r' || l <= l' The latter specifies a lexicographic order: Constructors are ordered by the order of their appearance the data declaration, and the arguments of a constructor are compared from left to right. Although I have tried to make sense what lexicographic order means I haven't figured out. Maybe an example with a simple application of this would be helpful. To be honest I can't understand what the symbol <= really means. Thanks -- View this message in context: http://www.nabble.com/lexicographic-order-tp16381434p16381434.html Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

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Possible to automatically determine typeclass membership?
by Ryan Ingram 31 Mar '08

31 Mar '08

Is it possible in Haskell + GHC extensions to use reflection techniques to determine typeclass membership? I'm thinking of things like the following: Idea 1: > data MaybeEq a = NoEq a | Eq a => HasEq a > checkEq :: (some typeclass constraint) => a -> MaybeEq a (such that checkEq x returns HasEq x if and only if x has an Eq instance.) Idea 2: > data HTrue > data HFalse > class MaybeEq a b | a -> b > instance Eq a => MaybeEq a HTrue > instance (otherwise) => MaybeEq a HFalse -- ryan

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