October 2009 - Haskell-Cafe

Newbie has trouble using QuickCheck
by steenreem 23 Oct '09

23 Oct '09

Hello. I have made a simple test.hs file that imports the module QuickCheck. However, when I load this module in ghci, it says that the module QuickCheck is not found. I have looked in my haskell platform folder and find the folder "QuickCheck 1.2.0.0". I also tried to :set -package QuickCheck. This added QuickCheck to the list of loaded packages. However when loading my test module it still says "Could not find module QuickCheck". Locations searched were "QuickCheck.hs" and "QuickCheck.lhs" What's wrong? Thanks, Remy. -- View this message in context: http://www.nabble.com/Newbie-has-trouble-using-QuickCheck-tp26024248p260242… Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

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ANN: Data.Stream 0.4
by Wouter Swierstra 23 Oct '09

23 Oct '09

I'm happy to announce a new release of the Data.Stream library. http://hackage.haskell.org/package/Stream The only change with the previous version has been to add irrefutable patterns to several function definitions. This is rather delicate design decision: too many irrefutable patterns could result in thunks not being evaluated; too few irrefutable patterns could cause your functions diverge. As a rule of thumb I've chosen only to use irrefutable patterns in functions that produce streams from streams. The operations that observe finite information (a prefix, the element at index i, etc.) do not have have irrefutable patterns and force evaluation to weak head normal form. I've uploaded a new version to Hackage. I'd be interested to hear if any existing code takes a performance hit as a result of these changes. Wouter

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What's this pattern called?
by Martijn van Steenbergen 23 Oct '09

23 Oct '09

Bonjour café, > data ExprF r > = Add r r > | Sub r r > | Mul r r > | Div r r > | Num Int This is a well-known pattern that for example allows nice notation of morphisms. But what is it called? I've heard fixed-point view, open datatypes and some others, but I'm curious where this pattern comes up in literature and what it is called there. Thanks, Martijn.

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TimeZone bug?
by Magicloud Magiclouds 23 Oct '09

23 Oct '09

Hi, As we know, "CST" could mean both +8 or -6 time zone. So I got a problem. In the same envoironment. When a ZonedTime is shown, I got "xxxxx CST". Now it means +8 time zone. Then I read it, stupidly, it turns into -6 time zone. How could I fix this? I am using ghc 6.10.4 in rhel 5. -- 竹密岂妨流水过山高哪阻野云飞

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Lazy problem ?
by zaxis 23 Oct '09

23 Oct '09

"ssqHitNum.txt" contains data as below: 6 7 18 24 30 32 9 4 12 20 25 28 29 16 3 5 11 12 31 32 11 2 9 13 15 19 24 3 5 17 21 25 27 32 14 5 9 15 21 26 31 13 12 16 25 26 27 31 05 ... good_ssq_red:: IO [Int] good_ssq_red = withFile "ssqHitNum.txt" ReadMode (\h -> do { samp <- fmap str2Ints $ hGetContents h; print samp; --without this line, the result will always [1..16] return $ statis samp; }) statis :: [Int] -> [Int] statis samp = take 16 $ map (\(a,b) -> a) $ sortBy (\a b-> if (snd a >= snd b) then LT else GT) $ times4n where times = map (\n -> (foldl (\acc x -> if x==n then acc+1 else acc) 0 samp)) [1..33] times4n = map (\n -> (n,times!!(n-1))) [1..33] Does it mean that the sampe will not be evalued with `print samp` line ? thanks! -- View this message in context: http://www.nabble.com/Lazy-problem---tp26021845p26021845.html Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

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Time and space complexity of "take k . sort"
by Paul Johnson 23 Oct '09

23 Oct '09

This question on StackOverflow asked about how to find the largest 100 items in a very long list: http://stackoverflow.com/questions/1602998/fastest-way-to-obtain-the-larges… I replied that you could do it with something like this (but here taking the k smallest to strip out some irrelevant complications): > takeLargest k = take k . sort Because "sort" is lazily evaluated this only does enough sorting to find the first k elements. I guess the complexity is something like O(n*k*log(k)). But of equal practical interest is the space complexity. The optimum algorithm is to take the first k items, sort them, and then iterate through the remaining items by adding each item to the sorted list and then throwing out the highest one. That has space complexity O(k). What does the function above do? Paul.

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why cannot i get the value of a IORef variable ?
by zaxis 23 Oct '09

23 Oct '09

> let aaa = unsafePerformIO $ newIORef [] > writeIORef aaa [1,2,3] > readIORef aaa [(),(),()] sincerely! -- View this message in context: http://www.nabble.com/why-cannot-i-get-the-value-of-a-IORef-variable---tp26… Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

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NetSnmp problem.
by Magicloud Magiclouds 22 Oct '09

22 Oct '09

Hi, I am trying to use NetSnmp to get some information from my switch. And I met this problem. After initialize, I used snmpWalk to get some information, and dealed with it. Everything is fine. Then I sleep for 5 mins. SnmpWalk again. But this time, all asnValues were unsupported format, which broke my dealing process. Why? I am sure my switch and OID, bluh bluh bluh are correct. They worked in ruby script. -- 竹密岂妨流水过山高哪阻野云飞

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cvs.haskell.org down? haskell-mode abandoned?
by Svein Ove Aas 22 Oct '09

22 Oct '09

Attempting to update haskell-mode, I've been getting authorization failures from the cvs.haskell.org pserver. I thought this might have some connection to the recent disk crash, and as nobody uses cvs anymore, it could have been overlooked. In any case, I need it fixed before I can work on haskell-mode. Preferably by migrating haskell-mode over to cvs. :-) On a different note, I've been unable to contact the apparent current maintainer of haskell-mode, while in a state of having a useful patch. Though it may already have made it to cvs, with me simply being unable to check that.. actually, who *is* the current maintainer? The last release was Dec. 2007. -- Svein Ove Aas

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Is there in Haskell the eval function?
by Waldemar Biernacki 22 Oct '09

22 Oct '09

Hello! I have a very stupid question. Sorry for that! Is there the eval function like in imperative languages? I'd like to write an application which has to be compiled to exec file. It is neccessary to performe some additional procedures which are unknown at the moment of the compilition. Regards wb.

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