January 2013 - Haskell-Cafe

ANN: Hquery 0.1
by Tycho Andersen 21 Jan '13

21 Jan '13

Hi cafe, I've written a library called hquery, which can be used to manipulate xmlhtml trees (and has a similar selector syntax to jquery, hence the name). This enables easy manipulation and content generation from html templates. If you're familiar with Lift's CssSels, this is basically an implementation of those in haskell. This is my first foray into haskell, so any critiques of the code or API are welcome :-) Hquery: http://hackage.haskell.org/package/hquery-0.1.0.1 Examples: http://hackage.haskell.org/packages/archive/hquery/0.1.0.1/doc/html/Text-Hq… \t

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forkProcess, forkIO, and multithreaded runtime
by Michael Snoyman 21 Jan '13

21 Jan '13

Hi all, I think I have a misunderstanding of how forkProcess should be working. Ultimately this relates to some bugs in the development version of keter, but I've found some behavior in a simple test program which I wouldn't have expected either, which may or may not be related. With the program at the end of this email, I would expect that, once per second, I would get a message printed from each forkIO'd green thread, the forked process, and the master process. And if I spawn 8 or less child threads that's precisely what happens. However, as soon as I up that number to 9, the child process is never run. The process is, however, created, as can be confirmed by looking at the process table. This only occurs when using the multithreaded runtime. In other words, compiling with "ghc --make test.hs" seems to always produce the expected output, whereas "ghc --make -threaded test.hs" causes the behavior described above. Having looked through the code for the process package a bit, my initial guess is that this is being caused by a signal being sent to the child process, but I'm not familiar enough with the inner workings to confirm or disprove this guess. If anyone has any ideas on this, I'd appreciate it. Michael import System.Posix.Process (forkProcess, getProcessID) import Control.Concurrent (forkIO, threadDelay) import System.IO (hFlush, stdout) import System.Posix.Signals (signalProcess, sigKILL) import Control.Exception (finally) main :: IO () main = do mapM_ spawnChild [1..9] child <- forkProcess $ do putStrLn "starting child" hFlush stdout loop "child" 0 print ("child pid", child) hFlush stdout -- I've commented out the "finally" so that the zombie process stays alive, -- to prove that it was actually created. loop "parent" 0 -- `finally` signalProcess sigKILL child spawnChild :: Int -> IO () spawnChild i = do _ <- forkIO $ loop ("spawnChild " ++ show i) 0 return () loop :: String -> Int -> IO () loop msg i = do pid <- getProcessID print (pid, msg, i) hFlush stdout threadDelay 1000000 loop msg (i + 1)

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Up-front Unit Testing in Haskell
by Kazu Yamamoto 21 Jan '13

21 Jan '13

#Resending due to the lack of URL. Hello cafe, Just FYI: I wrote an tutorial, "Up-front Unit Testing in Haskell". It is about how to use doctest, hspec and Cabal. https://github.com/kazu-yamamoto/unit-test-example/blob/master/markdown/en/… Enjoy! --Kazu

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Up-front Unit Testing in Haskell
by Kazu Yamamoto 20 Jan '13

20 Jan '13

Hello cafe, Just FYI: I wrote an tutorial, "Up-front Unit Testing in Haskell". It is about how to use doctest, hspec and Cabal. Enjoy! --Kazu

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Re: [Haskell-cafe] [Haskell] Silent error
by Henning Thielemann 20 Jan '13

20 Jan '13

On Mon, 21 Jan 2013, miphis(a)meta.ua wrote: > The program looks like this: > > --*********************** > factorial :: Int->Int > factorial n = product [1..n] > > main = do > print $ factorial 50 > --*********************** > > And that yields "0" (no errors). Is it a bug or feature? :) This question is certainly better posted to haskell-cafe. The answer is: Int has limited bitsize (32 ord 64 bit depending on your machine), thus it computes factorial modulo 2^32 or 2^64. You can get the correct result by replacing Int by Integer.

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"Function decoration pattern" on wiki
by Derek Elkins 20 Jan '13

20 Jan '13

I created the following page on the wiki to capture one pattern that occurs often in Edward Kmett's lens library. http://www.haskell.org/haskellwiki/Function_decoration_pattern Feel free to rename it or add comments, examples, clarifications, additional notes or extensions to the technique.

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Duplicates in arbitrary instances for collections
by Tom Murphy 20 Jan '13

20 Jan '13

Every QuickCheck tutorial makes the point that, given a property on some type, the Arbitrary instance for that type is written in such a way that nearly all of the interesting edge-cases of that type are tried. So e.g. for numeric types, positive, negative, and zero numbers are tried. For collections, I think the case of duplicate elements is insufficiently covered. Say we have a list difference function: badDiff :: Eq a => [a] -> [a] -> [a] badDiff l r = filter (\e -> e `notElem` r) l And a test for it: prop_wrong :: [Int] -> [Int] -> Bool prop_wrong a b = (a `badDiff` b) == (a \\ b) `quickCheck prop_wrong` passes about 80% of the time! Obviously it shouldn't: > badDiff [1,2,3,3,3] [2,3] [1] > (\\) [1,2,3,3,3] [2,3] [1,3,3] But even with maxSuccess at 1000, this property passes at least 2/3 of the times it's run (on my machine, where (maxBound :: Int) is large). The reason is that `arbitrary` for [a] is just `sized $ \n -> choose (0,n) >>= \k -> sequence [arbitrary | _ <- [1..k]]` It would be trivial to duplicate values in the collection a certain percentage of the time (possibly sometimes adjacent). Is there a good reason not to? It shouldn't require an Eq constraint. Tom

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Container libraries
by Alexander Solla 20 Jan '13

20 Jan '13

Hi everybody, What is the state of container libraries? I am looking for a library which provides: * unordered containers (for "operational" type safety, I don't want to impose orders on things that don't have them -- unordered containers does this) * can handled nested containers (containers does that) * can be serialized easily, or better yet, has Generic instances Does such a library exist? What are you using for Set and Map needs? I have been fond of unordered-containers for a while, but found difficulty in creating HashSets of, for example, HashMaps, since there are no Hashable instances for their polymorphic types. I even dug into the code, looking at the possibility of submitting a patch, but gave up when I ran into funky #-suffixed values, which stymied my initial attempt.

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[ANNOUNCE] JuicyPixels 3.0
by Vincent Berthoux 20 Jan '13

20 Jan '13

Hello haskellers, I'm presenting the third version of JuicyPixels. As the version number indicate, this is a major version with backward compatibility break. The major reason behind this breakage is the introduction of High Dynamic Range images. High Dynamic Range images are used in photography, and in some APIs like OpenGL and DirectX for some fancy effects. Version 3.0 changelog : - Simplification of the 'Pixel' typeclass, removed many unused part. - Removal of some Storable instances for pixel types. - Amelioration of the documentation. - Support for High Dynamic range images. - Support for the Radiance file format (or RGBE, file extension .pic and .hdr). - Dropped the cereal library in favor of the last version of Binary, present in the Haskell platform. Every dependencies are now present in the platform. As usual, this version is available in Hackage and on GitHub : - Hackage : http://hackage.haskell.org/package/JuicyPixels - GitHub : https://github.com/Twinside/Juicy.Pixels Thanks Vincent Berthoux

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Inferring rewrite rules for higher order functions
by Joachim Breitner 20 Jan '13

20 Jan '13

Dear list, with rewrite rules I can tell the compiler to replace "map id" by "id", but I still have to write that rule by hand. Does anyone of you know about research into inferring such a rewrite rule automatically? I could imagine that for a function of type "f :: some -> args -> T a -> T b" and not-too-bad type constructors T it might be possible under what conditions on "some" and "args" infer when "f arg1 arg2 = id" holds, and if the conditions are fulfilled by functions like "id", generate a rewrite rule "f id id = id" automatically. Thanks, Joachim -- Joachim "nomeata" Breitner mail(a)joachim-breitner.de | nomeata(a)debian.org | GPG: 0x4743206C xmpp: nomeata(a)joachim-breitner.de | http://www.joachim-breitner.de/

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