January 2010 - Haskell-Cafe

status of wash?
by Günther Schmidt 13 Jan '10

13 Jan '10

Hi, is WASH still active, what's the current status of it? I'm looking for a Haskell version of Smalltalk's Seaside, at first glance WASH seems to come close. Günther

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ghc -e
by Tony Morris 13 Jan '10

13 Jan '10

Can I import a module when using ghc -e? e.g. ghc -e "import Control.Monad; forM [[1,2,3]] reverse" -- Tony Morris http://tmorris.net/

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what is *hack*?
by Günther Schmidt 13 Jan '10

13 Jan '10

Hi, References to a Hack. module came in the responses to my posts on HTML-GUIs. What is Hack then? Günther

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What is the difference between Strachey diff from Scott Semantic Domain?
by Daryoush Mehrtash 13 Jan '10

13 Jan '10

On Stack overflow page Conal Elliot says: > Beware that denotational semantics has two parts, from its two founders > Christopher Strachey and Dana Scott: the easier & more useful Strachey part > and the harder and less useful (for design) Scott part. > http://stackoverflow.com/questions/1028250/what-is-functional-reactive-prog… > Any more information on how the Scott version is different from Starchey? On Tue, Jan 12, 2010 at 3:24 PM, Raoul Duke <raould(a)gmail.com> wrote: > http://lambda-the-ultimate.org/node/1665#comment-55086 > <http://perlustration.blogspot.com/> Daryoush

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Xhtml?
by Günther Schmidt 13 Jan '10

13 Jan '10

Hi, apologies upfront. As time presses I decided to post questions immediately as soon as I run into dead-ends. I just don't want to give the impression that I'm not willing to do my homework. I'm trying to find documentation on Xhtml, the site on hackage refers to http://www.cse.ogi.edu/~andy/html/intro.htm for an introduction to the library, the link is dead. I managed to locate Andy Gill's current homepage (http://www.cse.ogi.edu/~andy/html/intro.htm) but there is no mention of his html combinatory library. Does anyone else know where an introduction to this library can be retrieved from? Günther

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QuickCheck: test passes in GHCi, fails when compiled
by Lauri Pesonen 13 Jan '10

13 Jan '10

I provided a Java solution to a problem of returning the first digit of an integer on StackOverflow and someone wondered if there were any floating point point problems with the solution, so I though I'd implement the algorithm in Haskell and run QuickCheck on it. Everything works fine on GHCi, but if I compile the code and run the test, it fails with -1000, 1000, -1000000. Any ideas why? In any case it seems that the commenter was right and there are some subtle problems with my solution. I'd just like to know more details... import Data.Char import Test.QuickCheck -- my solution getFirstDigit :: Int -> Int getFirstDigit 0 = 0 getFirstDigit x = let x' = abs x digits = (floor $ logBase 10 $ fromIntegral x') in x' `div` (floor $ 10 ** (fromIntegral digits)) -- two reference implementation that agree with each other getFirstDigitRef1 :: Int -> Int getFirstDigitRef1 x = digitToInt $ head $ show $ abs x getFirstDigitRef2 :: Int -> Int getFirstDigitRef2 x | x < 0 = getFirstDigitRef2 (-x) | x < 10 = x | otherwise = getFirstDigitRef2 $ x `div` 10 myTest x = getFirstDigit x == getFirstDigitRef1 x --myTest x = getFirstDigitRef2 x == getFirstDigitRef1 x myCheck n = check (defaultConfig { configMaxTest = n }) myTest main = myCheck 1000000000 -- ! Lauri

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overloaded overloading?
by Alberto G. Corona 12 Jan '10

12 Jan '10

Hi, I sometimes strumble on the same quiestion that forces me to insert functions that process objects of a certain class inside their class definition. This occurs when a computation uses the object internally, neiter as parameter or as a return value or in the case of existential types. An example of the first: class Example a where irec :: IO a pr :: a → IO String sample2 :: a → IO () sample2 _ = do x ← irec :: IO a pr x return () sample :: Example a ⇒ a → IO () sample _ = do x ← irec :: IO a pr x return () With the flag -fglasgow-exts, the following error below appears in sample. without the flag, the error appears in both sample and sample2. I´m too lazy to find what concrete extension is involved and why, anyhow, in the case of sample, the compiler must generate a new type a1 with no context. Could not deduce (Example a1) from the context () arising from a use of `irec' at Control\Workflow\Users.hs:73:7-10 Possible fix: add (Example a1) to the context of an expression type signature In a stmt of a 'do' expression: x <- irec :: IO a In the expression: do x <- irec :: IO a pr x return ()

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ANN: afv-0.0.0
by Tom Hawkins 12 Jan '10

12 Jan '10

Hi, Here is the first release of Atom's Formal Verifier (AFV) [1], a tool intended to verify Atom -- or human -- generated C code. With the help of the Yices SMT solver [2], AFV uses bounded model checking and k-induction to verify assertions in iteratively called C functions, such as an embedded control loop. For each assertion in a design, AFV will return a pass, a fail, or an inconclusive result -- or it will die trying. In addition to Yices, AFV needs GCC for C preprocessing. This is a very early release. There are many things missing including counter example generation, finite precision modeling, and basic model reduction. And they is a chance -- say about 100% -- that there are bugs with the modeling transformations. In addition, the list of unsupported C syntax is embarrassingly long: - recursive functions - non-unique top level names (e.g. top level names hidden with static) - loops - switch statements - pointers - arrays - type casting - non void functions - functions with arguments - structs - enums - unions - typedefs Still, AFV works well for the handful of tests I have tried so far. AFV comes with a simple example. To run: $ afv example # This generates afv.h and example.c shown below... // Provides assert and assume functions. #include "afv.h" // Periodic function for verification. Implements a simple rolling counter. void example () { // The rolling counter. static int counter = 0; // Assertions. GreaterThanOrEqualTo0: assert(counter >= 0); // The 'counter' must always be greater than or equal to 0. LessThan10: assert(counter < 10); // The 'counter' must always be less than 10. // Implementation 1. if (counter == 10) counter = 0; else counter++; // Implementation 2. // if (counter == 9) // counter = 0; // else // counter = counter + 1; // Implementation 3. // if (counter >= 9) // counter = 0; // else // counter++; // Implementation 4. // if (counter >= 9 || counter < 0) // counter = 0; // else // counter++; // Implementation 5. // counter = (counter + 1) % 10; } To run the verification on the example: $ afv verify example -k 20 example.c # Which returns... parsing example.c ... verifying assertion example.GreaterThanOrEqualTo0.assert ... inconclusive: unable to proved step up to max k = 20 verifying assertion example.LessThan10.assert ... FAILED: disproved basis in k = 11 These results state AFV was unable to prove the "GreaterThanOrEqualTo0" assertion and it disproved the "LessThan10" assertion. The different implementations in the example yield different results. Any feedback or questions are more than welcome. I would like to thank Benedikt Huber and friends for the language-c library [3], Ki Yung Ahn for the yices library [4], and the folks at SRI for their work on the Yices SMT solver. Without these tools and libraries, AFV would not have been possible. -Tom [1] http://hackage.haskell.org/package/afv [2] http://yices.csl.sri.com/ [3] http://hackage.haskell.org/package/language-c [4] http://hackage.haskell.org/package/yices

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Windows Automation - COM package - HaskellDirect
by Günther Schmidt 12 Jan '10

12 Jan '10

Hi all, I want to use Sigbjorn's com-package to do some automation under Windows. There seems to be some program "HaskellDirect" wich can create Haskell modules from type-libs, it's mentioned in the examples to the com package on Sigbjorn's site http://haskell.forkio.com/com-examples . Cabal install com does not seem to build / install the program though. Does anybody know how to get it? Günther

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short licensing question
by Sebastian Fischer 12 Jan '10

12 Jan '10

Hello Café, when writing a Haskell library that uses two other Haskell libraries -- one licensed under BSD3 and one under LGPL -- what are allowed possibilities for licensing the written package? PublicDomain? BSD3? LGPL? Sebastian -- Underestimating the novelty of the future is a time-honored tradition. (D.G.)

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