- Haskell-Cafe - Haskell.org

floating point operations and representation
by Jacob Schwartz 17 Mar '08

17 Mar '08

I have two questions about using the Double data type and the operations in the Floating typeclass on a computer that uses IEEE floating point numbers. I notice that the Floating class only provides "log" (presumably log base 'e') and "logBase" (which, in the latest source that I see for GHC is defined as "log y / log x"). However, in C, the "math.h" library provides specific "log2" and "log10" functions, for extra precision. A test on IEEE computers (x86 and x86-64), shows that for a range of 64-bit "double" values, the answers in C do differ (in the last bit) if you use "log2(x)" and "log10(x)" versus "log (x) / log(2)" and "log(x) / log(10)". I am under the restriction that I need to write Haskell programs using Double which mimic existing C/C++ programs or generated data sets, and get the same answers. (It's silly, but take it as a given requirement.) If the C programs are using "log2", then I need "log2" in the Haskell, or else I run the risk of not producing the same answers. My first thought is to import "log2" and "log10" through the FFI. I was wondering if anyone on Haskell-Cafe has already done this and/or has a better suggestion about how to get specialized "log2" and "log10" (among the many specialized functions that the "math.h" library provides, for better precision -- for now, I'm just concerned with "log2" and "log10"). My second question is how to get at the IEEE bit representation for a Double. I am already checking "isIEEE n" in my source code (and "floatRadix n == 2"). So I know that I am operating on hardware that implements floating point numbers by the IEEE standard. I would like to get at the 64 bits of a Double. Again, I can convert to a CDouble and use the FFI to wrap a C function which casts the "double" to a 64-bit number and returns it. But I'm wondering if there's not a better way to do this natively in Haskell/GHC (perhaps some crazy use of the Storable typeclass?). Or if someone has already tackled this problem with FFI, that would be interesting to know. Thanks. Jacob

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STM example code
by Galchin Vasili 17 Mar '08

17 Mar '08

Hello, I am playing around with the STM API. I would like to see examples of STM other than the Santa.hs as I am having problems with STM vs IO. Thanks, Vasili

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Re: [Haskell-cafe] QuickCheck
by rodrigo.bonifacio 17 Mar '08

17 Mar '08

Hi all, Is it possible to define a limit for the size of children list bellow? I've tried: children <- resize (10 (listGen featureGenNormal)) But it didn't work. Thanks a lot, Rodrigo. > Sebastian Sylvan: > >> featureGenNormal = do > >> id <- stringGen > >> name <- stringGen > >> featuretype <- arbitrary > >> grouptype <- arbitrary > >> children <- arbitrary > >> properties <- listGen stringGen > >> return (Feature id name featuretype grouptype children properties) > > Ryan Ingram wrote: > > Also, you can shorten this significantly with liftM or ap (from Control.Monad): > > True, but in this case I like being able to see meaningful > names for each parameter of the constructor. > ----------------------------------- Rodrigo Bonifácio de Almeida Universidade Católica de Brasília - Grupo de Engenharia de Software - JavaComBr (www.ucb.br/java)

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Re: Haskell-Cafe Digest, Vol 55, Issue 20
by Maverick 17 Mar '08

17 Mar '08

Thanks Sterl, you're right, this was because I have don´t compiled with the -threaded flag, this works fine now. > This answer may be way off base, but if differences appear between > ghci and compiled versions, I've often found its as simple as > remembering to compile with the -threaded flag. The ghci runtime is > threaded by default, as I understand it, while compiled binaries are > not, and IO operations will block in very different fashions (i.e. in > their own thread, or stalling the entire app) depending on the runtime. > Regards, > sterl. Alvaro ______________________________________________ ¿Con Mascota por primera vez? Sé un mejor Amigo. Entra en Yahoo! Respuestas http://es.answers.yahoo.com/info/welcome

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QuickCheck
by rodrigo.bonifacio 17 Mar '08

17 Mar '08

Hi all, I'm trying to use the quick-check library for checking some properties of a user defined data type. Bellow the target data type: data Feature = Feature Id Name FeatureType GroupType Children Properties | FeatureError where: Id = String Name = String FeatureType = int GroupType = int Children = [Feature] Propertyes = [String] I've written the following quick-check property: prop_AlternativeFeature :: Feature -> Feature -> QuickCheck.Property prop_AlternativeFeature fm fc = length (children fc) == 0 ==> length (checkAlternativeFeature fm fc) > 0 When I try to check such property, the result is: ERROR "./EshopModelChecking.hs":11 - Type error in instance member binding *** Term : arbitrary *** Type : Feature *** Does not match : Gen Feature I think that I need to write some arbitrary or generator functions, but I didn't realize how to do that with the availalble quick-checking documentation. Any help will be welcome. Thanks in advance. Rodrigo.

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Building HaXML 1.13.3 on Windows fails
by Dusan Kolar 17 Mar '08

17 Mar '08

Hello all, I'm trying to build HaXML 1.13.3 on Windows using build.bat - I have modified it even on places referred as "should work" (SRCS and OBJS variables); now it works till the last command: ghc-pkg register pkg.conf The error is: Reading package info from "pkg.conf" ... ghc-pkg.exe: Line 68: The field main-is was already defined on line 62 Taking a look into the pkg.conf says there are several lines main-is. I'm using ghc 6.6 to build the stuff. What shall I do? Thanks and regards, Dusan

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GHC threaded RTS will call the finaliser of a ForeignPtr while references still exist
by Adam Langley 17 Mar '08

17 Mar '08

I believe that I've managed to distill a crash which has been driving me crazy for a few days into a short enough test case (22 lines) that it might be useful. In short: the threaded RTS will call the finialiser of a ForeignPtr while an exception handler still holds a reference to it: % ghc -make fptest.hs cbits.c -threaded [1 of 1] Compiling Main ( fptest.hs, fptest.o ) Linking fptest ... % ./fptest New object getting created: 66f010 Finaliser getting called for 66f010 Use called for 66f010 cbits.c: http://hpaste.org/6420 fptest.hs: http://hpaste.org/6421 I'm hoping that this is useful to someone who knows the RTS. Cheers, -- Adam Langley agl(a)imperialviolet.org http://www.imperialviolet.org

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"GADT" rhymes with "cat"
by Ashley Yakeley 17 Mar '08

17 Mar '08

"GADT" rhymes with "cat". The "d" is silent, like the Danish "godt", or the German "Stadt", or the American trademark "Bundt". -- Ashley Yakeley

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Re: [Haskell-cafe] QuickCheck
by rodrigo.bonifacio 16 Mar '08

16 Mar '08

Dear Sebastian Sylvan, Thanks for your datailed answer. It saved me a lot of time. Best regards, Rodrigo. > On Sun, Mar 16, 2008 at 5:42 PM, rodrigo.bonifacio < > rodrigo.bonifacio(a)uol.com.br> wrote: > > > Hi all, > > > > I'm trying to use the quick-check library for checking some properties of > > a user defined data type. Bellow the target data type: > > > > data Feature = > > Feature Id Name FeatureType GroupType Children Properties | > > FeatureError > > > > where: > > > > Id = String > > Name = String > > FeatureType = int > > GroupType = int > > Children = [Feature] > > Propertyes = [String] > > > > > > I've written the following quick-check property: > > > > prop_AlternativeFeature :: Feature -> Feature -> QuickCheck.Property > > prop_AlternativeFeature fm fc = length (children fc) == 0 ==> length > > (checkAlternativeFeature fm fc) > 0 > > > > When I try to check such property, the result is: > > > > ERROR "./EshopModelChecking.hs":11 - Type error in instance member binding > > *** Term : arbitrary > > *** Type : Feature > > *** Does not match : Gen Feature > > > > I think that I need to write some arbitrary or generator functions, but I > > didn't realize how to do that with the availalble quick-checking > > documentation. > > > > Any help will be welcome. > > > > > You use the available functions to build up a generator for your data type. > > First, let's give the instanc itself. For this I'm just going to use the > frequency function to use "featureGenNormal" five times more often than > "return FeatureError". This means that will get a FeatureError every now and > then, but mostly you'll get featureGenNormal (see below). You can change > these frequences, of course. > > instance Arbitrary Feature where > arbitrary = do > frequency [ (5, featureGenNormal), (1, return FeatureError) ] > > In order to write featureGenNormal, we need to be able to generate random > values of each of the parts of the data type. Often these types will already > have Arbitrary instances, so generating an isntance for your type is quite > often just a matter of calling "arbitrary" for each component, and then > returning a datatype. However, there is no Arbitrary instance for String, > which is a bit annoying, so let's write our own generator for strings. > > First a generator for a single letter: > > letterGen = oneof $ map return $ ['a'..'z'] ++ ['A'..'Z'] > > Then a combinator for generating a list of values given a generator for a > single value: > > listGen :: Gen a -> Gen [a] > listGen g = do > x <- g > xs <- frequency [ (1, return []), (10, listGen g) ] > return (x:xs) > > And then we use this to build our "stringGen" generator. > > stringGen :: Gen String > stringGen = listGen letterGen > > Now, we have all we need to write the featureGenNormal generator: > > featureGenNormal = do > id <- stringGen > name <- stringGen > featuretype <- arbitrary > grouptype <- arbitrary > children <- arbitrary > properties <- listGen stringGen > return (Feature id name featuretype grouptype children properties) > > > Note that we use "arbitrary" to generate the list of children recursively. > > -- > Sebastian Sylvan > +44(0)7857-300802 > UIN: 44640862 > ----------------------------------- Rodrigo Bonifácio de Almeida Universidade Católica de Brasília - Grupo de Engenharia de Software - JavaComBr (www.ucb.br/java)

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ANNOUNCE: omnicodec
by Magnus Therning 15 Mar '08

15 Mar '08

I've just uploaded to hackage a package containing two command line utilities for encoding and decoding data, called omnicodec. It's built on top of dataenc. /M -- Magnus Therning (OpenPGP: 0xAB4DFBA4) magnus＠therning．org Jabber: magnus．therning＠gmail．com http://therning.org/magnus What if I don't want to obey the laws? Do they throw me in jail with the other bad monads? -- Daveman

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