March 2008 - Haskell-Cafe

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 16 Mar '08

16 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 16 Mar '08

16 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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Re: [Haskell-cafe] A question about "monad laws"
by askyle 15 Mar '08

15 Mar '08

> My favorite presentation of the monad laws is associativity of Kliesli > composition: > (a1 >=> a2) x = a1 x >>= a2 -- predefined in 6.8 control.monad > -- The laws > return >=> a = a > a >=> return = a > a >=> (b >=> c) = (a >=> b) >=> c If you use this presentation you also need the following law: (a . b) >=> c = (a >=> c) . b that is, compatibility with ordinary function composition. I like to call this "naturality", since it's instrumental in proving return and bind to be natural transformations. The law looks less alien if we use a flipped version of (>=>): (<=<) = flip (>=>) {- Monad Laws in terms of (<=<) -} return <=< f = f <=< return = f -- Identity f <=< (g <=< h) = (f <=< g) <=< h -- Associativity f <=< (g . h) = (f <=< g) . h -- Naturality (which happens to be my favorite presentation of the laws, followed by the derivations that lead to the 'do' notation, which lead to various 'ah' moments from unsuspecting FP-challenged friends) ----- Ariel J. Birnbaum -- View this message in context: http://www.nabble.com/A-question-about-%22monad-laws%22-tp15411587p15975734… Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

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Implementing fixed-sized vectors (using datatype algebra?)
by Alfonso Acosta 15 Mar '08

15 Mar '08

Hi, The EDSL implementation (system design) I'm working on would really benefit from an implementation of fixed-sized vectors. I thought this would be a generally desired ADT but it turned out I wasn't able to find an implementation. I was thinking about using datatype algebra plus GADTs to implement a type-level parameter indicating the size of the vector. I'm a total noob with regard to GADTs and type-level algebra trickery. So my questions are: 1) Do you think it is feasible? Can you think about a better approach? 2) An implementation of type-level Naturals would really help. What has already been done? Thanks in advance, Fons

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Gtk2hs
by Andrew Coppin 15 Mar '08

15 Mar '08

Just a short one... gtk2hs won't build on my [Linux] laptop. What's the best channel for seeking help with this?

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Type system
by Andrew Coppin 15 Mar '08

15 Mar '08

Haskell has an expressive and powerful type system - which I love. It also has a seemingly endless list of weird and obscure type system extensions. And there are various things you can do in Haskell which *require* some pretty serious type system hackery. And yet, none of this happens in any other programming language I've met. Take Java for example. In Java, a "type" is just an identifier. It has no further structure than that. It does have the added semantics that it refers to that class *or any subclass*, but that's about it. Even taking an extreme example like Eiffel [which supports generics], complex types are extremely rare. And yet they commonly pop up in Haskell. Can anybody put their finger on precisely why that is? Is it because Haskell is used by more PhDs? Is it because Haskell actually allows you to implement constructs that are impossible in other languages? Is it because Haskell really provides greater type safety? Is it something else? Not trying to suggest that either system is "superior", I'm just curios as to the origin of the difference...

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thanks for your feedback concerning tech documentation.
by michael＠schmong.org 14 Mar '08

14 Mar '08

Thanks. I was encouraged by this response I got. I'm ready to go. Since I'm trapped in the space-time continuum like most people, I can't do it all at once. I would like to. Anything that supports haskell is okay by me. My first area of interest is HAppS. I wrote some e-mail to them yesterday, but have yet to hear anything. Should I belt out some e-mail to specific people? By the looks of things, the traffic on that list is pretty slow right now. Michael Litchard

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