February 2010 - Haskell-Cafe

Re: [Haskell-cafe] Testing and module export lists
by Magnus Therning 25 Feb '10

25 Feb '10

On Wed, Feb 24, 2010 at 10:41, Arnaud Bailly <arnaud.oqube(a)gmail.com> wrote: > Maybe you just want to test what's in your export list which > represents the public interface of your code. And if you cannot write > a test that exercise private implementation through the public > interface, then maybe there is a design problem... Possibly! The specific case I have is a module for parsing a small language. The module exports a single function, a function that takes a string and returns a tree. Of course the parser is built using parser combinators. I'd like to test each combinator I've built, but I don't want to export them all. The only way I see of achieving this is to split each module in two, one public and one internal, where the public one just re-exports the relevant pieces of the internal one. That's workable, but hardly aesthetically pleasing ;-) /M -- Magnus Therning (OpenPGP: 0xAB4DFBA4) magnus＠therning．org Jabber: magnus＠therning．org http://therning.org/magnus identi.ca|twitter: magthe

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Interested in the Dutch HUG Day (24 April 2010)?
by Sean Leather 25 Feb '10

25 Feb '10

In honor of the first anniversary of the Dutch Haskell Users Group (formed at Hac5 in Utrecht), we are planning a mini symposium to bring together Haskell and functional programming enthusiasts in the Netherlands and surrounding area. *The Dutch HUG Day will be held on Saturday, 24 April.* Please mark that date in your calendar. The remaining details are being worked out. In order to help us with this process, we have created a short survey. If you are even remotely considering attending, please take a moment to fill out a form with your name and the activities that interest you. * http://spreadsheets.google.com/viewform?formkey=dGVuVTdnZEl0ZDE3MEVweUtRdml… Note that this does not commit you to anything. We will only use this information to help plan the event (e.g. determine the number of seats at the location and the types of activities). For more information about the Dutch HUG, visit the wiki page and sign up for the mailing list. * http://www.haskell.org/haskellwiki/Dutch_HUG * http://groups.google.com/group/dutch-hug Thanks for your time! Sincerely, The Dutch HUG Day planning team

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Category Theory woes
by Mark Spezzano 25 Feb '10

25 Feb '10

Hi all, I'm trying to learn Haskell and have come across Monads. I kind of understand monads now, but I would really like to understand where they come from. So I got a copy of Barr and Well's Category Theory for Computing Science Third Edition, but the book has really left me dumbfounded. It's a good book. But I'm just having trouble with the proofs in Chapter 1--let alone reading the rest of the text. Are there any references to things like "Hom Sets" and "Hom Functions" in the literature somewhere and how to use them? The only book I know that uses them is this one. Has anyone else found it frustratingly difficult to find details on easy-to-diget material on Category theory. The Chapter that I'm stuck on is actually labelled Preliminaries and so I reason that if I can't do this, then there's not much hope for me understanding the rest of the book... Maybe there are books on Discrete maths or Algebra or Set Theory that deal more with Hom Sets and Hom Functions? Thanks, Mark Spezzano.

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Heterogeneous Data Structures - Nested Pairs and functional references
by Günther Schmidt 25 Feb '10

25 Feb '10

Hi, I've been thinking a lot lately about heterogeneous and extensible data structures for which HList certainly offers a solution. While HList is implemented through type-level programming I wonder if I can achieve similar results through value-level programming alone. This is where I was thinking of functional references. I wonder if or rather how one could do this: Let's say there was some clever monad ... someMonad = do h1 <- add "twenty" h2 <- add False h3 <- add 16 ..... modify h2 True and get a ("twenty",(True, 16)) back. And while *in* the monad some accessors available. Now come to think of it I think I actually read about this somewhere so I doubt this is truly my idea. Anybody got some thoughts on this? Günther

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Some great results on fused code with the LLVM backend
by Don Stewart 25 Feb '10

25 Feb '10

I tried out some of the vector and uvector fusion benchmarks with the new LLVM backend http://donsbot.wordpress.com/2010/02/21/smoking-fast-haskell-code-using-ghc… and got some great results for the tight loops generated through fusion. Up to 2x faster than gcc -O3 in some cases. The LLVM backend looks very promising -- considering we've not even begun to explore the optimization pipeline at the level. -- Don

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Re: Bug#570284: O: washngo
by Joachim Breitner 25 Feb '10

25 Feb '10

Hi, Am Mittwoch, den 17.02.2010, 15:35 -0600 schrieb John Goerzen: > I no longer use WASH, nor do I have time to maintain it anymore. > Orphaning it, and CCing debian-haskell in case someone there has > interest in it. we have two option: Taking it over by the Haskell Group, or dropping it from Debian. Therefore my question: Is anyone using Wash? Is it still useful, three years after the last change and with HAppS around? Greetings, Joachim -- Joachim "nomeata" Breitner Debian Developer nomeata(a)debian.org | ICQ# 74513189 | GPG-Keyid: 4743206C JID: nomeata(a)joachim-breitner.de | http://people.debian.org/~nomeata

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RE: gtk2hs and ghc-6.10.4 on Windows (Vista)
by gladstein＠gladstein.com 24 Feb '10

24 Feb '10

In correspondance with Alistair Bayley I learned that there is in fact a gtk2hs installer for 6.10.4. One can find it by Googling "gtk2hs 6.10.4" (which I did not think to do). The URL is in http://www.mail-archive.com/gtk2hs-devel@lists.sourceforge.net/msg00340.html The installer doesn't work if the Haskell Platform is installed in the default location, which has spaces in the pathname. Reinstalling Haskell in, say, c:\HaskellPlatform makes it all work.

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Re: Testing and module export lists
by John Lato 24 Feb '10

24 Feb '10

> From: Magnus Therning <magnus(a)therning.org> > > How do people who like unit testing / property testing deal with export lists? > > I often find that I do want an export list to reduce clutter in the > finished code, but for testing I'd like to expose everything in a > module. Is there a nice way to deal with this (using the C > pre-processor would not qualify as "nice" ;-)? > Maybe there's a switch that causes GHC to simply ignore the export > list of a module and export everything? That would be a nice feature to deal with this problem. The best solution I have is to separate functions that don't belong in the public interface into separate modules that aren't exposed (they're listed in the other-modules section of the .cabal file). I then add the test executable to the .cabal file, so when it's built it can access the hidden modules (or I run the test from ghci). Of course this only works if it makes sense to structure the code this way. Cheers, John

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Lazy IO with recursive reads?
by Fabian Roth 24 Feb '10

24 Feb '10

Hi all, I am trying to read structured data from a socket and return a lazy list of records. However, the socket reading operation seems to be strict and never returns (until stack overflow). Here's some simplified code to reproduce the problem: -------------------- import Control.Monad main = do messages <- readLazy mapM_ (\x -> putStr $ show x ++ "\n") $ messages return () where readLazy :: IO [String] readLazy = do c <- fancyIORead liftM2 (++) (return c) readLazy fancyIORead :: IO [String] fancyIORead = return ["aa","bb"] -------------------- In my implementation fancyIORead reads blocks from the socket and returns a list of records. But it seems readLazy doesn't ever return. What could be the problem here? Also, if anyone has a better solution to write this thing, pls let me know. Thanks! Fabian

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ANN: concurrent-extra-0.2
by Roel van Dijk 24 Feb '10

24 Feb '10

Hello, We would like to announce an update of concurrent-extra [1]. Bug fixes: - A bug in RLock.acquire (thanks to Felipe Lessa). New features: - Broadcast: Wake multiple threads by broadcasting a value. This is a generalisation of Event. - Thread: Threads extended with the ability to wait for their termination. - delay: Arbitrarily long thread delays. - timeout: Wait arbitrarily long for an IO computation to finish. The lightweight thread wrapper was inspired by the threadmanager package [2]. The main advantage of our implementation is that we don't maintain an internal mapping from ThreadId to ThreadStatus. Instead we rely on the forked thread to broadcast its status to interested listeners. This should result in better performance (no lookup required). Every exported symbol is now documented. Regards, Roel & Bas van Dijk [1] http://hackage.haskell.org/package/concurrent-extra-0.2 [2] http://hackage.haskell.org/package/threadmanager-0.1.3

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