February 2009 - Haskell-Cafe

Hac5: April 17-19, Utrecht -- Book Now!
by Sean Leather 23 Feb '09

23 Feb '09

Twelve days ago, we emailed you about a particular event... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ The 5th Haskell Hackathon April 17 - 19, 2009 Utrecht, The Netherlands ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ We have some updated information for you. If you don't want to read the details, then the important points are: * Hotel or hostel accommodation may be hard to come by, so book your room now! * Check out the website for lots of useful information. As I mentioned before, you can find out more about the Hackathon here: http://haskell.org/haskellwiki/Hac5 The site is a wealth of useful information gratefully provided by the Utrecht organizing team. If you haven't seen it recently, check again. In particular, however, we'd like to inform you of some newly obtained information. We unknowingly scheduled the Hackathon on the same weekend as a popular record and CD fair. http://www.recordplanet.nl/ So, not only do you get the chance to hack on projects involving your favorite purely functional programming language, you also have the opportunity to visit "the most impressive record fair of the world." However, this does have some drawbacks... Rooms in hotels and hostels in Utrecht are often notoriously difficult to come by. It's already a crowded city sometimes with too few places to stay (esp. if you want cheap). Therefore, if you've decided to join us or have already registered, we highly recommend booking a room at your earliest convenience. (If you haven't decided to visit Utrecht, then you should hurry up and agree to it already!) For this purpose, we have updated the main Hac5 page with information about accommodations. http://haskell.org/haskellwiki/Hac5#Accommodation We also have a Google map with information about places to stay and transportation. http://haskell.org/haskellwiki/Hac5#Map And finally, don't forget that you can use either the Attendees page, the Hackathon mailing list, or the IRC channel to communicate with other (potential) attendees or the organizers about sharing rooms, finding a place, or anything else. http://haskell.org/haskellwiki/Hac5/Attendees http://haskell.org/mailman/listinfo/hackathon #haskell-hac5 (on freenode) Good luck and happy hacking! == Organizers == Andres Löh, Utrecht University (UU) José Pedro Magalhães, UU Sean Leather, UU Eelco Lempsink, UU + Tupil Chris Eidhof, UU + Tupil ... and more ...

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Stacking StateTs
by Luis O'Shea 23 Feb '09

23 Feb '09

I've been experimenting with the state monad and with StateT, and have some questions about how to combine one state with another. This email is literate Haskell tested on GHCi, version 6.10.1. Also, sigfpe's post on monad transformers (http://blog.sigfpe.com/2006/05/ grok-haskell-monad-transformers.html) was very helpful. > import Control.Monad.State My question is basically whether the function modifyT (below) makes sense, whether some form of it already exists in a standard library, and (most importantly) whether it actually indicates that I'm thinking about StateT all wrong. > modifyT :: Monad m => > (s -> StateT t m s) > -> StateT t (StateT s m) () > modifyT f = do > x <- get > y <- lift get > (y',x') <- lift $ lift $ runStateT (f y) x > lift $ put y' > put x' Some context may be useful, so here is how I ended up thinking I needed modifyT. The state monad makes it easy to write stateful computations. For example here is a computation that has an Integer as its state and returns a String: > test1 :: State Integer String > test1 = do > modify (+ 1) > a <- get > return $ "foobar" ++ (show a) If the computation wants to do some IO then it makes sense to start with the IO monad and then apply the StateT transformer to it: > test2 :: StateT Integer IO String > test2 = do > modify (+ 1) > a <- get > lift $ print a > return $ "foobar" ++ (show a) So from now on I won't actually do any IO and will replace IO with an arbitrary monad m. Also instead of the fixed string "foobar" I'll have it take a String as a parameter: > test3 :: Monad m => String -> StateT Integer m String > test3 s = do > modify (+ 1) > a <- get > return $ s ++ (show a) A nice feature of all this is that it is easy to combine these computations: > test4 :: Monad m => StateT Integer m (String,String) > test4 = do > s1 <- test3 "foo" > s2 <- test3 "bar" > return $ (s1,s2) Now seeing as test3 takes a String and returns another String you can imagine using it to transform a String state. (I'm also going to assume that test3 is in another library so we don't want to alter how it's written.) So here is how you could use test3 in a computation that has (String,Integer) as its state: > test5 :: (Monad m) => m Integer > test5 = do > (s1,x1) <- runStateT (test3 "") 0 > (s2,x2) <- runStateT (test3 s1) (2*x1 + 1) > (s3,x3) <- runStateT (test3 s2) (x2*x2) > return x3 Then running test5 >>= print gives 17. The problem with test5, of course, is that we have manually threaded the state, with all the problems that implies. For example nothing prevents you from erroneously misthreading the state: > test5bad :: (Monad m) => m Integer > test5bad = do > (s1,x1) <- runStateT (test3 "") 0 > (s2,x2) <- runStateT (test3 s1) (2*x1 + 1) > (s3,x3) <- runStateT (test3 s1) (x2*x1) > return x3 Running test5bad >>= print gives 5. Obviously we want operate in a State monad with more state. One way to do this is to stack two StateTs on top of m. This is, finally, where I need the modifyT that we defined above -- it lets us "lift" test3 to a function that modifies the state of the top *two* StateTs. Now let's use it to rewrite test5: > test6 :: (Monad m) => StateT Integer (StateT String m) Integer > test6 = do > modifyT test3 > modify $ \x -> 2*x + 1 > modifyT test3 > modify $ \x -> x*x > modifyT test3 > x <- get > return x > > test7 :: (Monad m) => m Integer > test7 = evalStateT (evalStateT test6 0) "" As expected, running test7 >>= print gives 17. So, given that modifyT seems to be useful, does it, or something like it, already exists in the standard libraries? More likely, am I making a mountain of a molehill and is there a better way to structure all this? Thanks, Luis > main = do > test5 >>= print > test5bad >>= print > test7 >>= print

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advancePtr for ForeignPtr
by Henning Thielemann 23 Feb '09

23 Feb '09

Is still someone on haskell.org ? ---------- Forwarded message ---------- Date: Thu, 19 Feb 2009 21:40:08 +0100 (CET) From: Henning Thielemann <lemming(a)henning-thielemann.de> To: ffi(a)haskell.org Subject: advancePtr for ForeignPtr I want to have an advancePtr on ForeignPtr in order create a subarray. Is this reasonable and possible? Do I have to use 'touchForeignPtr' as finalizer of the subarray's ForeignPtr in order to assert that the superarray lives at least as long as the subarray? Anyway I'd prefer a function in the standard modules than doing such things by myself.

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looking for a 2d graphics package
by Martin DeMello 23 Feb '09

23 Feb '09

What 2d graphics library would be the best fit for this sort of thing? http://www.superliminal.com/geometry/tyler/gallery/circular/tiles060.html I took a look at diagrams but it seemed more geared towards things that fit on a rectangular layout. martin

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Semantic web
by gregg reynolds 23 Feb '09

23 Feb '09

Anybody implementing rdf or owl stuff in haskell? Seems like a natural fit. G

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Fwd: [plt-scheme] Vancouver Lisp Users Group meeting for March 2009 - Haskell for Lisp Programmers
by Tim Wawrzynczak 23 Feb '09

23 Feb '09

I thought you guys might find this interesting: Haskell for Lisp programmers ---------- Forwarded message ---------- From: Bill Clementson <billclem(a)gmail.com> Date: Mon, Feb 23, 2009 at 11:12 AM Subject: [plt-scheme] Vancouver Lisp Users Group meeting for March 2009 - Haskell for Lisp Programmers To: undisclosed-recipients Hi all, Haskell is a polymorphically statically typed, lazy, purely functional language based on the lambda calculus. As such, it shares some things in common with some dialects of Lisp but differs in other regards. Our March lispvan speaker will present an introduction to Haskell geared towards Lisp programmers. If you want to read up about Haskell before the meeting, a good starting point is the Haskell Wiki. Here's the "official" meeting notice: Topic: Haskell for Lisp Programmers Presenter: Erik Charlebois Date: Wednesday, March 4th, 2009 Time: 7pm - 10pm (or whenever) Venue: The Hackery, 304 Victoria Dr (entrance off Franklin), Vancouver (see map) Summary: Haskell is a lazy, pure, statically-typed functional programming language enjoying a lot of attention these days. Its strict approach to side effects is seen as one of the viable approaches to making parallel programming tractable. Erik will talk about the core differences between Haskell and Lisp, the actions the Haskell community is taking to manage this growth, and some neat applications of the language already in the wild. * Differences between Haskell and Lisp * Syntax * Static typing * Side effects with monads * The Haskell Platform * Build, Package, Distribute * Community * Haskell Dog and Pony Show Bio: Erik was a compiler developer for 3 years at IBM working on the XL Fortran and C/C++ compilers for the Cell processor. He is currently employed at Slant Six Games doing gameplay programming. For the past 2 years, he has been studying programming languages and databases in a search for smarter ways to build soft real-time applications like games. If possible, I will record the presentation and post it on my blog after the meeting for those who are unable to attend. Join us for a beer (bring your own - there's a fridge) and a chance to learn what static typing and monads are all about! Any updates will be posted on my blog entry for the meeting: http://bc.tech.coop/blog/090223.html -- Bill Clementson _________________________________________________ For list-related administrative tasks: http://list.cs.brown.edu/mailman/listinfo/plt-scheme

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Online GHC documentation
by Maurício 23 Feb '09

23 Feb '09

Just a small tip: I think it would be nice if GHC online documentation were built with links to the source code, like in hackage. Best, Maurício

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Fwd: cabal-install and bash scripts in windows- Was: Re: pkgenv - disposable, isolated pkg environments
by Alberto G. Corona 22 Feb '09

22 Feb '09

Forwarded to the list: By the way, there are important packages such are haskell plugins, that have also this same single obstacle for workining under Windows. Many have basically the same bash script that test the type of c compiler, which fails miserably under windows: example output: checking for C compiler default output file name... configure: error: C compiler cannot create executables See `config.log' for more details. cabal: Error: some packages failed to install: plugins-1.4.0 failed during the configure step. The exception was: exit: ExitFailure 77 Could be possible to check if this problem can be solved with cabal-install?. 2009/2/22 Duncan Coutts <duncan.coutts(a)worc.ox.ac.uk> > On Sun, 2009-02-22 at 18:38 +0100, Paolo Losi wrote: > > Duncan Coutts wrote: > > > > > Would it be useful to have cabal-install check a env var for its config > > > file? > > > > Definitely yes. > > This would allow dropping the dependency on bash aliasing and > > make porting to Windows fairly easy. > > Ok, I've filed a ticket so hopefully someone will pick it up. It should > be pretty easy. Just a few lines of code. > > http://hackage.haskell.org/trac/hackage/ticket/505 > > Duncan > > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe >

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Practise of hiding contsructors and serialization
by Marc Weber 22 Feb '09

22 Feb '09

While working on extending scion I got some trouble and wonder which is the best way to do this? I have to pass a compilation result from a scion process instance to the scion daemon process. Of course I'd just like to use a simple Read Show for that. However I can't because two datatypes are NominalDiffTime and Bag ErrMsg: newtype NominalDiffTime = MkNominalDiffTime Pico deriving (Eq,Ord) data ErrMsg = ErrMsg { errMsgSpans :: [SrcSpan], errMsgContext :: PrintUnqualified, errMsgShortDoc :: Message, errMsgExtraInfo :: Message } Both are fine and both don't export all contstructors so you can't just let ghc derive instances for it? So the only way is to duplicate those datatypes in order to derive some Show, Read instances? On the one hand I do understand that you don't want users to access constructors directly. On the other hand it would be desirable to derive Read, Show instances here. Is there a way to get all? a) hiding constructors so that you don't have to expose implementation details to the user. b) still allow deriving serialization instances? Is there a better way than using toRational to get a type which can be serialzied quickly? I don't like Rational because the unit information 1e-12[s] is no longer included in the type.. Sincerly Marc Weber

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Error on instance of Control.Exception.Exception
by Maurício 22 Feb '09

22 Feb '09

Hi, I read here: www.haskell.org/ghc/docs/latest/html/libraries/base/Control-Exception.html that this is declared: --- class (Typeable e, Show e) => Exception e where toException :: e -> SomeException fromException :: SomeException -> Maybe e --- I understand this means Excepton is a class. But after I try: --- instance Exception DbError where --- GHC says: Type constructor `Exception' used as a class In the instance declaration for `Exception DbError' Isn't Exception a class? If it is, what is the proper way to have an Exception instance for my type? Thanks, Maurício

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