- Haskell-Cafe - Haskell.org

Confused by SYB example with zipping
by Henry Laxen 16 Feb '09

16 Feb '09

Dear Group, When trying to run the example at: http://www.cs.vu.nl/boilerplate/testsuite/gzip/Main.hs ghc 6.10.1 says A pattern type signature cannot bind scoped type variables `a' unless the pattern has a rigid type context In the pattern: f :: a -> a -> a In the definition of `mkTT': mkTT (f :: a -> a -> a) x y = case (cast x, cast y) of { (Just (x' :: a), Just (y' :: a)) -> cast (f x' y') _ -> Nothing } In the definition of `main': main = print $ gzip (mkTT maxS) genCom1 genCom2 where genCom1 = everywhere (mkT (double "Joost")) genCom genCom2 = everywhere (mkT (double "Marlow")) genCom double x (E (p@(P y _)) (S s)) | x == y = E p (S (2 * s)) double _ e = e maxS (S x) (S y) = S (max x y) .... Failed, modules loaded: CompanyDatatypes. --------------------------------------------------------------------- I must admit I don't really know what to make of this. Any insights would be appreciated. Thanks. Henry Laxen

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Parameterized monad transformers
by Miran Lipovača 16 Feb '09

16 Feb '09

Howdy y'all! I was reading sigfpe's Beyond Monads post (http://blog.sigfpe.com/2009/02/beyond-monads.html) about parameterized monads. Great post, makes you go: this is so elegant and simple, why didn't I think of that? Anyway, I was wondering whether those monads have corresponding transformers, and if so, what they look like and what they are good for? Is there any weight to the idea at all? I don't know much about monad transformer theory, but I do imagine that for transformers to be useful, they have to be stackable, so the underlying monad of a parameterized monad also has to be a parameterized monad. I asked around in #haskell and ski_ and I had a couple of ideas exploring the concept (although he did most of the exploring, I just carried his luggage). Seeing how parameterized monads are gotten by just loosening some restrictions on normal ones and drawing then parallels between them, I think the same methodology could be applied to transformers. So there should be a transformer version t of a parameterized monad m and a lifting operation plift that lifts m to t. At first I did what sigfpe did with non-transformer monads, which was to just implement the appropriate functions for the more general monads and see what GHCI tells us about the type. I first thought about having a parameterized StateT, PStateT: newtype PStateT m s1 s2 a = PStateT { runPStateT :: s1 -> m (a, s2) } along with a lifting operation plift :: (PMonad m) => m s1 s2 a -> t (m s1 s2) s3 s3 a but I don't think that's cool because of the different kind of the underlying monad. So then ski_ suggested newtype PStateT m (s1,s1') (s2,s2') a = PStateT { runPStateT :: s1 -> m s1' s2' (Pair a s2) } where (s1,s1') and (s2,s2') stand for the pair of types and not a (,) type constructor with two types applied as type parameters. That's why the pair in the result is represented as Pair a s2. Anyway, s1' and s2' represent the category tail and head of the underlying parameterized monad. With that, the lifting operation would have a type of plift :: (PMonad pm) => pm s1 s2 a -> t pm (s, s1) (s, s2) a basically I'm just wondering how you guys think the transformer versions of parameterized monads work and what their types should be. And if you think they're feasible at all. So basically just talk about parameterized monad transformers here :] Cheerio!

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Looking for pointfree version
by Edsko de Vries 16 Feb '09

16 Feb '09

Hi, Is there a nice way to write down :: Focus -> [Focus] down p = concat [downPar p, downNew p, downTrans p] in point-free style? (In doesn't make much difference what these functions do; if it helps, their types are downPar, downNew, downTrans :: Focus -> [Focus]). Ideally, I would like to write something like down = downPar ... downNew ... downTrans but I'm not sure what should be on the dots. This works: down = concat . flip map [downPar, downNew, downTrans] . flip ($) but is extremely ugly and doesn't really explain what's going on :) (It seems to me I should be able to take advantage of the list monad, somehow). Pointers appreciated! Edsko

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Help needed with apache config for hackage
by Duncan Coutts 16 Feb '09

16 Feb '09

Hi folks, Does anyone have any experience with apache configuration, particularly mime types and handling browser quirks and would like to help us with an issue we have on hackage? http://hackage.haskell.org/trac/hackage/ticket/498 The problem is described in the ticket but basically IE6 gets confused by the Content-Type and Content-Encoding and ends up saving ".tar.gz" files with the wrong name ".tar.tar". We need help working out how to configure apache to use a workaround and with testing that the solution actually works. Thanks! Duncan

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ANNOUNCE: spacepart-0.1.0.0 (was called data-spacepart)
by Corey O'Connor 16 Feb '09

16 Feb '09

0.1.0.0 - Renamed Math.Geometry to Data.SpacePart.AABB - Renamed Data.QuadTree to Data.SpacePart.QuadTree - Added Data.SpacePart.QuadTree.query. Returns all elements that intersect a given boundary. - The inclusive nature of the boundary's min extent should take precedence of the exclusive nature of the max extent. Before this change many of the tests failed when boundaries of 0 area were involved. One case that did not work was constructing a quadtree containing elements of 0 area. This change corrected this. The tests all_elements_inserted_query_prop and element_bounds_query_is_element_prop still fail if any element involved is of 0 area. - Cannot create quadtrees with initial bounds of 0 area. - Removed requirement on elements being an instance of the Intersectable class. The only required instance is of Data.SpacePart.AABB.HasBoundary. - Changed package name to spacepart instead of data-spacepart. The last release of data-spacepart used a data based version number. This version number policy did not work well with the standard package version policy. - Added some QuickCheck based checks. Run with test/run_verify - Cleaned up the module exports. Cheers, Corey O'Connor

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Type families not as useful over functions
by John Ky 16 Feb '09

16 Feb '09

Hi Haskell Cafe, I tried using type families over functions, but when I try it complains that the two lines marked conflict with each other. class Broadcast a where type Return a broadcast :: a -> Return a instance Broadcast [a -> r] where type Return [a -> r] = a -> [r] -- Conflict! broadcast fs a = [] instance Broadcast [a -> b -> r] where type Return [a -> b -> r] = a -> b -> [r] -- Conflict! broadcast fs a b = [] Given that in Haskell, every function of n+1 arguments is also a function of n arguments, this is likely the cause of the conflict. In this case, currying is not my friend. Unfortunately this means I'm stuck with numbered function names: bc0 :: [r] -> [r] bc0 rs = rs bc1 :: [a -> r] -> a -> [r] bc1 [] a = [] bc1 (r:rs) a = (r a):bc1 rs a bc2 rs a b = rs `bc1` a `bc1` b bc3 rs a b c = rs `bc1` a `bc1` b `bc1` c -- etc Cheers, -John

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Haddock Markup
by David Waern 16 Feb '09

16 Feb '09

I got the subject line wrong, so I'm reposting this: ==== Hi everyone, I received this question from Lennart Augustsson (via Simon M) and thought I'd send out an inquiry to the Haskell community in general (Lennart, I hope you don't mind): Lennart writes: > We have some local patches for haddock that extends the <<blah>> > syntax so you can put TeX formulae in the documentation. > It looks like, <<! LaTeX stuff here !>>, but I'd like to extend it so > you can process the string with any command. > > Are you interested in folding this into the main branch? So the question is about extending the Haddock markup language. When modifying the language we should think about the tension between familiarity, presentation features (pictures, math, whatever) and visual portability across different mediums (HTML, ghci, IDE tooltips, etc). And here I should say that Haddock already supports pictures using the << url >> syntax. IMHO, adding <<! LaTeX !>> for TeX math is fine, because: - math in documentation is often useful - if you're going to write math, you need a format, even when the medium is plain text as in ghci. - TeX formulae seem to be relatively widely used and understood. As for running arbitrary commands, I think we are opening up to a lot of unfamiliar syntax. I'd like to hear what everyone thinks about that. There was also a thread about Haddock markup on haskell-cafe@ about a year ago, which originated with the interesting idea of using Markdown (or a Pandoc-extended version of it) instead of the current language: http://www.mail-archive.com/haskell-cafe@haskell.org/msg38054.html I think the original idea there is pretty nice, but let's first focus on the current markup language in order to answer Lennart's question. That thread contains some useful opinions on this matter, also. So, any comments? :) David

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The wheather is ⊥
by Henk-Jan van Tuyl 16 Feb '09

16 Feb '09

The temperature is now NaN°: http://traviswalters.net/?p=58 -- Met vriendelijke groet, Henk-Jan van Tuyl -- http://functor.bamikanarie.com http://Van.Tuyl.eu/ --

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Need some ideas to map XML Schema types to Haskell types
by Praki Prakash 16 Feb '09

16 Feb '09

Hi, I need to work with a web service that has a few thousand types in its schema. I am trying to automate the generation of data types and serializers from the schema. My first thought was to map each data type to a Haskell record and generate pickle methods thereafter for serialziation/deserialization. However, I am seeing a few challenges in mapping Schema types to Haskell records. First issue is the naming conflicts with record fields. If the number of types were smaller, I could have manually mangled the name with a unique prefix. Using a type class for shared name fields seems like an option but it looks like quite a bit of work. Is there any other technique I can use? Second issue is with capturing the Schema type hierarchy. Is there any way to relate types? If not, what would be a good technique to represent type hierarchies? Thanks, Praki

1 0

Epic failure
by Andrew Coppin 16 Feb '09

16 Feb '09

OK, what did I do wrong here? module Main () where import Data.List top = 10 ^ 8 :: Int main = do let numbers = [1 .. top] print $ foldl' (+) 0 numbers -- Runtime is 20 seconds. #include <stdio.h> int main(void) { int total = 0; int n; for (n = 1, n < 100000000; n++) total += n; printf("%d", n); } // Runtime is 0.8 seconds. module Main () where import Data.List top = 10 ^ 8 :: Int kernel i o = if i < top then o `seq` kernel (i+1) (o+i) else o main = do print $ kernel 1 0 -- Runtime is 0.5 seconds. Clearly these two nearly identical Haskell programs should have exactly the same runtime. Instead, one is 40x slower. Clearly something has gone catastrophically wrong here. The whole point of GHC's extensive optimiser passes are to turn the first example into the second example - but something somewhere hasn't worked. Any suggestions?

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Help with Stack Space Overflow / Memory Issues
by Travis B. Hartwell 16 Feb '09

16 Feb '09

Hello, I'm writing a small program to process Delicious [1] RSS feeds. I like look at the recent feeds to see what others have bookmarked recently. But, there are a lot of duplicates in the recent feeds as an entry is shown for each person who bookmarks an individual URL. I decided to write a small program that would trim out those that I've seen before. I wrote a small program that read a feed (initially just a on-disk copy of an RSS feed) and removed the duplicate items just within that feed. It worked great. Then, I wanted to add persistence, so this would maintain state from one run to the next. I decided to use Data.Binary to serialize the Data.Map I was using and re-load it each time. Unfortunately, making this change caused a "Stack Space Overflow" error and I couldn't track down what was wrong. This was with GHC 6.8.2. I recently upgraded to GHC 6.10.1 and the memory just grows unbounded, until it actually locks up my machine. This happens even when I comment out the code for the serialization / de-serialization of the map, so essentially the only difference from my prior version is the function where the map is initialized returns IO [Item] instead of [Item]. The latest version of my code is up on github [2], and the sample RSS feed I was processing is included in the repo. I'd appreciate some help in how to attack this problem. I've even tried profiling this (back when I was using 6.8.2) and there was nothing enlightening there, at least with my limited Haskell experience. I am unsure of how to get this to work, or if the problem is even my code. Additionally, I am unsure if my serialization code would work anyway. Because Haskell is not pass-by-reference, would the changes to the seenMap propogate back to my deDupWithSerializedMap function where it is serialized? If not, how would I go about doing this? I think part of my problem might be the difference between pure and impure code and how to separate it. Thanks for the help! [1] http://www.delicious.com/ [2] http://github.com/Nafai77/recent-feeds/tree/master --------------- Travis B. Hartwell Software Toolsmith Blog: http://www.travishartwell.net/blog Where to find me: http://www.travishartwell.net/blog/static/where_to_find_me

1 0

Interactive debugging?
by Colin Paul Adams 16 Feb '09

16 Feb '09

Can this be done with Haskell? In particular, I'm using ghc 6.10.1. Can I get symbols for use with gdb, for instance? -- Colin Adams Preston Lancashire

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Intergalactic Telefunctors
by Gregg Reynolds 16 Feb '09

16 Feb '09

Came up with an alternative to the container metaphor for functors that you might find amusing: http://syntax.wikidot.com/blog:9

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Race condition possible?
by Peter Verswyvelen 16 Feb '09

16 Feb '09

Consider the following code stamp v x = do t <- getCurrentTime putMVar v (x,t) Is it possible - with GHC - that a thread switch happens after the t <- getCurrentTime and the putMVar v (x,t)? If so, how would it be possible to make sure that the operation of reading the current time and writing the pair to the MVar is an "atomic" operation, in the sense that no thread switch can happen between the two? Would this require STM? Thanks again for sharing your wisdom with me :) Peter

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Telnet handshaking
by Martijn van Steenbergen 15 Feb '09

15 Feb '09

Hello, Are there any Haskell libraries out there that implement telnet? Bare sockets are a good approximation, but I'm looking for a library that understands the escape sequences specific to telnet. Thanks, Martijn.

2 2

Help with Menu item handlers in gtk2hs with glade
by Lakshmi Narasimhan 15 Feb '09

15 Feb '09

I designed a basic GUI with Menu bar and menu items in glade. I use the glade bindings in haskell to get the menu item using xmlGetWidget and castToMenuItem function. I set a *onActivateItem* handler for this menuitem. However this is not working. I saw the glade file and it has a signal function in the xml. I tried to use the same function name, but it doesn't work either. Any suggestions on what I might be doing wrong here and how to get menu item handlers working.

2 1

Delimited continuations: please comment
by Cristiano Paris 15 Feb '09

15 Feb '09

Hi, I'm experimenting with delimited continuations in the effort to understand how they work and when it's convenient to use them. Consider this piece of code (install the CC-delcont before running it): ---- {-# LANGUAGE NoMonomorphismRestriction #-} import Control.Monad.CC import Control.Monad.Trans -- why do I have to import this? data Monad m => Susp m a b = Stop | Susp (a -> m (Susp m a b)) job = reset $ \p -> let askMsg = shift p $ \k -> return $ Susp $ k . return in do x <- askMsg liftIO $ putStrLn $ "x was " ++ show x y <- askMsg liftIO $ putStrLn $ "y was " ++ show y return Stop scheduler j = do Susp nj <- j Susp nj <- nj "Hello!" nj "World!" return undefined main = runCCT $ scheduler job ---- which produces the output: ---- [paris@bagend haskell]$ runhaskell dc.hs x was "Hello!" y was "World!" [paris@bagend haskell]$ ---- The goal of this is to have a test-case implementation of the system call mechanism found in operating systems, like the one described by Oleg in (see page 3): http://okmij.org/ftp/papers/context-OS.pdf In effect, this is a bit different from the syscall service routine described by Oleg, as the scheduler function reacts in different ways for subsequent calls (the first time feeds "Hello!", the second one "World!", in a nice monad style). Yet, I liked the separation between the scheduler and the job, which are two completely different values and which I tried to keep. As this is (almost) my first time using delconts, could you provide feedback, comments, opinions about my piece of code and the topic in general (convenience, performances, alternatives and so on)? Thank you, Cristiano

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Feynman on Mathematics and Physics
by Daryoush Mehrtash 15 Feb '09

15 Feb '09

Just came across this 6 part video of Feynman lecture on relationship of mathematics and physics. http://www.youtube.com/watch?v=W1jIPzjFU_U&e Enjoy, Daryoush

1 0

A day with xmonad
by Colin Paul Adams 15 Feb '09

15 Feb '09

I have blogged my experience of trying out xmonad with Fedora 10 at http://colina.demon.co.uk/?q=node/534 . -- Colin Adams Preston Lancashire

1 0

ANNOUNCE: haha-0.1 - Animated ascii lambda
by Sebastiaan Visser 15 Feb '09

15 Feb '09

Always wanted to have an full-color rotating vector based ascii art lambda on your terminal? This is your chance, installing `haha' will do the trick! This is very minimal vector based ascii art library written just for fun. There is a sample program called `rotating-lambda' which does exactly what is says. Make sure your terminal window is at least 80x40 and supports the most basic ANSI escape sequences before trying the demo. -- Sebastiaan

3 2

Iteratee-based IO and lightweight monadic regions in the wild
by Artyom Shalkhakov 15 Feb '09

15 Feb '09

Hello, Is anybody planning to use these shiny new ways for doing IO? I'm also interested in a fair comparison of ByteString/Binary with iteratee-based IO. Has anybody done this? Regards, Artyom Shalkhakov.

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Re: [Haskell-beginners] Re: permuting a list
by Daniel Fischer 15 Feb '09

15 Feb '09

Am Samstag, 14. Februar 2009 16:37 schrieb Heinrich Apfelmus: > > That of course begs the question whether there is a faster but purely No, it didn't beg any question. (Sorry for being a humourless pedant here) > functional algorithm for generating random permutations without indexes > and arrays? > > The answer is a resounding "yes" and the main idea is that shuffling a > list is *essentially the same* as sorting a list; the minor difference > being that the former chooses a permutation at random while the latter > chooses a very particular permutation, namely the one that sorts the input. > > For the full exposition, see > > http://apfelmus.nfshost.com/random-permutations.html Excellent work, thanks. > > > Regards, > apfelmus Cheers, Daniel

2 1

Possible bug?
by Peter Verswyvelen 15 Feb '09

15 Feb '09

Could it be considered a bug when a function compiles fine without type signature, but when you add the type signature that GHCi reports with :type, it fails to compile? I am using functional dependencies, and I know these are not the best way to do it. Can all functional dependencies be completely replaced with associated types when using GHC 6.10.1? I tried once but I failed.

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space leak with 'concat' ?
by Henning Thielemann 15 Feb '09

15 Feb '09

$ ghc +RTS -M16m -c30 -RTS -e 'concat $ repeat "bla"' This breaks down after a while, also if I increase the memory restriction: ... ablablablablablablablablablablablablablablablablablablablablablaHeap exhausted; Current maximum heap size is 15998976 bytes (15 Mb); use `+RTS -M<size>' to increase it. Whereas this one works: $ ghc +RTS -M16m -c30 -RTS -e 'cycle "bla"' 'concat' seems to be the culprit. What's so bad about it?

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IO semantics and evaluation - summary
by Gregg Reynolds 15 Feb '09

15 Feb '09

Many thanks to everybody who tried to set me straight on the thread about IO monad and evaluation semantics. I've begun summarizing the info, and I believe I've come up with a much better way of explaining IO; just flip the semantic perspective, and think in terms of interpretations instead of actions. Voila! Oxymoron (values that perform actions) eliminated. See the "Computation considered harmful" and "Fixing Haskell IO" articles at http://syntax.wikidot.com/blog Naturally I would be grateful for any corrections/comments. Thanks, gregg

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Class Instances
by Cetin Sert 14 Feb '09

14 Feb '09

Hi, class Processor a where ready :: (forall b c. a → [b → c]) {- instance Processor (b → c) where ready = repeat instance Processor [b → c] where ready = id-} doSth :: (Show p, Processor p) ⇒ p → IO () doSth p = print p ------------------------------- Why can I not declare the above instances and always get: message.hs:229:10: Couldn't match expected type `b' against inferred type `b1' `b' is a rigid type variable bound by the instance declaration at message.hs:228:20 `b1' is a rigid type variable bound by the type signature for `ready' at message.hs:226:19 Expected type: b -> c Inferred type: b1 -> c1 In the expression: repeat In the definition of `ready': ready = repeat message.hs:229:10: Couldn't match expected type `c' against inferred type `c1' `c' is a rigid type variable bound by the instance declaration at message.hs:228:24 `c1' is a rigid type variable bound by the type signature for `ready' at message.hs:226:21 Expected type: b -> c Inferred type: b1 -> c1 In the expression: repeat In the definition of `ready': ready = repeat message.hs:232:10: Couldn't match expected type `b1' against inferred type `b' `b1' is a rigid type variable bound by the type signature for `ready' at message.hs:226:19 `b' is a rigid type variable bound by the instance declaration at message.hs:231:20 Expected type: [b1 -> c] Inferred type: [b -> c1] In the expression: id In the definition of `ready': ready = id message.hs:232:10: Couldn't match expected type `c1' against inferred type `c' `c1' is a rigid type variable bound by the type signature for `ready' at message.hs:226:21 `c' is a rigid type variable bound by the instance declaration at message.hs:231:24 Expected type: [b -> c1] Inferred type: [b1 -> c] In the expression: id In the definition of `ready': ready = id Is there a way around this? Regards, CS

2 4

Another point-free question (>>=, join, ap)
by Edsko de Vries 14 Feb '09

14 Feb '09

Hi, I can desugar do x' <- x f x' as x >>= \x -> f x' which is clearly the same as x >>= f However, now consider do x' <- x y' <- y f x' y' desugared, this is x >>= \x -> y >>= \y' -> f x' y' I can simplify the second half to x >>= \x -> y >>= f x' but now we are stuck. I feel it should be possible to write something like x ... y ... f or perhaps f ... x ... y the best I could come up with was join $ return f `ap` x `ap` y which is not terrible but quite as easy as I feel this should be. Any hints? Edsko

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Language popularity
by Henk-Jan van Tuyl 14 Feb '09

14 Feb '09

Yesterday I saw Haskell mentioned for the first time in a magazine, Bits & Chips. It is a magazine for professionals, about hardware and software; the article was about the domain specific language Cryptol from Galois. In the same issue of the magazine, there was an article saying that the company Tiobe has proclamed C the language of the year 2008, because of it's growth in popularity of almost two percent. The current statistics can befoud at [1]. The most popular functional language at the moment is LOGO [2], at the 15th place in the top 50 (from 22nd place a year ago). Haskell is at the 35th place (no indication of the score last year). A quick search in the Web Archive [3] reveals that Haskell was at the 41st place in the index in June 2007. My own research, using Google: Search Hits ------------------------------- Java programming 20.400.000 LOGO programming 14.600.000 Haskell programming 250.000 [1] http://www.tiobe.com/index.php/content/paperinfo/tpci/index.html [2] http://en.wikipedia.org/wiki/Logo_(programming_language) [3] http://web.archive.org/web/20070606231519/www.tiobe.com/?tiobe_index -- Regards, Henk-Jan van Tuyl -- http://functor.bamikanarie.com http://Van.Tuyl.eu/ --

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Changing version numbering schemes for HackageDB packages?
by Corey O'Connor 13 Feb '09

13 Feb '09

I released a new version of data-spacepart that resolved some of the issues with the previous release. One issue I had was the previous release used the version numbering scheme I use at work: [date].[release] Which does not appear to work as well as the traditional X.Y.Z release numbering scheme with Cabal. As part of the new release I changed the version numbering scheme. An *obviously* bad idea if I thought it through. Any [date].[release] style version number is greater than a X.Y.Z version number until X gets rather large. So what to do? Continue using the [date].[release] version numbering scheme? Or is there a way to coax HackageDB to ignore the old release? EG: The new release is: http://hackage.haskell.org/cgi-bin/hackage-scripts/package/data-spacepart-0… The old release is: http://hackage.haskell.org/cgi-bin/hackage-scripts/package/data-spacepart-2… The "latest" release according to HackageDB is: http://hackage.haskell.org/cgi-bin/hackage-scripts/package/data-spacepart Which points to the old release. Cheers, -Corey O'Connor

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How to deal with utf-8?
by Magicloud 13 Feb '09

13 Feb '09

Hi, I am using Text.CSV to read and using gtk2hs to display csv files using utf-8 encode. Well, it displays broken strings, seems like it cannot deal with utf-8. What should I do? Thanks.

3 2

Is using Data.Dynamic considered a no-go?
by Peter Verswyvelen 13 Feb '09

13 Feb '09

Haskell seems to have pretty strong support for dynamic casting using Data.Typeable and Data.Dynamic. All kinds of funky dynamic programming seems to be possible with these "hacks". Is this considered as being as bad as - say - unsafePerformIO? What kind of evil is lurking here? Cheers, Peter

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Re: [Haskell] Google Summer of Code 2009
by Jamie 13 Feb '09

13 Feb '09

What I would like to see is H.264 video codec in Haskell. H.264/MPEG-4 is getting very popular nowadays and it would be great to have encoder and decoder in haskell. Can use x264 (encoder) and ffmpeg (en/de coder) as a base to start with. Jamie

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Re: Type families not as useful over functions
by oleg＠okmij.org 13 Feb '09

13 Feb '09

John Ky wrote: > Is there a way to define type r to be all types except functions? Perhaps the following article How to write an instance for not-a-function http://okmij.org/ftp/Haskell/typecast.html#is-function-type answers your question. It shows several complete examples.

1 0

Is this related to continuations somehow?
by Andrew Wagner 13 Feb '09

13 Feb '09

So, I was reading a bit about continuations the other day, and, since I've been thinking about good ways of expressing chess strategies in Haskell, I thought that I'd play around a bit with something like continuations for game-playing strategies. The idea is that you have combinators that allow you full access to the strategies which remain to be applied. In this way, strategies can "activate" and "de-activate" other strategies. Here's a simple little toy app for Tic-Tac-Toe (Naughts and Crosses): http://codepad.org/nN9JsxFK You can run main on 'example', and see that it searches every line and fails. And, as you can see, it aborts after finding a win in example2. This would be easily extensible to say things like "if you've seen a blocking move, and you don't have a win, then play the blocking move", and of course the other deep intricacies of the game. My question is, is this, in fact, related to continuations somehow? Could continuations simplify it? Or am I doing something completely different?

1 0

Re: Google Summer of Code 2009
by Matthew Elder 13 Feb '09

13 Feb '09

> So in general, think hard about missing capabilities in Haskell: > > * tools > * libraries > * infrastructure > > that benefit the broadest number of Haskell users or developers. > > Another route is to identify a clear niche where Haskell could leap > ahead of the competition, with a small investment. > > -- Don > One capability I would love to see as a SoC project is the ability to generate javascript (a small subset) as an additional output target from ghc. Something like this was already done with yhc here<http://www.haskell.org/haskellwiki/Yhc/Javascript>but if it could be ported to GHC and cabalized, this would really be a boon to the web dev community; javascript is quickly becoming one of the most predominant languages used for GUI creation (mainly via the web). -- Need somewhere to put your code? http://patch-tag.com Want to build a webapp? http://happstack.com

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Re: [Haskell-cafe] Google Summer of Code 2009
by Andrzej Jaworski 13 Feb '09

13 Feb '09

> If you have ideas for student projects that you think would benefit the > Haskell community, now is the time to start discussing them on mailing Here is an idea that if done right might bootstrap Haskell real world applications with the help of greed and adrenaline:-) The ignition: (0) Bind Haskell to an automatic trading platform [API] (1) write real-time streamer and stock scanner. [PType] should offer more than was demonstrated in [F#] (2) Join [apps] from any angle (3) Consider a [DSL] for data analysis or write an [EasyLanguage] [API] http://www.interactivebrokers.com/en/p.php?f=programInterface&ib_entity=llc [F#] http://channel9.msdn.com/pdc2008/TL11/ [PType] http://research.nii.ac.jp/~hu/pub/aplas04-xu.pdf [apps] http://www.interactivebrokers.com/en/general/poll/ibconsultants.php?accept_… [DSL] http://www.cc.gatech.edu/~saswat/research/one.pdf [EasyLanguage] https://www.tradestation.com/support/books/pdf/EL_Essentials.pdf https://www.tradestation.com/support/books/pdf/EL_FunctionsAndReservedWords…

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Re: [Haskell] Google Summer of Code 2009
by Andrzej Jaworski 13 Feb '09

13 Feb '09

> If you have ideas for student projects that you think would benefit the > Haskell community, now is the time to start discussing them on mailing Here is an idea that if done right might bootstrap Haskell real world applications with the help of greed and adrenaline:-) The ignition: (0) Bind Haskell to an automatic trading platform [API] (1) write real-time streamer and stock scanner. [PType] should offer more than was demonstrated in [F#] (2) Join [apps] from any angle (3) Consider a [DSL] for data analysis or write an [EasyLanguage] [API] http://www.interactivebrokers.com/en/p.php?f=programInterface&ib_entity=llc [F#] http://channel9.msdn.com/pdc2008/TL11/ [PType] http://research.nii.ac.jp/~hu/pub/aplas04-xu.pdf [apps] http://www.interactivebrokers.com/en/general/poll/ibconsultants.php?accept_… [DSL] http://www.cc.gatech.edu/~saswat/research/one.pdf [EasyLanguage] https://www.tradestation.com/support/books/pdf/EL_Essentials.pdf https://www.tradestation.com/support/books/pdf/EL_FunctionsAndReservedWords…

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can't figure out a type
by John Lato 13 Feb '09

13 Feb '09

Hello, I'm working on some code like the following: > class Chunkable c el | c -> el where > cLength :: c -> Int > cHead :: c -> Maybe el I want to be able to map over this type, like this: > cMap :: Chunkable c' el' => (el -> el') -> c -> c' but this isn't quite right. c' shouldn't be any instance of Chunkable, it should be the same instance except parameterized over a different type. Another approach would be something like: class (Functor c) => Chunkable c el ... except that's not right either. I think c has the wrong kind to be a Functor instance. I expect there's something very basic I'm missing. Could anyone point in the proper direction of how to do this? Can this be expressed with associated types, perhaps? Thanks, John Lato

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Re: Graph library, was: Haskell.org GSoC
by Louis Wasserman 13 Feb '09

13 Feb '09

fgl uses pretty much the most beautiful way of abstracting graphs I've seen; a brief review: type Context a b -- a collected representation of a vertex's number, label, and all information on edges going into and out of that vertex match :: Graph gr => Node -> gr a b -> (Maybe (Context a b), gr a b) -- if a vertex is in the graph, extracts its adjacency information in a Context and returns the graph without that vertex (&) :: DynGraph gr => Context a b -> gr a b -> gr a b -- melds a vertex and its adjacency information into the graph I've been doing a huge amount of messing around with graph algorithms recently, and this inductive style of graph querying and modification make several algorithms far more intuitive than most imperative implementations I've seen. In addition, both of these lend themselves well to use in a State monad. Take the following code to extract the connected component of a vertex in an undirected graph: extractComponentM :: DynGraph gr => gr a b -> Node -> State (gr a b) (gr a b) extractComponentM partialComponent v = State (match v) >>= maybe (return partialComponent) expandContext where expandContext cxt = liftM (cxt &) (foldM extractComponentM partialComponent (neighbors' cxt)) extractComponent :: DynGraph gr => gr a b -- the initial graph -> Node -- the node whose component to find -> (gr a b, gr a b) -- the original graph without the component, and the component extractComponent g v = runState (extractComponentM empty v) g That's far more compact -- and intuitive to someone familiar with both fgl and the State monad -- than a standard connected-component finder in an imperative language, speaking as someone who wrote code along these lines imperatively for several years. (Fun fact: I've been working on a nice encapsulation of priority queues as monad transformers to make other common graph algorithms both efficient and make them this pretty to read, too. Any thoughts on that would be appreciated.) I strongly feel that fgl, though it could be improved in some areas, makes a better truly general graph abstraction than anything else I've seen. Areas that could specifically be improved include the stuff that uses LRTrees, and other stuff that isn't especially intuitive, elegant, or even efficient in fgl's implementation. Louis Wasserman wasserman.louis(a)gmail.com

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Re: Google Summer of Code 2009
by John Lato 13 Feb '09

13 Feb '09

Johan Tibell wrote: > On Thu, Feb 12, 2009 at 2:12 AM, Felipe Lessa <felipe.lessa(a)gmail.com> wrote: >> Do we already have enough information to turn >> http://okmij.org/ftp/Haskell/Iteratee/ into a nice, generic, cabalized >> package? I think Iteratees may prove themselves as useful as >> ByteStrings. > > I still haven't figured out what the "correct" definition of Iteratee > would look like. The Iteratee code that Oleg wrote seems to have the > structure of some kind of "two level" monad. I think that's the reason > for the frequent occurrences of >>== and liftI in the code. There > seems to be some things we yet have to discover about Iteratees. > I concur. I've recently been involved with several discussions on this topic, and there are some issues that remain. The "two level monad" part doesn't bother me, but I think the type should be slightly more abstract and I'm not sure of the best way to do so. IMO liftI is used more because of Oleg's particular style of coding than anything else. I don't think it need be common in user code, although it may be more efficient. I think that, if a GSOC project were to focus on Iteratees, it would need to look at issues like these. I can't judge as to whether this is an appropriate amount of work for GSOC, however simply packaging and cabal-izing Oleg's Iteratee work (or Johan's, or my own) is likely of too small a scope. John Lato

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CFP: 5th Haskell Hackathon, April 17-19, Utrecht
by Sean Leather 12 Feb '09

12 Feb '09

Call for participation: ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ The 5th Haskell Hackathon April 17 - 19, 2009 Utrecht, The Netherlands ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ The Haskell Hackathon is a collaborative coding festival with a simple focus: build and improve Haskell libraries, tools, and infrastructure. Of course, we have fun doing it, too! All hackers are welcome to visit and work on various Haskell-related projects. You decide what you want to work on! Lists of projects and concerted efforts are collected on the Hac5 website. == General Information == Information about the projects as well as practicalities such as traveling, accommodation, etc. is available from the Hac5 website: http://haskell.org/haskellwiki/Hac5 == Registration == Please register by following the instructions on the registration page: http://haskell.org/haskellwiki/Hac5/Register Once you've registered, please add your info to the attendees self-organizing page: http://haskell.org/haskellwiki/Hac5/Attendees If you are looking to share costs or want to meet up prior to the Hackathon with other attendees, feel free to organize via the Attendees wiki page or join the Hackathon IRC channel on freenode: #haskell-hac5 == 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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TASE 2009 - CALL FOR PAPERS
by CRACIUN F. 12 Feb '09

12 Feb '09

TASE 2009 - Final CALL FOR PAPERS ****************************************** * 3rd IEEE International Symposium on * Theoretical Aspects of Software Engineering * (TASE 2009) * 29-31 July 2009, Tianjin, China * http://www.dur.ac.uk/ieee.tase2009 * * For more information email: IEEE.TASE2009(a)durham.ac.uk ********************************************************** Large scale software systems and the Internet are of growing concern to academia and industry. This poses new challenges to the various aspects of software engineering, for instance, the reliability of software development, web-oriented software architecture and aspect and object-orientation techniques. As a result, new concepts and methodologies are required to enhance the development of software engineering from theoretical aspects. TASE 2009 is a forum for researchers from academia, industry and government to present ideas, results, and ongoing research on theoretical advances in software engineering. TASE 2009 is the third in a series of conference, sponsored by IEEE CS and IFIP. The first TASE conference was held in Shanghai, China, in June 2007. The second TASE conference was held in Nanjing, China, in June 2008. Topics of Interest: Authors are invited to submit high quality technical papers describing original and unpublished work in all theoretical aspects of software engineering. Topics of interest include, but are not limited to: * Requirements Engineering * Specification and Verification * Program Analysis * Software Testing * Model-Driven Engineering * Software Architectures and Design * Aspect and Object Orientation * Embedded and Real-Time Systems * Software Processes and Workflows * Component-Based Software Engineering * Software Safety, Security and Reliability * Reverse Engineering and Software Maintenance * Service-Oriented Computing * Semantic Web and Web Services * Type System and Theory * Program Logics and Calculus * Dependable Concurrency * Software Model Checking Program Co-Chairs ----------------- Wei-Ngan Chin (National Univ. of Singapore, Singapore) Shengchao Qin (Durham University, UK) Program Committee ----------------- Bernhard Aichernig (Graz University of Technology, Austria) Stefan Andrei (Lamar University, USA) Keijiro Araki (Kyushu University, Japan) Farhad Arbab (CWI and Leiden University, Netherlands) Jonathan Bowen (King's College London, UK) Michael Butler (University of Southampton, UK) Juan Chen (Microsoft Research, USA) Tyng-Ruey Chuang (Academica Sinica, Taiwan) Jim Davies (University of Oxford, UK) Zhenhua Duan (Xidian University, China) Xinyu Feng (Toyota Technological Inst. at Chicago, USA) Dieter Gollmann (Hamburg University of Technology, Germany) Tetsuo Ida (University of Tsukuba, Japan) Radu Iosif (Verimag, CNRS, France) Xuandong Li (Nanjing University, China) Kung-Kiu Lau (University of Manchester, UK) Shaoying Liu (Hosei University, Japan) Dorel Lucanu (University of Iasi, Romania) Tom Maibaum (McMaster University, Canada) Darko Marinov (Univ. of Illinois at Urbana-Champaign, USA) Hong Mei (Peking University, China) Huaikou Miao (Shanghai University, China) Peter Mueller (ETH Zurich, Switzerland) Viet Ha Nguyen (Vietnam National University, Vietnam) Sungwoo Park (Pohang Univ. of Science and Technology, Korea) Corneliu Popeea (MPI-SWS, Germany) Geguang Pu (East China Normal University, China) Zongyan Qiu (Peking University, China) Volker Stolz (UNU/IIST, Macau) Jing Sun (University of Auckland, New Zealand) Jun Sun (National Univ. of Singapore, Singapore) Kenji Taguchi (National Institute of Informatics, Japan) Yih-Kuen Tsay (National Taiwan University, Taiwan) Elizabeth Vidal (San Agustin National University, Peru) Ji Wang (National University of Defense Technology, China) Linzhang Wang (Nanjing University, China) Xianbing Wang (Wuhan University, China) Wang Yi (Uppsala University, Sweden) Jim Woodcock (University of York, UK) Hongyu Zhang (Tsinghua University, China) Jian Zhang (Chinese Academy of Sciences, China) Jianjun Zhao (Shanghai Jiao Tong University, China) Hong Zhu (Oxford Brookes University, UK) Huibiao Zhu (East China Normal University, China) Important Dates: February 20, 2009: Title and abstract submission deadline February 27, 2009: Paper submission deadline April 20, 2009: Acceptance/rejection notification May 11, 2009: Camera-ready version due July 29 - 31, 2009: TASE 2009

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HXT + Segmentation Fault
by rodrigo.bonifacio 12 Feb '09

12 Feb '09

Hi all, I'm trying to parse some XML files using HXT. However, even the examples available on the twiki fail. I guess that the problem is related to some library version, but I'm not sure. The error reported is: "Segmentation fault". Thanks in advance. Compiling with GHC-6.8.3 running on MAC/OS Tiger Packages: Cabal-1.2.4.0, HUnit-1.2.0.0, QuickCheck-1.1.0.0, array-0.1.0.0, base-3.0.2.0, bitset-0.6, bytestring-0.9.0.1.1, cgi-3001.1.6.0, containers-0.1.0.2, curl-1.3.3, directory-1.0.0.1, fgl-5.4.2.0, filepath-1.1.0.0, funsat-0.5.1, ghc-6.8.3, haskell-src-1.0.1.2, haskell98-1.0.1.0, hpc-0.5.0.1, html-1.0.1.1, hxt-8.1.0, (mtl-1.1.0.1), mtl-1.1.0.2, (network-2.2.0.0), network-2.2.0.1, old-locale-1.0.0.0, old-time-1.0.0.0, packedstring-0.1.0.0, parallel-1.0.0.1, parse-dimacs-1.2, parsec-2.1.0.1, parsec-3.0.0, pcre-light-0.3.1, pretty-1.0.0.0, process-1.0.0.1, random-1.0.0.0, readline-1.0.1.0, regex-base-0.72.0.1, regex-compat-0.71.0.1, regex-posix-0.72.0.2, rts-1.0, stm-2.1.1.1, tagsoup-0.6, template-haskell-2.2.0.0, time-1.1.2.1, unix-2.3.0.1, xhtml-3000.2.0.0

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Re: [Haskell] Google Summer of Code
by John Lato 12 Feb '09

12 Feb '09

Hi Bulat, I know you've talked about performance in the past, and I don't want to start a huge argument, but do you have recent data to back this up? IIRC you're using ghc 6.6, yes? I haven't looked at H.264 (and I realize it's compressed, so the situation is different from my work), however ghc can generate very fast code for binary I/O. Check out (shameless self-promotion) http://johnlato.blogspot.com for my recent writeup on creating a high-performance, pure-Haskell, Iteratee-based WAVE file reader. Sincerely, John Lato > > anyway it's impossible due to slow code generated by ghc > > > -- > Best regards, > Bulat mailto:Bulat.Ziganshin@gmail.com >

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Writing a generic event handler
by John Ky 12 Feb '09

12 Feb '09

Hi Haskell Cafe, I'm interested in writing some events and event handlers in Haskell. I already have a Loop data structure, and I intend to use it for this purpose: -- Create event tEvent <- newLoop (return ()) -- Register event handlers tHandler1 <- newLoop (putStrLn "Handler1") tHandler2 <- newLoop (putStrLn "Handler2") splice tEvent tHandler1 splice tEvent tHandler2 -- Fire event action <- doLoop tEvent action doLoop :: Monad m => TVar (Loop (m ())) -> STM (m ()) doLoop tLoop = do aLoop <- readAsList tLoop return $ sequence_ aLoop My question is: Is it possible to write a generic doLoop that works over arbitrary functions? For instance the following code wouldn't work because the event provides one argument and the handler takes one argument: -- Create event tEvent <- newLoop (\x -> return ()) -- Register event handlers tHandler1 <- newLoop (\x -> putStrLn ("Handler1" ++ show x)) tHandler2 <- newLoop (\x -> putStrLn ("Handler2" ++ show x)) splice tEvent tHandler1 splice tEvent tHandler2 -- Fire event action <- doLoop tEvent action 123 Thanks, -John Full source code for Loop type: module Fx.STM.Loop where import Control.Monad import Fx.STM.Util import GHC.Conc import System.IO.Unsafe -- Transactional loop. A loop is a circular link list. data Loop a = ItemLink { item :: a , prev :: TVar (Loop a) , next :: TVar (Loop a) } -- Create a new empty transactional loop. newLoop :: a -> STM (TVar (Loop a)) newLoop item = do tLoop <- newTVar undefined writeTVar tLoop (ItemLink item tLoop tLoop) return tLoop -- Splice two transactional loops. This will join two loops if they were -- originally separate, or split a single loop if the links were originally -- part of the same loop. No change occurs if the two links are identical. splice :: TVar (Loop a) -> TVar (Loop a) -> STM () splice tLink0 tLink1 = do aLink0 <- readTVar tLink0 aLink1 <- readTVar tLink1 let tLink0Prev = prev aLink0 let tLink1Prev = prev aLink1 writeTVar tLink0 aLink0 { prev = tLink1Prev } writeTVar tLink1 aLink1 { prev = tLink0Prev } aLink0Prev <- readTVar tLink0Prev aLink1Prev <- readTVar tLink1Prev writeTVar tLink0Prev aLink0Prev { next = tLink1 } writeTVar tLink1Prev aLink1Prev { next = tLink0 } return () -- Unlink a single link from a transactional loop. unlink :: TVar (Loop a) -> STM () unlink tLink = do (ItemLink item tLinkPrev tLinkNext) <- readTVar tLink aLinkPrev <- readTVar tLinkPrev writeTVar tLinkPrev aLinkPrev { next = tLinkNext } aLinkNext <- readTVar tLinkNext writeTVar tLinkNext aLinkNext { prev = tLinkPrev } writeTVar tLink (ItemLink item tLink tLink) return () -- Read the length of the loop. readLength :: TVar (Loop a) -> STM Int readLength tLink = do list <- readAsList tLink return $ length list readLinks :: TVar (Loop a) -> STM [TVar (Loop a)] readLinks tLink = readLinksUntil tLink tLink readLinksUntil :: TVar (Loop a) -> TVar (Loop a) -> STM [TVar (Loop a)] readLinksUntil tLink tLinkEnd = do (ItemLink _ tLinkPrev tLinkNext) <- readTVar tLink return [] if tLinkNext == tLinkEnd then return [tLink] else do tail <- readLinksUntil tLinkNext tLinkEnd return $ tLink:tail -- Read the elements of the loop as a list starting from tLink. readAsList :: TVar (Loop a) -> STM [a] readAsList tLink = readAsListUntil tLink tLink -- Read the elements of the loop as a list starting from tLink -- and terminating non-inclusively at tLinkEnd. readAsListUntil :: TVar (Loop a) -> TVar (Loop a) -> STM [a] readAsListUntil tLink tLinkEnd = do (ItemLink item tLinkPrev tLinkNext) <- readTVar tLink if tLinkNext == tLinkEnd then return [item] else do tail <- readAsListUntil tLinkNext tLinkEnd return $ item:tail -- Create a new loop from a list. newFromList :: [a] -> STM (TVar (Loop a)) newFromList [item] = newLoop item newFromList (item:items) = do tLink <- newLoop item tLinkRest <- newFromList items splice tLink tLinkRest return tLink doLoop :: Monad m => TVar (Loop (m ())) -> STM (m ()) doLoop tLoop = do aLoop <- readAsList tLoop return $ sequence_ aLoop

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The Haskell re-branding exercise
by Paul Johnson 12 Feb '09

12 Feb '09

16 32

(Off-topic) CUDA
by Andrew Coppin 12 Feb '09

12 Feb '09

And so, inspired by the marketing litrature, I just spent £££ on a very expensive new GPU that supports CUDA. The only problem is... I can't seem to get any software to use it. Does anybody know how to make this stuff actually work? (Also... Haskell on the GPU. It's been talked about for years, but will it ever actually happen?)

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Re: [Haskell] Google Summer of Code 2009
by Benja Fallenstein 12 Feb '09

12 Feb '09

Hi all, On Tue, Feb 10, 2009 at 3:26 PM, Malcolm Wallace <Malcolm.Wallace(a)cs.york.ac.uk> wrote: > If you have ideas for student projects that you think would benefit the > Haskell community, now is the time to start discussing them on mailing > lists of your choice. We especially encourage students to communicate > with the wider community (...) I have a project that I've tinkered with doing for a while, but have never actually gotten around to, and I'm wondering whether there would be interest from the community in using it. Actually, two related projects: (1) A library for working with hashable data structures, including a hashable finger tree with sequence, map and set types based on it. Surprisingly, there does not seem to be much library support for working with hashes in Haskell beyond the bare-bones, imperative Data.HashTable, even though pure functional programming and hashes Go Well Together. I'm pulling these bounds out of my intuition, but hashable collections should give amortized O(1) comparison for equality and amortized O(log n) comparison for order, and you could use them as set values and map keys without efficiency blowing up in your face. The library should probably also include support for hashtables based on the various kinds of pure and monad-encapsulated arrays we have in Haskell, and maybe include support for interning values; unsafePerformIO, weak references and friends were originally introduced with the intention to support interned values in a way that programmers can tailor to their own needs, but it would be nice to have some default library support. (2) Based on this, a serialization library that would recognize if it has already written a particular value from memory to the same file, and write a pointer to the first occurrence of that value instead of serializing the actual value again, making it efficient to serialize versioned data structures. The idea is to make something similar to HAppS' MACID, but where MACID serializes the different kinds of updates that you can do for your data, this library would simply do the update in memory, then serialize the whole updated value to disk, but actually *writing* only the *new* parts of the data structure. (I think this could be simpler to use, and would be in a sense more elegant and 'haskelly' than the MACID approach. On the other hand, we already *have* MACID.) So, anybody out there who looks at these things and thinks "this could be useful"? :-) Thanks, - Benja

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ANN : happs-tutorial 0.7
by Creighton Hogg 12 Feb '09

12 Feb '09

Hello, I'm pleased to announce the release of happs-tutorial 0.7 on Hackage. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/happs-tutorial This is the first release of happs-tutorial built against the new Happstack project. Not much has changed in content since the last release except a few minor cleanups & a little bit of reorganization. The 0.8 release will consist of more extensive additions, with the primary focus being a walk through of multimaster. I've taken over the development of the tutorial from Thomas Hartman, who has already put a rather massive effort into making HAppS/Happstack more accessible. Please feel free to e-mail me with any comments, errata, or threats of bodily harm. Cheers, Creighton Hogg

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GMP on Mac OS X linked statically by default
by David Leimbach 12 Feb '09

12 Feb '09

Was there a reason for this? If so, it'd be nice if the package that was build explained why... otherwise it feels kind of arbitrary, and would be nice if there was documentation available to make it link dynamically in case someone didn't want to LGPL their program. Anyone know the steps to make it link dynamically? Dave

2 2