- Haskell-Cafe - Haskell.org

Haskell Weekly News: Issue 247
by Daniel Santa Cruz 10 Oct '12

10 Oct '12

Welcome to issue 247 of the HWN, an issue covering crowd-sourced bits of information about Haskell from around the web. This issue covers the week of September 30 to October 6, 2012. Quotes of the Week * monochrom: 98% of people lack the property that they're members of the other 2% * ManuelChakravarty: We used to say, "Learn Haskell, it makes you a better Java programmer." Now we say, "Learn Agda, it makes you a better Haskell programmer." * Cale: data Nature a = Rock a | Stream (Nature a) | Bush (Nature a) (Nature a) * rwbarton: type inference is supposed to be the compiler's job, not the reader's job * ShelSilverstein: Lazy Lazy Lazy Lazy Lazy Lazy Jane. She wants a drink of water so she waits and waits and waits and waits and waits for it to rain. * Cale: We're up to 950 users which apparently means that any beginner question is now answered by 4 people. * YayMe: When I first started playing with haskell I could barely sleep for 2 days because of how confused it made me * EvanLaforge: Then we can tell them about how we had to walk in the snow both ways uphill fighting space leaks with bare hands while they just have to read about it. * nand: every time I try arguing semantics, I come to the conclusion that every word is the exact same thing and refers to anything * bfig: i have been reborn as a haskell programer after finding Debug.Trace Top Reddit Stories * Factorization diagrams Domain: mathlesstraveled.com, Score: 82, Comments: 11 On Reddit: [1] http://goo.gl/Jrk0M Original: [2] http://goo.gl/kXkiV * The New Cloud Haskell Domain: well-typed.com, Score: 73, Comments: 21 On Reddit: [3] http://goo.gl/hcRJJ Original: [4] http://goo.gl/0oWfj * GHC HEAD now features Agda-like Holes Domain: hpaste.org, Score: 66, Comments: 43 On Reddit: [5] http://goo.gl/J8bIu Original: [6] http://goo.gl/297jv * FizzBuzz Revisited using Monoids Domain: dave.fayr.am, Score: 55, Comments: 68 On Reddit: [7] http://goo.gl/nqIsE Original: [8] http://goo.gl/995Zq * Which advanced Haskell topics interest you? (Survey) Domain: docs.google.com, Score: 39, Comments: 12 On Reddit: [9] http://goo.gl/Djy63 Original: [10] http://goo.gl/K5pRm * A fast new SipHash implementation Domain: serpentine.com, Score: 34, Comments: 8 On Reddit: [11] http://goo.gl/ZGSxD Original: [12] http://goo.gl/GiD3F * Communication Patterns in Cloud Haskell (Part 1): master-slave, work-stealing and work-pushing Domain: well-typed.com, Score: 33, Comments: 10 On Reddit: [13] http://goo.gl/IRr8s Original: [14] http://goo.gl/6vUyY * Category and lenses Domain: web.jaguarpaw.co.uk, Score: 28, Comments: 2 On Reddit: [15] http://goo.gl/r6Ajl Original: [16] http://goo.gl/VwtQP * Yesod Tutorial 1 at FP Complete Domain: fpcomplete.com, Score: 28, Comments: 15 On Reddit: [17] http://goo.gl/wnUkh Original: [18] http://goo.gl/kvCMu * The usual type signatures for foldl and foldr are confusing Domain: self.haskell, Score: 27, Comments: 63 On Reddit: [19] http://goo.gl/XGHq8 Original: [20] http://goo.gl/XGHq8 * Haskell Symposium 2012 (with youtube links) Domain: haskell.org, Score: 27, Comments: 1 On Reddit: [21] http://goo.gl/7Dz89 Original: [22] http://goo.gl/I1XXa * Solving a Partial Differential Equation Comonadically Domain: idontgetoutmuch.wordpress.com, Score: 27, Comments: 2 On Reddit: [23] http://goo.gl/Qxwjq Original: [24] http://goo.gl/lBm12 * Need a math related Haskell project suggestion Domain: self.haskell, Score: 22, Comments: 20 On Reddit: [25] http://goo.gl/o3lYl Original: [26] http://goo.gl/o3lYl * Algebraically Interpreting Polymorphism Domain: stackoverflow.com, Score: 21, Comments: 12 On Reddit: [27] http://goo.gl/3SCA9 Original: [28] http://goo.gl/wC6rm Top StackOverflow Questions * Why is seq bad? votes: 18, answers: 1 Read on SO: [29] http://goo.gl/IwVYS * Can I speed up this Haskell algorithm? votes: 18, answers: 3 Read on SO: [30] http://goo.gl/PSVUB * Is there a library that uses ConstraintKinds to generalize all the base type classes to allow constraints? votes: 15, answers: 1 Read on SO: [31] http://goo.gl/MfTMC * What are skolems? votes: 15, answers: 2 Read on SO: [32] http://goo.gl/qEnEI * What does it usually mean when a Haskell record accessor leads with an underscore? votes: 14, answers: 3 Read on SO: [33] http://goo.gl/3YD1n * How does this piece of obfuscated Haskell code work? votes: 12, answers: 1 Read on SO: [34] http://goo.gl/rLtFG * How to implement a game loop in reactive-banana? votes: 12, answers: 1 Read on SO: [35] http://goo.gl/nx3zR * How do Haskell currying and pattern matching work together? votes: 9, answers: 1 Read on SO: [36] http://goo.gl/jBCz0 * How does the constraints package work? votes: 9, answers: 1 Read on SO: [37] http://goo.gl/mb2na * In haskell, What does :+: mean in data type definition? votes: 9, answers: 1 Read on SO: [38] http://goo.gl/kv0Yn * Monad m => a -> [a -> m a] -> m a votes: 9, answers: 1 Read on SO: [39] http://goo.gl/ns8FG * How to decorate a Tree in Haskell votes: 7, answers: 2 Read on SO: [40] http://goo.gl/kBn66 Until next time, Daniel Santa Cruz References 1. http://mathlesstraveled.com/2012/10/05/factorization-diagrams/ 2. http://www.reddit.com/r/haskell/comments/1117h2/factorization_diagrams/ 3. http://www.well-typed.com/blog/70 4. http://www.reddit.com/r/haskell/comments/10y089/the_new_cloud_haskell/ 5. http://hpaste.org/75668 6. http://www.reddit.com/r/haskell/comments/10u7xr/ghc_head_now_features_agdal… 7. http://dave.fayr.am/posts/2012-10-4-finding-fizzbuzz.html 8. http://www.reddit.com/r/haskell/comments/10zlyy/fizzbuzz_revisited_using_mo… 9. https://docs.google.com/spreadsheet/viewform?formkey=dE1QZFNRLTFMdkllYWIyR2… 10. http://www.reddit.com/r/haskell/comments/10r2yk/which_advanced_haskell_topi… 11. http://www.serpentine.com/blog/2012/10/02/a-fast-new-siphash-implementation… 12. http://www.reddit.com/r/haskell/comments/10u381/a_fast_new_siphash_implemen… 13. http://www.well-typed.com/blog/71 14. http://www.reddit.com/r/haskell/comments/10zo6t/communication_patterns_in_c… 15. http://web.jaguarpaw.co.uk/~tom/blog/2012/09/30/category-and-lenses.html 16. http://www.reddit.com/r/haskell/comments/10ph2c/category_and_lenses/ 17. http://fpcomplete.com/yesod-tutorial-1-my-first-web-site/ 18. http://www.reddit.com/r/haskell/comments/10seol/yesod_tutorial_1_at_fp_comp… 19. http://www.reddit.com/r/haskell/comments/10q2ls/the_usual_type_signatures_f… 20. http://www.reddit.com/r/haskell/comments/10q2ls/the_usual_type_signatures_f… 21. http://www.haskell.org/haskell-symposium/2012/index.html 22. http://www.reddit.com/r/haskell/comments/10xafx/haskell_symposium_2012_with… 23. http://idontgetoutmuch.wordpress.com/2012/04/01/solving-a-partial-different… 24. http://www.reddit.com/r/haskell/comments/10zy51/solving_a_partial_different… 25. http://www.reddit.com/r/haskell/comments/10zf9c/need_a_math_related_haskell… 26. http://www.reddit.com/r/haskell/comments/10zf9c/need_a_math_related_haskell… 27. http://stackoverflow.com/questions/10453558/algebraically-interpreting-poly… 28. http://www.reddit.com/r/haskell/comments/10rt4y/algebraically_interpreting_… 29. http://stackoverflow.com/questions/12687392/why-is-seq-bad 30. http://stackoverflow.com/questions/12730503/can-i-speed-up-this-haskell-alg… 31. http://stackoverflow.com/questions/12697348/is-there-a-library-that-uses-co… 32. http://stackoverflow.com/questions/12719435/what-are-skolems 33. http://stackoverflow.com/questions/12746229/what-does-it-usually-mean-when-… 34. http://stackoverflow.com/questions/12659951/how-does-this-piece-of-obfuscat… 35. http://stackoverflow.com/questions/12685430/how-to-implement-a-game-loop-in… 36. http://stackoverflow.com/questions/12658936/how-do-haskell-currying-and-pat… 37. http://stackoverflow.com/questions/12728159/how-does-the-constraints-packag… 38. http://stackoverflow.com/questions/12733171/in-haskell-what-does-mean-in-da… 39. http://stackoverflow.com/questions/12755655/monad-m-a-a-m-a-m-a 40. http://stackoverflow.com/questions/12658443/how-to-decorate-a-tree-in-haske…

1 0

ANNOUNCE: luachunk-0.1
by Anupam Jain 10 Oct '12

10 Oct '12

Hi all, I just released luachunk-0.1 on Hackage (http://github.com/ajnsit/luachunk) Luachunk is a small library to read and write Lua 5.1 bytecode chunks. It is modeled after ChunkSpy.lua (http://luaforge.net/projects/chunkspy/) though the code is written from scratch. A pretty listing printer for lua bytecode is in the works. I used LuaChunk in a project to generate Lua bytecode via a DSL, and then interpret it with HsLua. I hope to release the DSL itself sometime in the future. -- Anupam

2 3

Re: [Haskell-cafe] CPS Streams
by jeff p 10 Oct '12

10 Oct '12

Here is an haste of the original message since it seems like the formatting was lost. http://hpaste.org/76082 sorry about that, Jeff

1 0

ANN: Groundhog 0.2 - now with database index support
by Boris Lykah 10 Oct '12

10 Oct '12

I am excited to announce a new release of Groundhog 0.2 - a library for high-level database access. * http://hackage.haskell.org/package/groundhog * http://hackage.haskell.org/package/groundhog-th * http://hackage.haskell.org/package/groundhog-postgresql * http://hackage.haskell.org/package/groundhog-sqlite * http://github.com/lykahb/groundhog Since the last release I got a lot of positive feedback which showed that many people are interested in the project. I want to thank them for their encouragement and constructive suggestions that helped me to adjust the development priorities. To help keeping track of the Groundhog project development, the most important changes of a release are now logged to the HISTORY files in repository. Along with many small improvements and refactorings, this major release has two novel features: * Support for database unique indexes. Now the unique keys can be created with either constraints or indexes. In the API the difference affects only the code generation settings which have a new field * Customizing column type name for schema migration. This contributes to compatibility with already existing databases and allows utilization of the database-specific features like limiting string size or number precision. See example at https://github.com/lykahb/groundhog/blob/master/examples/dbSpecificTypes.hs Package groundhog-th has improved error reporting. Now it precisely reports the line and position where parsing of the YAML-based settings failed. This required a patch in the yaml dependency, so other yaml package users have this fix as well. The full description of the YAML settings format is available at http://hackage.haskell.org/packages/archive/groundhog-th/0.2.0/doc/html/Dat… I appreciate your feedback.

1 0

Image processing using Repa
by Janek S. 10 Oct '12

10 Oct '12

I'm playing a bit with Repa library and its DevIL bindings. I tried to modify one of the examples from tutorial on HaskellWiki. I want to load an image, rotate it and save it to disk. I managed to write something like this: import Foreign.Ptr import System.Environment import Data.Array.Repa as R hiding ((++)) import qualified Data.Array.Repa.Repr.ForeignPtr as RFP import Data.Array.Repa.IO.DevIL main = do [f] <- getArgs (RGB v) <- runIL $ readImage f RFP.computeIntoP (RFP.toForeignPtr v) (rot180 v) runIL $ writeImage ("flip-"++f) (RGB v) return () rot180 g = backpermute e flop g where e@(Z :. x :. y :. _) = extent g flop (Z :. i :. j :. k) = (Z :. x - i - 1 :. y - j - 1 :. k) This is obviously wrong, because the foreign pointer used as a data source is at the same time used as destination, so the data gets overwritten before it is used. Does this mean that I have to allocate foreign memory buffers on my own? If so, than it feels kind of painfull to go through the hassle of allocating foreign pointers, converting between many different representations and so on. Am I doing something wrong and if not is there a more painless way of working with images and repa in Haskell? Jan

2 2

CPS Streams
by jeff p 10 Oct '12

10 Oct '12

{- I've always thought that the essence of iteratees is just CPS (I have no desire to argue the merits of this statement, and I preemptively concede to anyone who disagrees with this), so I decided to experiment with a direct CPS version of streams. The resulting code turned out nicer than I'd expected, so I'm writing about it in case anyone else finds it interesting or instructive (it seems to me that Haskellers tend to shy away from CPS). I'm pretty sure this technique is not new, but a cursory search didn't turn up quite the same thing. I'd be interested in any pointers to this somewhere else, as well any general feedback about this. -} {-# LANGUAGE ScopedTypeVariables, TupleSections #-} import Control.Applicative hiding (empty) import Control.Arrow import Control.Exception import Control.Monad (unless) import Data.Attoparsec.Text import Data.Text import Data.Text.IO import Prelude hiding (catch, getLine, lines, length, null, putStrLn, readFile, splitAt) import qualified Prelude as P import System.IO (IOMode(..), hClose, hIsClosed, withFile) import System.IO.Error hiding (catch) {- A stream is a function which takes a stream continuation and gives a result. A stream continuation takes an input (the next element of the stream) and the rest of the stream and returns a result. If the continuation is given Nothing, then the stream it was given to is done. This intuition can be realized with the following type which will let us do compositional incremental input processing in a direct functional style. -} newtype Stream0CPS a r = Stream0CPS { stream0CPS :: Maybe (a, Stream0 a r) -> r } type Stream0 a r = Stream0CPS a r -> r -- Stream the lines of a file. -- The finally statement is not necessary, but it lets us observe when files are closed. fileStream0 :: FilePath -> Stream0 Text (IO r) fileStream0 fileName k = withFile fileName ReadMode $ \h -> do P.putStrLn $ "___opening "++fileName finally (go h k) (do isClosed <- hIsClosed h unless isClosed $ hClose h >> P.putStrLn ("___finally closing "++fileName) ) where go h k = stream0CPS k . fmap (, go h) =<< safeNextLine h safeNextLine h = catch (Just <$> hGetLine h) $ \e -> if isEOFError e then hClose h >> P.putStrLn ("___EOF closing "++fileName) >> return Nothing else ioError e -- Print elements of a stream. printStream0 :: Show a => Stream0 a (IO ()) -> IO () printStream0 xs = xs $ Stream0CPS $ maybe (P.putStrLn "NOTHING") (\(x, xs') -> P.putStrLn (show x) >> printStream0 xs') -- Append two streams together. appendStream0 :: Stream0 a r -> Stream0 a r -> Stream0 a r appendStream0 xs ys k = xs $ Stream0CPS cont where cont Nothing = ys k cont (Just (v, xs')) = stream0CPS k $ Just (v, appendStream0 xs' ys) -- Create a stream with only the first n elements of a given stream. takeStream0 :: Int -> Stream0 a r -> Stream0 a r takeStream0 n xs k | n > 0 = xs $ Stream0CPS $ stream0CPS k . fmap (id *** takeStream0 (n - 1)) | otherwise = stream0CPS k Nothing {- Suppose we have a file named testWords with the words "one" through "five" on separate lines, and a file called testNums with the numbers 1 though 5 on separate lines. Then we can do the following: *Main> printStream0 $ appendStream0 (fileStream0 "testWords") (fileStream0 "testNums") ___opening testWords "one" "two" "three" "four" "five" ___EOF closing testWords ___opening testNums "1" "2" "3" "4" "5" ___EOF closing testNums NOTHING which is as expected, however, we also see: *Main> printStream0 $ appendStream0 (takeStream0 3 $ fileStream0 "testWords") (fileStream0 "testNums") ___opening testWords "one" "two" "three" ___opening testNums "1" "2" "3" "4" "5" ___EOF closing testNums NOTHING ___finally closing testWords which is not ideal since we'd like testWords to be closed as soon as we're done with it. In order to accomplish this, we have to add a way to tell a stream to close down. This is easily accomplished by giving an extra argument to streams, which tells them what to do. While below we have only allowed for streams to continue and stop, there is no reason the StreamAction type, and the stream functions, couldn't be expanded to allow for dynamically changing stream behavior, e.g. a variable sized chunker, or a tunable filter. -} newtype StreamCPS a r = StreamCPS { streamCPS :: Maybe (a, Stream a r) -> r } type Stream a r = StreamAction -> StreamCPS a r -> r data StreamAction = Continue | Stop {- Now we just have to modify the previous functions to explain what to do when asked to stop. -} fileStream :: FilePath -> Stream Text (IO r) fileStream fileName action k = withFile fileName ReadMode $ \h -> do P.putStrLn $ "___opening "++fileName finally (go h action k) (do isClosed <- hIsClosed h unless isClosed $ hClose h >> P.putStrLn ("___finally closing "++fileName) ) where go h Continue k = streamCPS k . fmap (, go h) =<< safeNextLine h go h Stop k = hClose h >> P.putStrLn ("___Stop closing "++fileName) >> streamCPS k Nothing safeNextLine h = catch (Just <$> hGetLine h) $ \e -> if isEOFError e then hClose h >> P.putStrLn ("___EOF closing "++fileName) >> return Nothing else ioError e printStream :: Show a => Stream a (IO ()) -> IO () printStream xs = xs Continue $ StreamCPS $ maybe (P.putStrLn "NOTHING") (\(x, xs') -> P.putStrLn (show x) >> printStream xs') -- In order to stop appendStream xs ys, stop xs then stop ys appendStream :: Stream a r -> Stream a r -> Stream a r appendStream xs ys Stop k = xs Stop $ StreamCPS $ \_ -> ys Stop k appendStream xs ys Continue k = xs Continue $ StreamCPS cont where cont Nothing = ys Continue k cont (Just (v, xs')) = streamCPS k $ Just (v, appendStream xs' ys) takeStream :: Int -> Stream a r -> Stream a r takeStream _ xs Stop k = xs Stop k takeStream n xs Continue k | n > 0 = xs Continue $ StreamCPS $ streamCPS k . fmap (id *** takeStream (n - 1)) | otherwise = xs Stop k {- Now we get the desired behavior: *Main> printStream $ appendStream (takeStream 3 $ fileStream "testWords") (fileStream "testNums") ___opening testWords "one" "two" "three" ___Stop closing testWords ___opening testNums "1" "2" "3" "4" "5" ___EOF closing testNums NOTHING Here are a few more stream functions to give a feel for what sorts of things can easily be done. -} emptyStream :: Stream a r emptyStream _ k = streamCPS k Nothing getLineStream :: Stream Text (IO r) getLineStream Stop k = streamCPS k Nothing getLineStream Continue k = streamCPS k . Just . (, getLineStream) =<< getLine listToStream :: [a] -> Stream a r listToStream _ Stop k = streamCPS k Nothing listToStream [] Continue k = streamCPS k Nothing listToStream (x:xs) Continue k = streamCPS k $ Just (x, listToStream xs) -- Stream consumers should have a general enough return type to work with streams in a monad. countStream :: Monad m => Stream a (m Int) -> m Int countStream xs = xs Continue $ StreamCPS $ go 0 where go n (Just (_, xs')) = xs' Continue $ StreamCPS $ go $ n + 1 go n Nothing = return n consStream :: a -> Stream a r -> Stream a r consStream _ s Stop k = s Stop k consStream x s Continue k = streamCPS k $ Just (x, s) mapStream :: (a -> b) -> Stream a r -> Stream b r mapStream f xs action k = xs action $ StreamCPS $ streamCPS k . fmap (f *** mapStream f) filterStream :: (a -> Bool) -> Stream a r -> Stream a r filterStream p xs action k = xs action $ StreamCPS $ maybe (streamCPS k Nothing) (\(x, xs') -> if p x then streamCPS k $ Just (x, filterStream p xs') else filterStream p xs' Continue k) dropStream :: Int -> Stream a r -> Stream a r dropStream _ xs Stop k = xs Stop k dropStream n xs Continue k = xs Continue newK where newK | n > 0 = StreamCPS $ maybe (streamCPS k Nothing) (\(_, xs') -> dropStream (n - 1) xs' Continue k) | otherwise = k -- Round robin interleaving of a list of streams. mergeStreams :: [Stream a r] -> Stream a r mergeStreams [] _ k = streamCPS k Nothing mergeStreams (s:ss) Stop k = s Stop $ StreamCPS $ \_ -> mergeStreams ss Stop k mergeStreams (s:ss) Continue k = s Continue $ StreamCPS $ maybe (mergeStreams ss Continue k) $ \(v,s') -> streamCPS k $ Just (v, mergeStreams $ ss++[s']) -- Run an attoparsec parser over the given stream. -- Note the parser can consume partial and/or multiple lines from the stream to generate its result. attoStream :: forall a r . Parser a -> Stream Text (IO r) -> Stream a (IO r) attoStream p = go (parse p) where go :: (Text -> Result a) -> Stream Text (IO r) -> Stream a (IO r) go _ xs Stop k = xs Stop $ StreamCPS $ \_ -> streamCPS k Nothing go f xs Continue k = xs Continue $ StreamCPS $ maybe (streamCPS k Nothing) $ \(line, xs') -> case f line of Done rest v -> streamCPS k $ Just (v, attoStream p $ consStream rest xs') f'@(Partial _) -> go (feed f') xs' Continue k f'@(Fail _ _ctxs err) -> P.putStrLn ("___Atto Error: "++err) >> go (feed f') xs' Stop k

2 1

object file cannot be loaded.
by Magicloud Magiclouds 09 Oct '12

09 Oct '12

Hi, I am installing postgres hackage (cannot remember the exact name right now). When it compiling the template haskell part I got the following error message. I tried to clear all user space hackages. Not helping. Loading package text-0.11.2.3 ... linking ... ghc: /home/magicloud/.cabal/lib/text-0.11.2.3/ghc-7.6.1/HStext-0.11.2.3.o: unknown symbol `bytestringzm0zi10zi0zi1_DataziByteStringziInternal_PS_con_info' ghc: unable to load package `text-0.11.2.3' -- 竹密岂妨流水过山高哪阻野云飞 And for G+, please use magiclouds#gmail.com.

2 4

OpenGL library will not load into GHCi 7.6.1 on Win7 x86-64
by Dan Haraj 09 Oct '12

09 Oct '12

This is a regression of some sort since the library operates fine within GHCi for previous versions of GHC. I don't know whether it is a problem with GHCi, the OpenGL library, or some third party. This is the error encountered when a module that uses OpenGL tries to load: Prelude Main> main Loading package OpenGLRaw-1.2.0.0 ... linking ... ghc.exe: unable to load package `OpenGLRaw-1.2.0.0' Prelude Main> :q Leaving GHCi. <interactive>: C:\...\OpenGLRaw-1.2.0.0\ghc-7.6.1\HSOpenGLRaw-1.2.0.0.o: unknown symbol `__imp_wglGetProcAddress' I have very little of an idea of what's going on here. It would be rather painful to debug this myself so any insight is appreciated.

2 1

ANN: New York Haskell Users Group -- First Meeting on Wed., Nov 14.
by Gershom B 09 Oct '12

09 Oct '12

The (arguably) greatest city in the U.S. now (finally) has a users group for (unarguably) the greatest programming language anywhere. We're very happy to announce the inaugural meeting of the New York Haskell Users Group. We intend this group to be welcoming to new developers, but with enough substance to keep old Haskell hands interested and amused. If you're in the area but can't make it, you can join the meetup regardless at http://www.meetup.com/NY-Haskell/ to stay informed of future events. We plan to have a mix of talks and experience reports at all levels, so if you will be passing through town and might have something to say, let us know! Two Haskell Talks: Practical Data Processing With Haskell _and_ Putting Cloud Haskell to Work Wednesday, November 14, 2012 7:00 PM To 9:00 PM Pivotal Labs, 841 Broadway New York, NY RSVP: http://www.meetup.com/NY-Haskell/ We're kicking off the New York Haskell Users Group with two great talks, one for people just getting started and one for those with some experience already. Both should be accessible and enjoyable no matter how much (or little) you already know. Food and refreshments will be provided, courtesy of the generosity of Pivotal Labs, and after the talks, we're planning to keep the discussion going over food and drink at a nearby establishment. 7pm - Practical Data Processing With Haskell: Ozgun Ataman will give an introductory talk on Haskell, diving right in with how you can start using Haskell for practical data manipulation tasks today. You'll be introduced to a typical setup for Haskell development and given a demonstration of how a common data format (CSV, JSON, etc.) can be parsed, processed and finally output using Haskell. The talk will include a small actual Haskell program to be modified live and some commentary around using Haskell in practical applications. 8pm - Putting Cloud Haskell to Work for Distributed Computing: Gershom Bazerman will give an overview of the new distributed-process library that implements Cloud Haskell, providing computation across heterogeneous nodes through a message passing interface. He will discuss what this new tool provides out of the box, and what you'll have to bring to the table. The talk will include some code samples using the new library, as well as some experience about what it's like to use Haskell for distributed computing in the real world. Speaker Biographies: Ozgun Ataman is the founder of Soostone, a managment consulting and analytics company built on Haskell. He is the author of many open-source Haskell libraries, and is a contributor to the Snap web framework. Gershom Bazerman is a developer at S&P Capital IQ. He is most well known in the Haskell community as the author of the JMacro library for programmatic generation of JavaScript, and for one well-received April fools joke.

1 0

total Data.Map.! function
by Henning Thielemann 09 Oct '12

09 Oct '12

I wondered whether there is a brilliant typing technique that makes Data.Map.! a total function. That is, is it possible to give (!) a type, such that m!k expects a proof that the key k is actually present in the dictionary m? How can I provide the proof that k is in m? Same question for 'lab' (import Data.Graph.Inductive(lab)). That is, can there be a totalLab, with (totalLab gr = fromJust . lab gr) that expects a proof that the node Id is actually contained in a graph?

4 4