June 2010 - Haskell-Cafe

Using Hint with a socket server
by Tom Jordan 17 Jun '10

17 Jun '10

I'm trying to receive small segments of Haskell code over a socket, and be able to evaluate them in real time in GHCI. I've already downloaded Hint and have run the test code, and it's working great. I'm also using the socket server code from Ch.27 of "Real World Haskell" and that is working well also. directly below is the function from the socket server code that handles the incoming messages. Instead of doing this: "putStrLn msg"... I want to send whatever is captured in "msg" to the GHC interpreter that is used in the Hint code, something like this: "eval msg". I'm not sure how to combine both of these functionalities to get them to work with each other.. -- A simple handler that prints incoming packets plainHandler :: HandlerFunc plainHandler addr msg = putStrLn msg Below is the full code for the socket server, then below that is "SomeModule" used in the Hint example test below that. -- file: ch27/syslogserver.hs import Data.Bits import Network.Socket import Network.BSD import Data.List type HandlerFunc = SockAddr -> String -> IO () serveLog :: String -- ^ Port number or name; 514 is default -> HandlerFunc -- ^ Function to handle incoming messages -> IO () serveLog port handlerfunc = withSocketsDo $ do -- Look up the port. Either raises an exception or returns -- a nonempty list. addrinfos <- getAddrInfo (Just (defaultHints {addrFlags = [AI_PASSIVE]})) Nothing (Just port) let serveraddr = head addrinfos -- Create a socket sock <- socket (addrFamily serveraddr) Datagram defaultProtocol -- Bind it to the address we're listening to bindSocket sock (addrAddress serveraddr) -- Loop forever processing incoming data. Ctrl-C to abort. procMessages sock where procMessages sock = do -- Receive one UDP packet, maximum length 1024 bytes, -- and save its content into msg and its source -- IP and port into addr (msg, _, addr) <- recvFrom sock 1024 -- Handle it handlerfunc addr msg -- And process more messages procMessages sock -- A simple handler that prints incoming packets plainHandler :: HandlerFunc plainHandler addr msg = putStrLn msg -- main = serveLog "8008" plainHandler ---------------------------------------------------------------------------------------------------------------- module SomeModule(g, h) where f = head g = f [f] h = f ---------------------------------------------------------------------------------------------------------------- import Control.Monad import Language.Haskell.Interpreter main :: IO () main = do r <- runInterpreter testHint case r of Left err -> printInterpreterError err Right () -> putStrLn "that's all folks" -- observe that Interpreter () is an alias for InterpreterT IO () testHint :: Interpreter () testHint = do say "Load SomeModule.hs" loadModules ["SomeModule.hs"] -- say "Put the Prelude, Data.Map and *SomeModule in scope" say "Data.Map is qualified as M!" setTopLevelModules ["SomeModule"] setImportsQ [("Prelude", Nothing), ("Data.Map", Just "M")] -- say "Now we can query the type of an expression" let expr1 = "M.singleton (f, g, h, 42)" say $ "e.g. typeOf " ++ expr1 say =<< typeOf expr1 -- say $ "Observe that f, g and h are defined in SomeModule.hs, " ++ "but f is not exported. Let's check it..." exports <- getModuleExports "SomeModule" say (show exports) -- say "We can also evaluate an expression; the result will be a string" let expr2 = "length $ concat [[f,g],[h]]" say $ concat ["e.g. eval ", show expr1] a <- eval expr2 say (show a) -- say "Or we can interpret it as a proper, say, int value!" a_int <- interpret expr2 (as :: Int) say (show a_int) -- say "This works for any monomorphic type, even for function types" let expr3 = "\\(Just x) -> succ x" say $ "e.g. we interpret " ++ expr3 ++ " with type Maybe Int -> Int and apply it on Just 7" fun <- interpret expr3 (as :: Maybe Int -> Int) say . show $ fun (Just 7) -- say "And sometimes we can even use the type system to infer the expected type (eg Maybe Bool -> Bool)!" bool_val <- (interpret expr3 infer `ap` (return $ Just False)) say (show $ not bool_val) -- say "Here we evaluate an expression of type string, that when evaluated (again) leads to a string" res <- interpret "head $ map show [\"Worked!\", \"Didn't work\"]" infer >>= flip interpret infer say res say :: String -> Interpreter () say = liftIO . putStrLn printInterpreterError :: InterpreterError -> IO () printInterpreterError e = putStrLn $ "Ups... " ++ (show e)

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coming soon: ICFP programming contest 2010
by J.Waldmann 17 Jun '10

17 Jun '10

http://icfpcontest.org/2010/ will start roughly 12 hours from now. 221022000022010112201010022001122011110220010

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Understanding GHC allocations
by Roman Cheplyaka 17 Jun '10

17 Jun '10

I'm trying to optimize the following program: http://github.com/feuerbach/particles/blob/303c8a17c9b732e22457b5409bdce4b7… Of course general suggestions are welcome (BTW I'm going to give a try to vector), but currently I'm concerned with two questions: 1. Heavy allocations in 'distance' function. Here is (part of) the profile: COST CENTRE MODULE %time %alloc ticks bytes d2 Main 9.0 22.0 290 600000000 d Main 8.6 65.9 278 1800000000 d1 Main 7.5 11.0 242 299700000 >From reading core I got the impression that everything is strict & unboxed. Perhaps this is related to creating some closures? How to get rid of those allocations? 2. Again from reading the core I learned that although 'l' and other constants are inlined, their type is boxed Double. This makes sense since CAFs are evaluated on demand, but obviously in this particular case it does not make sense, so can I somehow make them unboxed? -- Roman I. Cheplyaka :: http://ro-che.info/ "Don't let school get in the way of your education." - Mark Twain

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What is Haskell unsuitable for? Dis-functional programming!
by caseyh＠istar.ca 17 Jun '10

17 Jun '10

:)

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Re: [Haskell-beginners] Re: Accounting Engine in Haskell
by Patrick LeBoutillier 17 Jun '10

17 Jun '10

Hi, > Keep in mind that you have to invest some time in learning Haskell > before you can reap the benefits. For an example of the latter, see also > > Paul Hudak, Mark P. Jones. > Haskell vs. Ada vs. C++ vs. Awk vs. ..., > An Experiment in Software Prototyping Productivity > http://haskell.org/papers/NSWC/jfp.ps This paper was very interesting to me. Does anyone know if the full source code for the Haskell prototype is available somewhere? Patrick -- ===================== Patrick LeBoutillier Rosemère, Québec, Canada

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Using the ContT monads for early exits of IO ?
by Günther Schmidt 16 Jun '10

16 Jun '10

Hi everyone, I'm about to write a rather lengthy piece of IO code. Depending on the results of some of the IO actions I'd like the computation to stop right there and then. Now I know in general how to write this but I'm wondering if this is one of those occasions where I should make use of the Cont monad to make an early exit. Günther

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Announce: hs2dot 0.1.1 - generate graphviz code by analyzing Haskell source code files
by Oscar Finnsson 16 Jun '10

16 Jun '10

Hi, I've just released hs2dot on hackagedb. It's a small tool that lets you automatically generate graphviz/dot code that visualize the relations between data types, types and type classes. Example usage: > hs2dot Hack.hs | dot -T pdf -o Hack.pdf Multiple files can be analyzed together as in > hs2dot Foo.hs Bar.hs | dot -T png -o FooBar.png so their data types can point to each other. Example diagrams generated using h2dot can be found at http://github.com/finnsson/hs2graphviz/tree/master/Examples/ Examples of real code analyzed: http://github.com/finnsson/hs2graphviz/raw/master/Examples/Control.pdf (from hpage) http://github.com/finnsson/hs2graphviz/raw/master/Examples/Hack.png (from Hack) Example of mock code analyzed: http://github.com/finnsson/hs2graphviz/raw/master/Examples/TestCode.pdf (three files analyzed together) The tool is still far from complete. Better handling of algebraic data types and instance declarations are a must. GADTs, data families and type synonym families are not yet implemented so they wont turn up. -- Oscar

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Announce: hlcm 0.2.2 - Parallel closed frequent itemsets mining
by Alexandre Termier 16 Jun '10

16 Jun '10

Dear all, I'm pleased to announce the release of hlcm on Hackage : http://hackage.haskell.org/package/hlcm-0.2.2 hlcm is data mining tool for computing closed frequent itemsets. This problem is famous as "market basket analysis": - given a list of transactions : [["bread", "butter","chocolate","tomato"] ,["bread","butter"] ,["bread","pencil","butter","chocolate"] ,["bread","butter","book"]] - and a minimal frequency threshold in [1..4], let's say 2: we want items that are sold together in at least 2 transactions hlcm will tell you that ["bread","butter","chocolate"] appears in 3 transactions and that ["bread","butter"] appears in 4 transactions. You can many funnier applications with your own data, for example log analysis, mining words in web pages, etc. You can see details on getting started with the program here: http://membres-liglab.imag.fr/termier/HLCM/hlcm/hlcm/Main.html The library documentation is here : http://membres-liglab.imag.fr/termier/HLCM/hlcm/HLCM.html hlcm is based on the most efficient algorithm for closed frequent itemset mining, LCM, which is much, much faster than the well-known Apriori algorithm (more details when following the pointers from the homepage: http://membres-liglab.imag.fr/termier/HLCM/hlcm.html) hlcm can also exploit parallelism through Strategies, with promising speedups. We still have more work to do in order to beat existing C/C++ implementations, but you can have a look at the paper that we submitted at Haskell Symposium this year for a detailed experimental study: http://membres-liglab.imag.fr/termier/HLCM/hlcm.pdf Don't miss out the section about the influence of RTS parameters on parallel performance. Feel free to send me an e-mail if you have any question about hlcm. Alexandre -- _____________________________________________________________ Alexandre Termier LIG (Laboratoire d'Informatique de Grenoble) Université Joseph Fourier 681 rue de la Passerelle B.P. 72, 38402 Saint Martin d'Hères (FRANCE) Phone: +33 4 76 82 72 07 Fax: +33 4 76 82 72 87 http://membres-liglab.imag.fr/termier/ _____________________________________________________________

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Terminology
by Emmanuel Castro 15 Jun '10

15 Jun '10

I am looking for the name of the property linking two functions f and g when : [f(a),f(b),f(c)] = g([a,b,c]) Is there a standard name? In practice, g is an optimised version of f when working on large amount of elements. Thank you

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learning advanced haskell
by Aran Donohue 15 Jun '10

15 Jun '10

Hi Cafe, I've been doing Haskell for a few months, and I've written some mid-sized programs and many small ones. I've read lots of documentation and many papers, but I'm having difficulty making the jump into some of the advanced concepts I've read about. How do people build intuitions for things like RankNTypes and arrows? (Is the answer "Get a PhD in type theory?") Normally I learn by coding up little exercise programs, but for these I don't have good intuitions about the kinds of toy problems I ought to try to solve that would lead me to understand these tools better. For systems like Template Haskell and SYB, I have difficulty judging when I should use Haskell's simpler built-in semantic abstractions like functions and typeclasses and when I should look to these other mechanisms. I understand the motivation for other concepts like iteratees, zippers and functional reactive programming, but there seem to be few entry-level resources. John Lato's recent Iteratee article is a notable exception*. Hints? Tips? "Here's how I did it"? _______ is a great program to write to get to learn ______? Thanks in advance, Aran * Even in this article, he busted out (>=>). And it appears that the iteratee library's actual design is more sophisticated than the design presented.

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