January 2012 - Haskell-Cafe

REMINDER: Hac Boston from January 20-22 at MIT
by Edward Z. Yang 18 Jan '12

18 Jan '12

I'd like to remind everyone that Hac Boston, a Haskell hackathon is being held January 20-22, 2012 at MIT (rooms 4-159 and 4-261) in Cambridge, MA. The hackathon will officially kick off at 2:30 Friday afternoon, and go until 5pm on Sunday with the occasional break for sleep. Everyone is welcome -- you do not have to be a Haskell guru to attend! Helping hack on someone else's project could be a great way to increase your Haskell skills. If you plan on coming, please officially register<http://haskell.org/haskellwiki/Hac_Boston/Register>, even if you already put your name on the wiki. Registration, travel, some information about lodging and many other details can now be found on the Hac Boston wiki <http://haskell.org/haskellwiki/Hac_Boston>. Edward Kmett tells me we still have space. We're also looking for a few people interested in giving short (15-20 min.) talks, probably on Saturday afternoon. Anything of interest to the Haskell community is fair game---a project you've been working on, a paper, a quick tutorial. If you'd like to give a talk, add it on <http://goog_145539007>the wiki <http://haskell.org/haskellwiki/Hac_Boston/Talks>. We look forward to seeing you at MIT! Cheers, Edward

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Encouraging performance with ghc 7.4
by Eugene Kirpichov 18 Jan '12

18 Jan '12

Hi cafe, Just wanted to inform you that I've been benchmarking my compute-intensive stuff on ghc 7.0.4 vs 7.4.0, and 7.4.0 gave a large speedup - one program that took 2.9s on a particular input is now taking 2.2s. This result is repeatable. So I encourage people to try out GHC 7.4.0. Some stuff may stop compiling (I had to do 3 basically one-line fixes in 3 packages before I got mine to compile), but I didn't encounter other problems. Below is +RTS -s of two runs: one compiled with ghc 7.4.0 and another with 7.0.4. I can make a more detailed comparison if it's useful and if someone tells me how - I thought about including +RTS -p, but Simon Marlow wrote recently that it has started giving meaningful results only in 7.4, so comparison with 7.0.4 would be unfair. However in this case I'm sure that the program is heavily compute-bound by the following two functions (because it's always compute-bound by them): 1) https://github.com/jkff/timeplot/blob/master/Tools/TimePlot/Plots.hs#L226 2) https://github.com/jkff/timeplot/blob/master/Tools/TimePlot/Conf.hs - localToUTC and strptime. jkff@jkff-laptop ~/projects/logs/... $ tplot -if e.trace -dk 'within[.] cumsum 10' -o e.png +RTS -s tplot -if e.trace -dk within[.] cumsum 10 -o e.png +RTS -s 2,751,756,296 bytes allocated in the heap 135,129,112 bytes copied during GC 33,149,720 bytes maximum residency (22 sample(s)) 1,755,868 bytes maximum slop 56 MB total memory in use (0 MB lost due to fragmentation) Generation 0: 4994 collections, 0 parallel, 0.22s, 0.23s elapsed Generation 1: 22 collections, 0 parallel, 0.08s, 0.09s elapsed INIT time 0.01s ( 0.00s elapsed) MUT time 2.61s ( 2.91s elapsed) GC time 0.30s ( 0.32s elapsed) EXIT time 0.00s ( 0.00s elapsed) Total time 2.91s ( 3.22s elapsed) %GC time 10.1% (9.8% elapsed) Alloc rate 1,051,262,083 bytes per MUT second Productivity 89.6% of total user, 80.9% of total elapsed jkff@jkff-laptop ~/projects/logs/... $ tplot -if e.trace -dk 'within[.] cumsum 10' -o e.png +RTS -s 2,161,811,620 bytes allocated in the heap 107,589,660 bytes copied during GC 34,799,400 bytes maximum residency (22 sample(s)) 1,721,152 bytes maximum slop 58 MB total memory in use (0 MB lost due to fragmentation) Tot time (elapsed) Avg pause Max pause Gen 0 3899 colls, 0 par 0.13s 0.14s 0.0000s 0.0003s Gen 1 22 colls, 0 par 0.08s 0.09s 0.0043s 0.0482s INIT time 0.00s ( 0.00s elapsed) MUT time 2.03s ( 2.28s elapsed) GC time 0.21s ( 0.23s elapsed) EXIT time 0.00s ( 0.00s elapsed) Total time 2.26s ( 2.51s elapsed) %GC time 9.3% (9.1% elapsed) Alloc rate 1,056,397,390 bytes per MUT second Productivity 90.7% of total user, 81.5% of total elapsed -- Eugene Kirpichov Principal Engineer, Mirantis Inc. http://www.mirantis.com/ Editor, http://fprog.ru/

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Lifting IO (IO a) -> IO a to m (m a) -> m a with monad-control
by Edward Z. Yang 18 Jan '12

18 Jan '12

Hello folks, I was curious whether or not it is possible to lift an arbitrary IO (IO a) -> IO a function to MonadBaseControl IO m => m (m a) -> m a. That is, implement a function: liftJoin :: MonadBaseControl mb m => (mb (mb (StM m a)) -> mb (StM m a)) -> m (m a) -> m a The difficulty seems to be that we can't extract the resumable state from the inner base monad action. If this is not possible, is there a suitable strengthening of MonadBaseControl that achieves this effect? Cheers, Edward [1] http://hackage.haskell.org/packages/archive/monad-control/latest/doc/html/C…

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Re: [Haskell-cafe] Need advice: Haskell in Web Client
by dokondr 18 Jan '12

18 Jan '12

On Wed, Jan 18, 2012 at 10:47 AM, John Lenz <lenz(a)math.uic.edu> wrote: > > > I don't see a great need of developing something like GWT for haskell, > since we already have good support for all sorts of existing tools that > span more than just haskell, like extjs, yui, and jqueryui. > Haskell makes my work doable in many areas where other PLs will take enormous efforts and time to achieve the same result. It would be great if I could write Web client code in pure Haskell (no HTML, no DOM, no Ajax, ..) compile it to Javascript and just run it in the browser and do all GUI and backend communication work. Simple as that. Regards, Dmitri.

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strict, lazy, non-strict, eager
by Albert Y. C. Lai 18 Jan '12

18 Jan '12

Most individuals of the Haskell community have long been maintaining a cognitive dissonance; some cases turn into plain hypocrisy. You might excuse it for its ancient and prominent origin: Richard Bird and/or Philip Wadler themselves wrote like "it is too lazy", "make it more strict" 13 years ago and surely more. But perpetuating it is not helping. I have not written this complaint until now because I have been waiting for unmistakable evidence, a smoking gun, a red hand so caught that you cannot explain away, for example you cannot explain that "one sentence is from one person, the other sentence is from a different person". So, on IRC in #haskell, from the same person, speaking on the same topic in the same context, in the same interval of 3 minutes (the first two sentences in the same minute): 1. a function f is strict if f ⊥ = ⊥ 2. ⊥ represents any computation which does not terminate, i.e. an exception or an infinite loop 3. "strict" describes the denotational semantics People, could you please make up your mind already? It has been more than 13 years. Denotational semantics: A. There are no computational steps. There is no termination, and there is no non-termination, since there are no steps to finish, and no steps to keep going. B. ⊥ represents "no information", not "non-termination". There is no "non-termination" to represent. C. fix id = ⊥ because ⊥ is the least fixed point of id, not because fix id non-terminates. There is nothing to terminate or non-terminate. D. You say strict, more strict, less strict; non-strict, more non-strict, less non-strict. You don't say eager, and you don't say lazy. Operational semantics: A. There is no ⊥; it does not appear in any sequence of computational steps, finitely long or infinitely long. B. You say eager, more eager, less eager; lazy, more lazy, less lazy; or speculative, more speculative, less speculative; or any other adjectives for evaluation strategies. You don't say strict, and you don't say non-strict. "Semantics", "semantically speaking": A. Which semantics, which semantically? There are two. B. Actually there are more, but apparently two is already enough to cause all kinds of incoherent statements. If I draw your attention to algebraic semantics, will you start saying "it is too lazy, need to make it more idempotent"?

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implementing a text editor swap file
by Martin DeMello 18 Jan '12

18 Jan '12

I'm writing a gtk2hs-based text editor, and would like to implement continuous (swap-file based) autosave the way vim and emacs do it. Any suggestions for how to implement this in a cross-platform manner? Also, is there a library that returns standard config file locations on a per-platform basis? martin

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haskell tcp server
by Alexander V Vershilov 18 Jan '12

18 Jan '12

Hello. I'm interested if there exists some library like warp but only for tcp. Or maybe some web page with skeleton for such server or with some variants, eg. concurent/single-threaded. I know that each service can have it's specific properties and realization, but there are many common things left. If it matter — my main task is to create server for protobuf protocol, maybe for such a task there are already created solutions. -- Alexander V Vershilov.

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How to make asynchronous I/O composable and safe?
by Joey Adams 18 Jan '12

18 Jan '12

I'm not happy with asynchronous I/O in Haskell. It's hard to reason about, and doesn't compose well. At least in my code. I'm currently trying to build a networking layer for my application using Network.TLS. Here is a rather minimalist API: newtype Connection = Connection (TLSCtx Handle) connectClient :: Handle -- ^ Connection handle, as returned by 'connectTo' -> X509 -- ^ TLS certificate (i.e. public key) -> IO Connection connectServer :: Handle -- ^ Connection handle, as returned by 'accept' -> X509 -- ^ TLS certificate (i.e. public key) -> TLS.PrivateKey -- ^ TLS private key -> IO Connection close :: Connection -> IO () sendMessage :: Connection -> Message -> IO () recvMessage :: Connection -> ByteString -> IO (Message, ByteString) The module provides little more than connection initialization and message serialization. I don't try to use locks or STM to multiplex the connection or, in the case of recvMessage, hide connection state. I just be sure to only use sendMessage in one thread at a time, only use recvMessage in one thread at a time, and marshal the "extra bytes" parameter of recvMessage from call to call (with the help of StateT). I wrote a simple "chat server" to test it. The client turned out okay: main :: IO () main = do cert <- getCertificate handle <- connectTo "localhost" (PortNumber 1337) conn <- connectClient handle cert _ <- forkIO $ forever $ do s <- getLine sendMessage conn $ TestMessage s forever $ flip runStateT B.empty $ do msg <- StateT $ recvMessage conn case msg of TestMessage s -> liftIO $ putStrLn s _ -> liftIO $ hPrintf stderr "Warning: unrecognized message from server: %s\n" (messageTypeName msg) The only glaring problem is that, if the user presses Ctrl+D, the forked (sending) thread dies, but the main (receiving) thread lingers. I'd have to add exception handlers to ensure that when one thread dies, the other thread dies too. However, the server is an abomination (see attachment). Unfortunately, it's not as simple as "spawn one thread per client". We need at least two threads, one to listen for messages from the client, and another to send messages to the client. GHC won't let us simultaneously, in the same thread, wait for input from a connection and wait for an STM transaction to succeed. Another source of complexity is: what if we throw an exception at a thread while it is in the middle of sending a packet? Then we can't shut down the connection properly (i.e. Network.TLS.bye), because the receiver might think the close_notify packet is part of the interrupted packet. Having a thread for each client is good, as it: * Lets us think about each client separately. No need to turn our code inside out or write one big loop that juggles all the clients. * Isolates exceptions. If sendMessage or recvMessage throws an exception, it doesn't bring the whole server down. On the other hand, having multiple threads interact with a single client is hard to think about: * We have to synchronize the threads (e.g. when one dies, kill the other one) * Multiple places where an exception can arise * Can multiple threads interact with the connection handle simultaneously? So why don't I make my connection API handle some of this? Well, I tried. There are so many ways to do it, and I couldn't find a way that simplified usage much. The approach used by Handle and by Network.TLS is to use MVars and IORefs to ensure that, if two threads access the same connection, the connection doesn't become totally corrupt. If I do the same, then I'll have *three* layers of locking under the hood. Worse, the locking done by Handle and Network.TLS doesn't guarantee much. I don't know if it's safe to have one thread sending and another thread receiving. Especially in the case of Network.TLS, where 'recvData' automatically handshakes in some cases, which sends packets. Since I don't know how much thread safety to expect, I can't write networking code and know for sure that it is safe. I'm certainly not protected from interleaved data if multiple threads send on the same handle. For example: import Control.Concurrent import System.IO main :: IO () main = do hSetBuffering stdout NoBuffering _ <- forkIO $ putStrLn "One sentence." putStrLn "Another sentence." produces: AnOonteh esre nsteenntceen.c e. That is, I can't rely on putStrLn being "atomic". To produce intelligible output (without changing the buffering mode), I have to "lock" the output each time I write something. putStrLn doesn't do it for me. === Summary === In Haskell, sound logic and a great type system lead to elegant, composable code in a variety of domains, such as: * Expression evaluation * Parsing * Concurrent programming (thanks to STM) Asynchronous I/O is tricky. However, Haskell currently does little to alleviate the complexity (at least for me). How can we structure network protocol APIs so that they stack well (e.g. only lock once, rather than locking each layer's connection state)? How can we deal with I/O errors without having to think about them at every turn? For now, how can I structure my application's communication API so it's less messy to use? Thanks, - Joey

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Book?
by Gregory Guthrie 17 Jan '12

17 Jan '12

I've seen a book: "The Practice Of Monadic Interpretation" Dan Popa Nov. 2008 Or "Practical Monadic Interpretation" Dan Popa Which seem that they might be the same book? As reported on Haskell Wiki/books as published in 2008, but Don't find it available anywhere under either title. Any pointers or references?

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Need advice: Haskell in Web Client
by dokondr 17 Jan '12

17 Jan '12

Hi all, I hope to use Haskell for graphics (charts) programming in Web client. My current implementation in brief: Server side, Haskell modules: 1) collecting various statistics from Twitter 2) generating text data for Gnuplot (http://www.gnuplot.info/) 3) Gnuplot creates png files with charts Web client: GWT (Google Web Toolkit) web UI that allows user to enter queries and see resulting charts in Web browser. Charts are png files generated by Gnuplot on the server side. Ideally, on the server side instead of using Gnuplot I would like Haskell to generate Javascript to be downloaded to Web client and draw charts in the browser. Something, very approximately, similar to what GWT does :) This code will be specific of course to plotting Javascript framework, no problem. Looking at "Haskell in web browser - HaskellWiki:" http://www.haskell.org/haskellwiki/Haskell_in_web_browser#Haskell_web_toolk… I feel a little confused about current state of the available Haskell tools for this task. Any ideas? Thanks! Dmitri

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