December 2010 - Haskell-Cafe

Re: [Haskell-cafe] DFAs and self-referential data
by oleg＠okmij.org 28 Dec '10

28 Dec '10

Maxime Henrion wrote: > I've been playing with some code to work with DFAs, but I'm now faced > with an implementation problem. In order to have states that can > transition to themselves, it seems I would need self-referential data; > otherwise I would need to separate those transitions from the rest and > handle them specially in the code. Perhaps an old article http://okmij.org/ftp/misc.html#ccard-transform might be of some help. The article describes not only running a finite automaton (represented as a cyclic graph) on given input but also printing the automaton out and determinizing it: converting NFA to an equivalent DFA. The latter operation converts one cyclic graph to another.

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How to "instance MonadIO Identity"?
by Magicloud Magiclouds 28 Dec '10

28 Dec '10

Hi, From another thread in this list, I got code as: > instance MonadIO Identity where > liftIO = id Well, it does not work for me as: Message.hs:22:12: Couldn't match expected type `Identity a' with actual type `IO a' Expected type: IO a -> Identity a Actual type: IO a -> IO a In the expression: id In an equation for `liftIO': liftIO = id -- 竹密岂妨流水过山高哪阻野云飞

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instance for (Show ([(String, Int)] -> Int))
by Aaron Gray 27 Dec '10

27 Dec '10

How do I code an 'instance' declaration for '(Show ([(String, Int)] -> Int))'. I am new to instance declarations and am following the Happy examples :- http://www.haskell.org/happy/doc/html/sec-using.html#sec-other-datatypes The compiler is requesting an instance declaration for Show :- expr-eval.hs:334:23: No instance for (Show ([(String, Int)] -> Int)) arising from a use of `print' at expr-eval.hs:334:23-27 Possible fix: add an instance declaration for (Show ([(String, Int)] -> Int)) In the first argument of `(.)', namely `print' In the second argument of `(>>=)', namely `print . calc . lexer' In the expression: getContents >>= print . calc . lexer Many thanks in advance, Aaron

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ANNOUNCE: yackage 0.0.0
by Michael Snoyman 27 Dec '10

27 Dec '10

Hi all, I was speaking with my coworker Yitz[1] about a project he's working on. Basically, he's going to end up with about 16 cabal packages that are not going to be deployed to Hackage, and wanted us to set up a Hackage server for our company to deploy these kinds of things. However, getting all the pieces of Hackage aligned properly for such a simple use case seemed a bit overkill. I then realized that I had the exact same problem during Yesod development: before I make a major release, I usually end up with about 10-15 packages that are not yet live on Hackage. It gets to be a real pain when suddenly wai-extra is depending on network 2.3 and authenticate requires network 2.2, and suddenly I need to manually recompile 10 packages. So I decided to write up a simple web service to act as a local Hackage server. It has no security (anyone with access can upload a package), doesn't build haddocks, doesn't show package descriptions, etc. All it does is: Show a list of uploaded packages/versions Links to the tarballs Allows you to upload new versions, which will automatically overwrite existing packages Provides the 00-index.tar.gz file needed by cabal-install, as well as the tarballs for all the packages In order to use this, just do the following: cabal install yackage run "yackage" Upload your packages Add remote-repo: yackage:http://localhost:3500/ to your ~/.cabal/config file cabal update Install your packages are usual You'll need to leave yackage running whenever you want to run an update or download new packages. A few other usage notes: If you overwrite a package, your cache folder will still have the old version. You might want to just wipe our your cache folder on each usage. Running cabal update will download the update for both yackage and the main hackage server; the latter can be a long process depending on your internet connection. Here's a little shell script that will disable the Hackage repo, wipe our the Yackage cache, update and re-enable the Hackage repo: #!/bin/sh CABAL_DIR=~/.cabal cp $CABAL_DIR/config $CABAL_DIR/config.sav sed 's/^remote-repo: hackage/--remote-repo: hackage/' < $CABAL_DIR/config.sav > $CABAL_DIR/config rm -rf $CABAL_DIR/packages/yackage cabal update cp $CABAL_DIR/config.sav $CABAL_DIR/config I hope others find this tool useful. Michael [1] http://www.haskellers.com/user/Yitz_Gale/

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ANNOUNCE: greg-client - a scalable distributed logger with a high-precision global time axis
by Eugene Kirpichov 27 Dec '10

27 Dec '10

Hi cafe, Let me bring to your attention this package developed by me and Dmitry Astapov: http://hackage.haskell.org/package/greg-client It is a Haskell binding to the "greg" logger ( http://code.google.com/p/greg , http://github.com/jkff/greg - "global registrator" ). You can read the project's description and motivation at the homepage http://code.google.com/p/greg , but here are some highlights: * It's designed for debugging lattency bottlenecks or synchronization issues in distributed systems. I've been using it on small and large clusters, Dmitry has been using it for debugging haskell-mpi if I'm correct (Dmitry, can you elaborate a bit?). * It's a client/server logger - you launch a server and clients log messages to it * It provides a high-precision global time axis by computing clock offset between server and each client (this allows to register events with their occurence timestamps, not arrival-to-server timestamps) * It's very fast (I squeezed 400,000 messages/sec = 20Mb/s out of the Java binding) * It can handle a huge number of clients (a much less scalable implementation was tested on a 2000-core cluster) * It's robust to intermittent deaths of clients, server or the link between them * It's very lightweight - no persistence infrastructure, piles of config files or whatever - just send him the messages, it will align them on the global axis and log them to console I found it especially useful in conjunction with the log visualization tools described here http://slideshare.net/jkff/two-visualization-tools . P.S. How to use it (while hackage hasn't yet rebuilt the docs): import System.Log.Greg main = withGregDo defaultConfiguration $ do .... ...logMessage "Something happened"... -- Eugene Kirpichov Senior Software Engineer, Grid Dynamics http://www.griddynamics.com/

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global, modifiable variable for debugging
by briand＠aracnet.com 27 Dec '10

27 Dec '10

Hi, I have a program with a debug flag in it (Strangely I've yet to be able to write bug-free code). I'd like to change the state of the debug flag based on command line args. I looked at IOVar but that would cause all the pure procedures to get swallowed by the IO Monad. Is a better way to get this behavior ? Thanks, Brian

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The Bay Area Haskell Hackathon is coming up: Feb 11-13, 2011
by Bryan O'Sullivan 27 Dec '10

27 Dec '10

Mark Lentczner and I are organizing, and it will be held at Hacker Dojo in Mountain View<http://maps.google.com/maps/place?cid=2122486601784397611&q=hacker+dojo&gl=…> . If you plan to attend, please fill in our sign-up form<https://spreadsheets.google.com/viewform?formkey=dEc5cW1fa3hjQ3JheVF5dHAwdT…>so we know we'll have room. Also, please consider joining the Hackathon's Google group<http://groups.google.com/group/bayhac> . We look forward to seeing you!

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UTF-8 in Haskell.
by Magicloud Magiclouds 27 Dec '10

27 Dec '10

Hi, Recently, I am reading ssh hackage (http://hackage.haskell.org/package/ssh) When at the part of deal with string, I got confused. I am not sure if this is a bug for the hackage, or I am just misunderstanding. An ascii char takes a Word8. So this works (LBS stands for Data.ByteString.Lazy): toLBS :: String -> LBS.ByteString toLBS = LBS.pack . map (fromIntegral . fromEnum) But a UTF-8 char takes a Int (Word32). Then I think the above code would break the data, right? If so, OK, then I think I could make a packInt which turns an Int into 4 Word8 first. Thus under all situation (ascii, UTF-8, or even UTF-32), my program always send 4 bytes through the network. Is that OK? -- 竹密岂妨流水过山高哪阻野云飞

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Internship in Tokyo
by employment 27 Dec '10

27 Dec '10

Tsuru Capital will be taking on interns again in 2011. The internship period is three months. There should be a position available again from January. The position is in Tokyo in a English speaking team of around 6 people. The company does algorithmic options market-making. The exact roles are not fixed yet but the projects will be something supportive to the trading activity. We are looking for Haskell programmers and preference will go to those with experience in Haskell programming. Without any experience it is unlikely that you will be selected. Our interns typically work on projects relating to reporting and statistics, using databases, web interfaces and iteratees on Linux systems. Performance tuning, optimization and testing are very important to us. We should emphasise that although we are doing trading the intern positions will be just tangential to the activity and we are really looking for people who want to do programming more than people who want to do finance. Those looking for actual trading experience may be disappointed. At the current time we are not looking expand the team working on the trading itself. Compensation would be a small package including flights and accommodation. If interested please send a resume to jobs(a)tsurucapital.com You can see more details about the company at tsurucapital.com and at the company page on LinkedIn For those who have applied before but would like to be considered again it may be worth just sending a reminder mail, giving the dates you are available.

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Intro to monad transformers
by michael rice 26 Dec '10

26 Dec '10

I lifted the code below from here: http://en.wikibooks.org/wiki/Haskell/Monad_transformers Since the wiki page doesn't say what needs to be imported, I'm guessing. Not sure what is happening. Maybe someone can tell me. Michael ============== import Control.Monad import Control.Monad.Trans.Class import Data.Char newtype (Monad m) => MaybeT m a = MaybeT { runMaybeT :: m (Maybe a) } instance Monad m => Monad (MaybeT m) where return = MaybeT . return . Just x >>= f = MaybeT $ do maybe_value <- runMaybeT x case maybe_value of Nothing -> return Nothing Just value -> runMaybeT $ f value instance Monad m => MonadPlus (MaybeT m) where mzero = MaybeT $ return Nothing mplus x y = MaybeT $ do maybe_value <- runMaybeT x case maybe_value of Nothing -> runMaybeT y Just value -> runMaybeT x instance MonadTrans MaybeT where lift = MaybeT . (liftM Just) instance Show (MaybeT m a) getValidPassword :: MaybeT IO String getValidPassword = do s <- lift getLine guard (isValid s) return s isValid :: String -> Bool isValid s = (length s > 8) && ((filter isAlphaNum s) == s) && any isDigit s && any isAlpha s askPassword :: MaybeT IO () askPassword = do lift $ putStrLn "Insert your new password:" value <- getValidPassword lift $ putStrLn "Storing in database..." ============= [michael@localhost ~]$ ghci GHCi, version 6.12.3: http://www.haskell.org/ghc/ :? for help Loading package ghc-prim ... linking ... done. Loading package integer-gmp ... linking ... done. Loading package base ... linking ... done. Loading package ffi-1.0 ... linking ... done. Prelude> :l test5 [1 of 1] Compiling Main ( test5.hs, interpreted ) Ok, modules loaded: Main. *Main> askPassword Loading package transformers-0.2.2.0 ... linking ... done. *** Exception: stack overflow *Main>

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