February 2009 - Haskell-Cafe

Help with Stack Space Overflow / Memory Issues
by Travis B. Hartwell 15 Feb '09

15 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 15 Feb '09

15 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

3 2

Intergalactic Telefunctors
by Gregg Reynolds 15 Feb '09

15 Feb '09

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

6 10

Race condition possible?
by Peter Verswyvelen 15 Feb '09

15 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

8 10

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

4 4

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 14 Feb '09

14 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