- Haskell-Cafe - Haskell.org

Concurrency question
by Dmitry Vyal 06 Sep '05

06 Sep '05

Hi, everyone! I have a function, which sometimes takes a long time to compute or even may loop forever. So I want to limit it in time somehow. I tried to run it in another thread in order to kill it after its time lapsed. But it seems to lock out other threads so they can't terminate it. I wonder is there some clever way of dealing with such situation (running a computation in background for specific time) ? Thanks a lot.

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haskell-mode in windows
by Abigail 06 Sep '05

06 Sep '05

Hello, It's not about Haskell directly but I would like to know how can I customize emacs in haskell-mode when i'm using emacs for windows. Please, I'm trying to do it but it doesn't do anything. Thanks. :D Abigail. __________________________________________________ Correo Yahoo! Espacio para todos tus mensajes, antivirus y antispam ¡gratis! Regístrate ya - http://correo.espanol.yahoo.com/

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[beta] cabal-get: install haskell packages automatically
by Isaac Jones 05 Sep '05

05 Sep '05

Beta testers wanted! Lemmih (mostly) has been working on a tool which will download and install Haskell packages and their dependencies. This tool should work for any cabal-compatible package. We're looking for beta testers to try out "cabal-get" (for downloading and installing) and "cabal-put" (for uploading to the central repository). We're also looking for more developers to work on this. The home, for now is here: http://hackage.haskell.org/ModHackage/Hackage.hs?action=home and to get cabal-get, grab the bootstrapper: darcs get http://hackage.haskell.org/darcs/cabal-get-bootstrap And execute the Install.lhs file. You may have to modify it slightly if you don't like the defaults. After you install cabal-get, you should be able to install cabal-put by saying: cabal-get update && cabal-get install cabal-put Darcs repositories for all the tools are here: http://hackage.haskell.org/darcs/ Questions or problems? Email me or Lemmih. peace, isaac

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A better, fair, and terminating backtracking monad [Was: Concurrency question]
by oleg＠pobox.com 05 Sep '05

05 Sep '05

Dmitry Vyal wrote: > Currently I'm playing with theorem-proving using resolution. So I need > some technique to break a computation, if takes too long. When it comes to resolution proofs, perhaps a better approach is to use a better monad that natively offers termination guarantees. For example, http://pobox.com/~oleg/ftp/Computation/monads.html#fair-bt-stream First of all, the monad can let succeed the computations that surely diverge in List and similar monads. As the code at the end of that file shows, the monad avoids depth-first traps, and can even handle left-recursion at the monadic level. Furthermore, the monad has the inherent `timing' metric: Incomplete eliminations. Each elimination is bounded in time. One can specify the upper bound on the number of eliminations and thus force the termination if no solution was found thus far. It is possible to modify the run function of the monad to return the continuation once the counter has expired. So, one can run the monad for a bit more, if desired. In contrast to iterative deepening, running the continuation does _not_ repeat any of the previous work. We have used the FBackTrack monad to implement a Datalog-like system (with top-down. resolution-based search however), and we could successfully solve the problem of finding the transitive closure in a _cyclic_ graph (the poster problem for post-Prolog systems like XSB). No tabling was needed. A scheme version of the monad was used in a leanTAP theorem prover (for the full first-order predicate logic): http://cvs.sourceforge.net/viewcvs.py/kanren/kanren/mini/leanTAP.scm http://cvs.sourceforge.net/viewcvs.py/kanren/kanren/mini/book-si.scm The original leanTAP prover was meant to be used with iterative deepening. Our implementation removes the limiting counter from the leanTAP (which throttles instantiations of a universally quantified formula); the monad has enough strength to find the solution anyway (Pelletier problems 43 and 38).

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Newbie question
by André Vargas Abs da Cruz 02 Sep '05

02 Sep '05

Hi everyone, I think this is a totally newbie question as i am a complete novice to Haskell. I am trying to write down a few programs using GHC in order to get used with the language. I am having some problems with a piece of code (that is supposed to return a list of lines from a text file) which I transcribe below: module Test where import IO readDataFromFile filename = do bracket (openFile filename ReadMode) hClose (\h -> do contents <- hGetContents h return (lines contents)) The question is: if i try to run this, it returns me nothing (just an empty list). Why does this happen ? When i remove the "return" and put a "print" instead, it prints everything as i expected. Thanks in advance André

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RE: [Haskell-cafe] Monadic vs "pure" style (was: pros and cons of sta tic typing and side effects)
by Bayley, Alistair 01 Sep '05

01 Sep '05

> From: Duncan Coutts [mailto:duncan.coutts@worcester.oxford.ac.uk] > > This is often a misconception, that just because you find you need to > 'do' something in the middle of your algorithm, that you need > to convert it wholly to monadic style. Yes. However, Wadler makes a convincing (at least to me) case that the monadic style is easier to extend. The code changes for the monadic style appear to be more localised. Something else I noticed about my non-monadic code was the way I was threading state through functions. I was tempted to introduce a State monad to make this easier to manage, but then I decided to try mutable arrays instead, so that experiment was not attempted. So it might well have been better in monadic style anyway, even with immutable arrays. I'm conscious that for most (?) monads, monadic code can be invoked from non-monadic code. I'm only aware of the IO monad as a one-way trap. So changing code from pure to monadic doesn't necessarily involve program-wide changes, unless the monad you're introducing happens to be IO. In my array example, I introduced STArrays, but the main interface remained pure (non-monadic), which was my goal. I was also wondering what the disadvantages of monadic style are? Are there compiler optimisations which are not possible with monadic code? Alistair. ----------------------------------------- ***************************************************************** Confidentiality Note: The information contained in this message, and any attachments, may contain confidential and/or privileged material. It is intended solely for the person(s) or entity to which it is addressed. Any review, retransmission, dissemination, or taking of any action in reliance upon this information by persons or entities other than the intended recipient(s) is prohibited. If you received this in error, please contact the sender and delete the material from any computer. *****************************************************************

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Re: [Haskell] pros and cons of static typing and side effects ?
by Martin Vlk 01 Sep '05

01 Sep '05

Hi folks, have you read this paper: http://www-i2.informatik.rwth-aachen.de/Staff/Current/michaelw/sttt-ml-hask… It contains descriptions of lots of real-world problems and how easily they are solved with Haskell (and ML, because the paper compares the two languages). Among other things it touches on the static typing pros and cons in a way that anyone who ever did serious programming will understand. vlcak

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embedding prolog in haskell.
by Keean Schupke 01 Sep '05

01 Sep '05

Does anyone know if the source code for the embedded prolog (by Silvija Seres & Michael Spivey) is available for download from anywhere? I have read the paper and found some of the types are wrong, some critical definitions are missing, and the definition of unify is missing. Regards, Keean.

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Haskell thread preemption
by Dmitry Vyal 31 Aug '05

31 Aug '05

Hello, all. I'm experimenting with concurrent haskell using GHC 6.4. I wrote these fuctions as described in "Tackling the Awkward Squad": par_io :: IO a -> IO a -> IO a par_io t1 t2 = do c <- newEmptyMVar :: IO (MVar a) id1 <- forkIO $ wrapper c t1 id2 <- forkIO $ wrapper c t2 res <- takeMVar c killThread id1 killThread id2 return res where wrapper :: MVar a -> IO a -> IO () wrapper mvar io = do res <- io putMVar mvar res timeout :: Int -> IO a -> IO (Maybe a) timeout n t = do res <- par_io thr timer return res where thr = do res <- t return $ Just res timer = do threadDelay n return Nothing Now, then I try something like timeout 1000000 (print $ cycle "test") it behaves correctly: repeatingly prints "test" for one second and then returns Nothing. But then I try this: timeout 1000000 (print $ 2^2^2^2^2^2) it hangs for about eight seconds. Currently I'm playing with theorem-proving using resolution. So I need some technique to break a computation, if takes too long. It seems that using "timeout" there does nothing. I waited for several minutes for invalid theorem and timeout didn't expire. "Control.Concurrent" page of "Haskell Hierarchical Libraries" says, that a thread may be pre-empted whenever it allocates some memory, and that tight loops which do no allocation tend to lock out other threads. But resolution function, i wrote, hogs a *lot* of memory and (print $ 2^2^2^2^2^2) does either. So i'm curious about this behavior of timeout. So what is the correct way of running such computations in parallel? Maybe I'm missing something?

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RE: [Haskell-cafe] Haskell poker server
by Bayley, Alistair 31 Aug '05

31 Aug '05

> From: Cale Gibbard [mailto:cgibbard@gmail.com] > > Well, here's an attempt at a start on a similar mechanism for Haskell: > > ---------- (start Packet.hs) > module Packet where > > import Data.Bits > import Data.Word > > class Packet a where > readPacket :: [Word8] -> (a, [Word8]) ... There's a request on LtU for a similar ability (somewhat wider in scope, perhaps): http://lambda-the-ultimate.org/node/view/938 There must be a "gap in the market" :-) Alistair. ----------------------------------------- ***************************************************************** Confidentiality Note: The information contained in this message, and any attachments, may contain confidential and/or privileged material. It is intended solely for the person(s) or entity to which it is addressed. Any review, retransmission, dissemination, or taking of any action in reliance upon this information by persons or entities other than the intended recipient(s) is prohibited. If you received this in error, please contact the sender and delete the material from any computer. *****************************************************************

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