September 2005 - Haskell-Cafe

[Announce] CabalFind 0.1
by Dimitry Golubovsky 10 Sep '05

10 Sep '05

Some time ago, I expressed an idea that certain search engines may be used to collect information about existing .cabal packages whose developers made them indexable by those engines. Finally, I have written an experimental library, CabalFind. The library provides a generalized interface to search engines, and also sample implementations for Google and Yahoo* I haven't set it up as a separate project, say, on SourceForge because I am not sure whether it is feasible to do so. Instead, I am suggesting that the developers of Haskell Library Infrastructure decide whether this library may serve as a basis for library infrastructure maintenance utilities. The library may be downloaded at darcs get http://www.golubovsky.org/repos/cabalfind/ or http://www.golubovsky.org/repos/cabalfind/cabalfind.tar.gz The .cabal file for the package is tailored for GHC 6.2.2. For use with 6.4, changes may be necessary, as the XML Toolbox for 6.4 no longer includes the HTTP module, so HTTP.cabal needs to be included in build-depends. More information provided at http://haskell.org/hawiki/CabalFind Any feedback is appreciated. Dimitry Golubovsky Middletown, CT ------ * Only these two search engines are found to be able to search by URI suffix (.cabal in our case); without this feature package search becomes very inefficient.

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ghc for sunos
by Brian McQueen 09 Sep '05

09 Sep '05

Is there a ghc for SunOS? I don't see on in the list. How hard would it be to make one? I'm surrounded by Sun boxes here.

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Re: [Haskell] Mixing monadic and non-monadic functions
by Ben Lippmeier 08 Sep '05

08 Sep '05

> Do you disagree with that > assertion? Or are you just saying that the syntax change as I propose > it is called "effect analysis"? Sorry, I thought you were talking about the usability of IO and State monads in particular, not more general ones. When I was talking about effect analysis it was along the same lines as how (fun1 :: a -> IO b) and (fun2 :: a -{IO}> b) are basically equivalent - though IMHO fun2 can be easier to use because it's type is a bit more orthognal to a "pure" function (fun3 :: a -> b). Ben.

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Practical introduction to monads
by Paul Moore 07 Sep '05

07 Sep '05

I've started learning Haskell, and I'm going through all the tutorial material I can find - there's a lot of good stuff available. One thing I haven't found a really good discussion of, is practical examples of building monads. There's plenty of discussion of the IO monad, and the state monad, and a lot of good theory on monads, but although I've seen tantalising statements about how powerful the ability to define your own monads can be, but no really concrete examples - something along the lines of - here is problem X - this might be our first cut at coding it - we can abstract out this stuff, as a monad - see how the code looks now, how much cleaner it is (I've seen this type of model developing a state monad, but I'm looking for a more application-specific approach). Also, a lot of (non-IO) monad code I've seen seems to revolve around higher order features like monad combinators, which, while certainly powerful, still make my head explode. On the other hand, monadic IO feels entirely comprehensible, revolving round do-notation, and looking reassuringly sequential and imperative :-) Surely there are other uses of monads which can feel familiar to non-functional programmers? Otherwise, I'm still left with the (inaccurate, and unhelpful) impression that "IO is a badly integrated exception"... I don't know if the above makes sense. I've struggled with this for a while now, and haven't managed to express my confusion any more clearly - for which I apologise. I'd appreciate any pointers or explanations. FWIW, I've read (among other papers) "Why Functional Programming Matters", "A Gentle Introduction to Haskell", Hal Daume's "Yet Another Haskell Tutorial", Simon Peyton Jones' "Tackling the Awkward Squad", and "Haskell: The Craft of Functional Programming". I don't claim to have digested all of this - indeed, there are probably many areas I've only skimmed - so pointers to areas in these documents I might have missed would be appreciated. On the other hand, pointers to papers I've missed altogether would also be gratefully received. Thanks in advance for any help, Paul. -- There is a theory which states that if ever anybody discovers exactly what the Universe is for and why it is here, it will instantly disappear and be replaced by something even more bizarre and inexplicable. There is another theory which states that this has already happened. -- Douglas Adams

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Re: [Haskell-cafe] Concurrency question
by Dmitry V'yal 07 Sep '05

07 Sep '05

Donald Bruce Stewart wrote: > Can I ask if just using a `seq` works as well? That was enough (now I > recall) for the watchdog code I posted earlier. > > -- Don In my case this doesn't work >timeout :: DeepSeq a => Int -> IO a -> IO (Maybe a) >timeout n t = do res <- par_io timer thr --timer > return res > where thr = do res <- t > return $! Just res > timer = do threadDelay $ n * 1000 > return Nothing It's probably because "resolve" function has type > resolve :: CNF -> (Bool, [(Int,Int)]) So it should be sequenced deeper.

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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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