September 2008 - Haskell-Cafe

control-timeout with gtk
by Marco Túlio Gontijo e Silva 20 Sep '08

20 Sep '08

Hello, I've written a simple sequencer[0] using gtk2hs and control-timeout[1]. In GHCi it works fine, but when I compile it, the timeouts generated by control-timeout are not executed. Actually, when I use the keyboard a lot, at some time they got executed. I thought it could be that these two packages are incompatible, but the I wondered why they work in GHCi. Any ideas? Greetings. 0: http://marcot.iaaeee.org/capoeira 1: http://hackage.haskell.org/cgi-bin/hackage-scripts/package/control-timeout -- marcot Página: http://marcotmarcot.iaaeee.org/ Blog: http://marcotmarcot.blogspot.com/ Correio: marcot(a)riseup.net XMPP: marcot(a)jabber.org IRC: marcot(a)irc.freenode.net Telefone: 25151920 Celular: 98116720 Endereço: Rua Turfa, 639/701 Prado 30410-370 Belo Horizonte/MG Brasil

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ANNOUNCE: Extensible and Modular Generics for the Masses (EMGM) 0.1
by Sean Leather 20 Sep '08

20 Sep '08

Extensible and Modular Generics for the Masses ============================================== Extensible and Modular Generics for the Masses (EMGM) is a library for generic programming in Haskell using type classes. This is the initial release of a maintained library for EMGM. Other versions have previously existed in various states from various sources. We plan to continue updating and maintaining this version. Visit the home page: http://www.cs.uu.nl/wiki/GenericProgramming/EMGM Features -------- The primary features of EMGM include: * Datatype-generic programming using sum-of-product views * Large collection of ready-to-use generic functions * Included support for standard datatypes: lists, Maybe, tuples * Easy to add support for new datatypes * Type classes make writing new functions straightforward in a structurally inductive style * Generic functions are extensible with ad-hoc cases for arbitrary datatypes * Good performance of generic functions The features of this distribution include: * The API is thoroughly documented with Haddock * Fully tested with QuickCheck and HUnit * Program coverage ensures that all useful code has been touched by tests * Tested on both Mac and Windows systems Requirements ------------ EMGM has the following requirements: * GHC 6.8.1 - It has been tested with versions 6.8.3 and 6.9.20080916. * Cabal library 1.2.1 - It has been tested with versions 1.2.3 and 1.4.0.1. Download & Source ----------------- Use caball-install: cabal install emgm Get the package: http://hackage.haskell.org/cgi-bin/hackage-scripts/package/emgm Get the source: svn checkout https://svn.cs.uu.nl:12443/repos/dgp-haskell/EMGM Examples -------- Check out the examples: https://svn.cs.uu.nl:12443/viewvc/dgp-haskell/EMGM/examples/ Bugs & Support -------------- Report issues or request features: http://code.google.com/p/emgm/ Discuss EMGM with the authors, maintainers, and other interested persons: http://www.haskell.org/mailman/listinfo/generics Credits ------- The research for EMGM originated with Ralf Hinze. It was extended with work by Bruno Oliveira and Andres Löh. More details of the library functionality were explored by Alexey Rodriguez. We are very grateful to all of these people for the foundation on which this library was built. The current authors and maintainers of EMGM are: * Sean Leather * José Pedro Magalhães * Alexey Rodriguez * Andres Löh

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performance of map reduce
by Manlio Perillo 20 Sep '08

20 Sep '08

Hi again. In http://book.realworldhaskell.org/read/concurrent-and-multicore-programming.… there is a map reduce based log parser. I have written an alternative version: http://paste.pocoo.org/show/85699/ but, with a file of 315 MB, I have [1]: 1) map reduce implementation, non parallel real 0m6.643s user 0m6.252s sys 0m0.212s 2) map reduce implementation, parallel with 2 cores real 0m3.840s user 0m6.384s sys 0m0.652s 3) my implementation real 0m8.121s user 0m7.804s sys 0m0.216s What is the reason of the map reduce implementation being faster, even if not parallelized? It is possible to implement a map reduce version that can handle gzipped log files? [1] These tests does not consider the "first run". For the first run (no data in OS cache), I have (not verified): 1) map reduce implementation, parallel with 2 cores real 0m3.735s user 0m6.328s sys 0m0.604s 2) my implementation real 0m13.659s user 0m7.712s sys 0m0.360s Thanks Manlio Perillo

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Hackage and HaXml situation
by John Goerzen 20 Sep '08

20 Sep '08

Hi everyone, We've got a bit of a problem in the community regarding HaXml. This has been brought up before, but never definitively resolved, and it's causing more and more trouble. Over at the HaXml homepage [1], it is stated that 1.13.3 is the most recent stable version of HaXml. I believe that Malcolm Wallace, HaXml maintainer, has confirmed this. Over at the HaXml hackage page [2], it would appear as if 1.19.4 is the most recent stable version of HaXml. There is no provision made to mark some versions as unstable, and to make 1.13.3 the default version downloaded from Hackage. There are API incompatibilities between 1.13.3 and 1.19.4 that make it impossible for my programs to support both. This is becoming a larger and more fragmenting problem. Arch Linux looked at the Hackage package and bundled up 1.19.4. Debian looked at the HaXml website and bundles 1.13.3. So we have a situation where I can write software that builds easily with one or the other, but not both. We really need a fix for this. Some things we could do would be: 1) Make a policy that Hackage is for the "most stable branch" of software only, and remove the 1.19.x series of HaXml. 2) Malcolm could decide to bless 1.19.x as stable, fixing the instant problem, but this would still leave the problem open for the future. 3) Add support to Hackage for labeling branches, and present the stable versions by default. Thoughts? [1] http://www.cs.york.ac.uk/fp/HaXml/ [2] http://hackage.haskell.org/cgi-bin/hackage-scripts/package/HaXml

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lazy strings and parallel read
by Manlio Perillo 20 Sep '08

20 Sep '08

Hi. After having read http://book.realworldhaskell.org/read/concurrent-and-multicore-programming.… I have a doubt about Data.ByteString.Lazy. Why getContents function don't use pread (or an emulation, if not available)? Thanks Manlio Perillo

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A round of golf
by Creighton Hogg 19 Sep '08

19 Sep '08

Hey Haskell, So for a fairly inane reason, I ended up taking a couple of minutes and writing a program that would spit out, to the console, the number of lines in a file. Off the top of my head, I came up with this which worked fine with files that had 100k lines: main = do path <- liftM head $ getArgs h <- openFile path ReadMode n <- execStateT (countLines h) 0 print n untilM :: Monad m => (a -> m Bool) -> (a -> m ()) -> a -> m () untilM cond action val = do truthy <- cond val if truthy then return () else action val >> (untilM cond action val) countLines :: Handle -> StateT Int IO () countLines = untilM (\h -> lift $ hIsEOF h) (\h -> do lift $ hGetLine h modify (+1)) If this makes anyone cringe or cry "you're doing it wrong", I'd actually like to hear it. I never really share my projects, so I don't know how idiosyncratic my style is.

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An interesting curiosity
by Andrew Coppin 19 Sep '08

19 Sep '08

I'm currently reading a C++ tutorial. (Don't ask!) Of course, C++ is that crazy language where *assignment* is actually an *operator*. Sick, sick people... The tutorial pointed out that "x = 4" is a legal assignment, but "4 = x" is not. "Obviously that would be silly." Now it occurs to me... in Haskell, "4 = x" is actually a perfectly legal thing to say. (!) It's pretty pointless, but it's legal. For example: foo x = let 4 = x in 2*x which, as best as I can tell, does exactly the same thing as (2*). In a similar vein, one might write foo x = do let 4 = x return (2*x) which also does more or less the same as (return). However, consider the following: foo :: Int -> Maybe Int foo x = do 4 <- return x return (2*x) This, it turns out, is an extremely odd way of checking whether x == 4. All of this works of course because in Haskell, "=" is not an assignment, it's a definition, and the RHS is not a variable, it's a pattern. And "4" is a perfectly legitimate pattern. Now, if only I could find a use for all this that borders on "useful"...! ;-)

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Re: Strongly Typed Memory Areas
by oleg＠okmij.org 19 Sep '08

19 Sep '08

John Van Enk wrote: > Was Iavor/Mark's paper ever implemented as a GHC extension? > "Strongly Typed Memory Areas" It turns out most of the functionality is already available in Haskell: Lightweight static resources, for safe embedded and systems programming http://okmij.org/ftp/Haskell/types.html#ls-resources Chung-chieh Shan's TFP2007 talk specifically contrasted code from Iavor/Mark's paper with Haskell code. Strongly typed memory areas are possible in Haskell today.

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Specifying a function type in a where clause.
by Rob deFriesse 19 Sep '08

19 Sep '08

I would like to know why I'm getting a particular compile time error message. In this program, I am specifying a function type on /combs'/ in the where clause: -------- module Main where import List (delete) combs :: Eq a => [a] -> Int -> [[a]] combs l 1 = map (\x -> [x]) l combs l n = foldl combs' [] l where combs' :: Eq a => [[a]] -> a -> [[a]] combs' acc x = let sl = delete x l in (map (\i -> x:i) $ combs sl (n-1)) ++ acc main = do print $ combs ["a","b","c","d"] 3 -------- I get this error message from GHC 6.8.3: cafe1.hs:9:43: Couldn't match expected type `a1' against inferred type `a' `a1' is a rigid type variable bound by the type signature for `combs'' at cafe1.hs:8:23 `a' is a rigid type variable bound by the type signature for `combs' at cafe1.hs:5:12 Expected type: [a1] Inferred type: [a] In the second argument of `delete', namely `l' In the expression: delete x l I don't understand why I'm seeing this. The type /a/ is the same type in /combs/ and /combs'/. I realize I can remove the type specification from /combs'/, and the code will compile. However, I'd like to get a better understanding of why GHC objects to this. Thank you, Rob.

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Re: HStringTemplate to generate html tables
by Sterling Clover 19 Sep '08

19 Sep '08

Hmm... an untested suggestion... <table> $cells:{ row | <tr>$row:{ cell | <td>$cell$</td>}$</tr>}$ </table> (i.e. nested iteration) or to encompass the table call inside, $cells:{ row | <tr>$row:{ cell | <td>$cell$</td>}$</tr>}:{<table>$it$</table>}$ Regards, Sterl. On Thu, Sep 18, 2008 at 8:16 AM, Thomas Hartman < thomashartman1(a)googlemail.com> wrote: > I am trying to use HStringTemplate for generating html tables, for use > in http://happstutorial.com:5001 > > The best I could do is below. Seems awfully kludgy. > > Can someone suggest a better way, which keeps me inside the > StringTemplate way of doing things? (Eg, no suggestions to use HTML.* > libraries, which are nice for some things but are a higher barrier to > entry than just jumping in and writing html.) > > Some kind of map structure... for each... ? > > Thanks for any help! > > thomas. > > {-# LANGUAGE NoMonomorphismRestriction #-} > import Text.StringTemplate > import Control.Applicative > import qualified Text.PrettyPrint as PP > > main = putStrLn . PP.render . toPPDoc $ tableFromCells cells > cells = [["mee","mah","moh"],["fee","fah","foh"]] > > -- This does generate an html table, but seems horribly obfuscated. > -- is there machinery for doing things like html table generation > -- in some better way than this hack where </end><start> > -- tags are used as a separator? > -- I would imagine something like > -- newSTMP "$cells:{<td>$it$</td>}:{<tr>$it</tr>}:{<table>$it</table>}" > -- which works a bit like a list comprehension > tableFromCells cells = setAttribute "cells" cells . optInsertTmpl > [("separator","</td><td>")] > $ newSTMP "$cells:\ > \{<tr><td>$it$</td></tr>}:\ > \{<table>$it$</table>}$"

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