November 2009 - Haskell-Cafe

About Regex question
by zaxis 05 Nov '09

05 Nov '09

import Text.Regex.Posix import Data.Char import Data.List guessKeys line = concat $ intersperse "-" (modifiers ++ [map toLower key]) where modifiers = map (!!1) (line =~ "(mod|shift|control)Mask") (_, _, _, [key]) = line =~ "xK_(\\w+)" :: (String, String, String, [String]) aaa line = map (!!1) (line =~ "(mod|shift|control)Mask") bbb line = line =~ "xK_(\\w+)" :: (String, String, String, [String]) In ghci >guessKeys "modF1" "*** Exception: Test.hs:8:10-85: Irrefutable pattern failed for pattern (_, _, _ , [key]) > aaa "modF1" <interactive>:1:0: No instance for (Text.Regex.Base.RegexLike.RegexContext Regex [Char] [[a]]) arising from a use of `aaa' at <interactive>:1:0-10 > bbb "modF1" ("modF1","","",[]) I just extract the functions from where clause of guessKeys. But why are there different running result ? Sincerely! ----- fac n = foldr (*) 1 [1..n] -- View this message in context: http://old.nabble.com/About-Regex-question-tp26211696p26211696.html Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

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Re: What's the deal with Clean?
by John Lato 05 Nov '09

05 Nov '09

Hi Bulat, I hope you don't mind me asking this, but when you state that "idiomatic haskell is much slower than idiomatic C", what do you mean by "much slower"? 2 times? 3 times? 20 times? Order of magnitude? I'm not going to disagree with you, however it seems to me the gap is closing. My opinion may be colored by having just examined Masayuki Takagi's SPH code, which I considered very idiomatic Haskell and for me performed equivalently to his C++ code with the changes I detailed in an earlier email (none of which involved any substantial changes to his code or style in my opinion). Best, John > From: Bulat Ziganshin <bulat.ziganshin(a)gmail.com> > > i don't say that haskell is slow for real programs. i say that > idiomatic haskell is much slower than idiomatic C and these papers > demonstrates how much, for simple computation-intensive loops > > -- > Best regards, > Bulat mailto:Bulat.Ziganshin@gmail.com >

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Base classes can be _ELIMINATED_ with interfaces
by Shelby Moore 05 Nov '09

05 Nov '09

I was correct before, except I conflated the word "extended" with "eliminated" in my mind: http://lambda-the-ultimate.org/node/1277#comment-51723 The most robust solution to Tim Sweeney's problem is to rethink what a "class" should be: http://www.haskell.org/haskellwiki/?title=Why_Haskell_matters&oldid=24286#H… In Haskel, subtyping is done with Module. A "type class" is a polymorphic (relative to data type) interface, and the polymorphism is strictly parameterized for the client/consumer of the interface, i.e. the data type is known to the function that inputs the interface AT COMPILE TIME: http://www.haskell.org/~pairwise/intro/section2.html#part3 If all the functions/methods of package Engine input exclusively interfaces (i.e. Engine.IActor), then client package GearsOfWar can create a new subtype GearsOfWar.Actor which also implements the interface Engine.IActor (and perhaps even more interfaces). A problem with virtual (runtime pointer) inheritance is that it hides the subclass from the compiler. Polymorphic interfaces are missing from C++ and one can sort of hack an emulation with abstract classes, private delegates, and templates: http://lambda-the-ultimate.org/node/1277#comment-51806 In Haskell, parameterized polymorphic interfaces can be inherited, and this is knowable to the client function at compile time. Fundamental theorems tell us that any membership rule for a set can not be an absolute point, thus we should strive for typing and classing architecture which is the most granular (these are my summaries): * Russell's Paradox: there is no rule for a set that does not cause it to contain itself, thus all sets are infinitely recursive. * Liskov Substition Principle: it is an undecidable problem that subsets inherit. * Linsky Referencing: it is undecidable what something is when it is described or perceived. * Coase Theorem: there is no external reference point, any such barrier will fail. * Godel's Theorem: any formal theory, in which all arithmetic truths can be proved, is inconsistent. * 1856 Thermo Law: entire universe (a closed system, i.e. everything) trends to maximum disorder.

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ANNOUNCE: feldspar-compiler
by Emil Axelsson 05 Nov '09

05 Nov '09

On behalf of my project colleagues at ELTE University, I'm happy to announce the release of feldspar-compiler: http://hackage.haskell.org/package/feldspar-compiler This package contains the C code back-end for the Feldspar EDSL, announced yesterday: http://hackage.haskell.org/package/feldspar-language For more information, see http://feldspar.sourceforge.net/ / Emil

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representing Haskell objects in a uniform way
by Pasqualino "Titto" Assini 05 Nov '09

05 Nov '09

Hi, I am writing a little IPC system to make Haskell values and functions remotely invokable. To do so, I need (or so I believe) to make my objects accessible via a generic interface as in: class AFun f where afun :: Data a => f -> ([Dynamic] -> a) So my generic object is something that takes an array of parameters, that being Dynamic can be anything, and returns a Data, that I can easily serialise and send back on the wire. I start by defining an instance for functions: instance (Typeable a,AFun b) => AFun (a->b) where afun f (p:ps) = let Just v = fromDynamic p in afun (f v) ps afun _ _ = error "Too few arguments" So far so good, but when I try to define an instance for values: instance Data v => AFun v where afun f [] = f afun _ _ = error "Too many arguments" I get: Couldn't match expected type `a' against inferred type `v' `a' is a rigid type variable bound by the type signature for `afun' at /home/titto/.quid2/state/ubuntu.local.8080/wikidata/haskell/package/haskelld/src/HaskellD/Test.hs:7:17 `v' is a rigid type variable bound by the instance declaration .... Why is that? a and v are both declared to be a Data, why should they not match? The full code follows: {-# LANGUAGE FlexibleInstances ,UndecidableInstances ,OverlappingInstances #-} import Data.Data import Data.Dynamic class AFun f where afun :: Data a => f -> ([Dynamic] -> a) instance (Typeable a,AFun b) => AFun (a->b) where afun f (p:ps) = let Just v = fromDynamic p in afun (f v) ps afun _ _ = error "Too few arguments" instance Data v => AFun v where afun f [] = f afun _ _ = error "Too many arguments" Thanks in advance, titto

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debugging
by Mohamed Ayed 05 Nov '09

05 Nov '09

can you please let me know how to debug Haskell programs? I got the observe module but it is complainnig about some "concurrent " moduel that it can not find . Also, how to change the path that Hugs uses to search for files ?

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question
by Mohamed Ayed 04 Nov '09

04 Nov '09

Hi, I want to write a program that will take an AST and a cost vector that represents a cost for each operation and produce a simplified version of the tree based on cost reduction. Can any one give me some ideas ? Thanks

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Re: How to optimize the performance of a code in Haskell?
by John Lato 04 Nov '09

04 Nov '09

For me, the Haskell version is about 1.5x slower than the C++ version, 19.6 seconds compared to 12.9. There wasn't an appreciable difference between -O and -O2. Which compiler (and version) are you using? I got: john$ g++ -O2 -o sph sph.cpp john$ time ./sph 300 real 0m12.914s user 0m11.015s sys 0m0.326s john$ g++ --version i686-apple-darwin9-g++-4.0.1 (GCC) 4.0.1 (Apple Inc. build 5490) Copyright (C) 2005 Free Software Foundation, Inc. and john$ ghc --make -O2 sph.hs john$ time ./sph 300 real 0m19.606s user 0m18.933s sys 0m0.304s john$ ghc --version The Glorious Glasgow Haskell Compilation System, version 6.10.4 on a recent model MacBook, OSX 10.5.8. Cheers, John > > Message: 4 > Date: Wed, 04 Nov 2009 17:53:49 +0100 > From: Kalman Noel <noel.kalman(a)googlemail.com> > Subject: [Haskell-cafe] Re: How to optimize the performance of a code > in Haskell? > To: Masayuki Takagi <kamonama2(a)gmail.com> > Cc: Haskell Cafe <haskell-cafe(a)haskell.org> > Message-ID: <4AF1B19D.2010403(a)googlemail.com> > Content-Type: text/plain; charset=ISO-8859-15; format=flowed > > (I take it you accidently wrote to fa.haskell, which is just a mirror of -cafe > and -beginners, so I'm cc-ing the Café with a full quote.) > > Masayuki Takagi: >> I'm writing fluid simulation programs with SPH(Smoothed particle >> hydrodynamics) in Haskell and C++. (The purpose that I write in two >> languages is to make a workflow that first i write in Haskell for >> rapid prototyping and accuracy then rewrite in C++ for performance.) >> >> I've compared them in performance. Then, although I have already done >> optimization with profiler, the Haskell code is 20 times slower than >> the C++ code. >> >> I think the performance of the Haskell code is so slow that there can >> be room for optimization. I'm happy if the Haskell code work 3 times >> slower than the C++ code at worst. >> >> How can I make the Haskell code faster? >> What information should I refer? >> >> The codes are here: >> http://kamonama.sakura.ne.jp/sph/20091101/sph.hs.zip >> http://kamonama.sakura.ne.jp/sph/20091101/sph.cpp >> >> To run the code in Haskell: >> $ ghc --make -O sph.hs >> $ ./sph 300 >> (300 is the time step to be conputed) >> >> To run the code in C++: >> $ g++ -O2 -o sph sph.cpp >> $ ./sph 300 >> (300 is the time step to be conputed) >> >> thanks >> > > I've not looked at the code, but you'll want ghc to do better optimizations > than -O. -O2 is what you should use in general. Also, number-crunching often > profits from -fexcess-precision. >

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Interactive chatbot
by Torsten Otto 04 Nov '09

04 Nov '09

Hi! My students have the task to program an interactive chatbot. We have run into a problem that I can't solve either: When we read the user's input through > t <- getLine it is not possible to delete typos before hitting enter and thereby sending the input off to the system (at least in OS X, bash). I didn't find that terribly problematic, but of course it is a bit of a show stopper from their point of view. The input is then used to generate a reply in purely functional code, and the reply sent to the command line via putStr. Is there a more clever way to interact with the user that would allow editing ones text before sending it to the bot? I guess we could try with a website, but don't know off hand how to do that, either, although I've seen beautiful webservers made in Haskell... Regards, Torsten Otto

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Announce: language-python version 0.2 now available
by Bernie Pope 04 Nov '09

04 Nov '09

I'm pleased to announce that version 0.2 of the language-python package is now available on hackage: http://hackage.haskell.org/package/language-python language-python provides lexical analysis and parsing for Python. Major features of this release: - Support for versions 2.x and 3.x of Python (previously only 3.x was supported). - Lexical tokens and AST nodes are annotated with accurate source span information. - Comments are retained as tokens, and are collected by the parser. Main shortcomings of this release: - Support for Unicode is limited (waiting on Unicode support in Alex). - It has only undergone minimal testing (testing infrastructure is still being built). I've also written a small client of the package, called language-python-colour, which renders Python source code as XHTML for colouring etc. The main purpose of this is to demonstrate how to use language-python, and the utility of accurate source spans. http://hackage.haskell.org/package/language-python-colour Example output: http://www.cs.mu.oz.au/~bjpop/code/lsystem.py.html Cheers, Bernie.

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