Re: [Haskell-cafe] pointer equality
On Wed, 20 Jul 2011 12:48:48 -0300 Thiago Negri evoh...@gmail.com wrote: Is it possible to implement (==) that first check these thunks before evaluating it? (Considering both arguments has pure types). E.g., Equivalent thunks, evaluates to True, does not need to evaluate its arguments: [1..] == [1..] Thunks are just expressions and equality of expressions is undecidable in any Turing-complete language (like any general-purpose programming language). Note that syntactical equality is not sufficient because (==) should be referentially transparent. Pedro ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Input and output of mathematical expressions
On Thu, 9 Jun 2011 16:23:20 +0200 Jacek Generowicz jacek.generow...@cern.ch wrote: Greetings Cafe, What would you recommend as a Haskell-based means of interactively reading and writing mathematical formulae? As a toy example, what might I use to write a program which presents the user with Please simplify the expression: \pi x^2 + 3\pi x^2 (Where the TeX-style expression would be presented with a greek pi and superscript twos on the xs.) The user should then have the ability to reply with something that looks like the result of TeXing 5 \pi x^2 Whatever means the user uses to enter this expression, he should be able to preview the typeset version of his input before submitting. Any ideas? Thanks. Regarding the rendering math formulas on web browsers: you might want to have a look at MathJax (http://www.mathjax.org/). You can use LaTeX or MathML and will work in most browsers (even if when they don't natively support MathML). Pedro ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Distributing Haskell GUI apps for Windows/MacOS?
Hello, I've implemented a Haskell program to play a modern board game. I've put it on Hackage (http://hackage.haskell.org/package/hstzaar) but would also like to distribute to people without the Haskell development tools. Are there any easy ways of making installers for Windows/MacOS (preferably using free or open source tools)? Best regards, Pedro ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Haskell on-line judge for Programming Challenges / Contests ?
On Thu, 14 Apr 2011 13:29:48 +0400 Dmitri O.Kondratiev doko...@gmail.com wrote: Hello, I am looking for a site providing on-line automatic judge for Programming Challenges that can be coded in Haskell. For example this site: http://www.programming-challenges.com provides lots of interesting programming provlems: http://www.programming-challenges.com/pg.php?page=index Yet, unfortunately only C, C++ and Java code can be submitted, judge on that site can't compile and run Haskell code. Is there any similar online judge for Haskell? Thanks, Dmitri Althpugh not Haskell-specific, the Mooshak system for managing programming contests on the Web allows you to configure most programming languages: http://mooshak.dcc.fc.up.pt/ It's been developed our CS department and used internally and externally for programming contests and courses. Pedro ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Proving correctness
On Sat, 12 Feb 2011 19:38:31 +0530 C K Kashyap ckkash...@gmail.com wrote: Anyway, how can one go about explaining to an imperative programmer with no FP exposure - what aspect of Haskell makes it easy to refactor? Like other people have said, the static type system is a major factor. It's true that Haskell's type system is more advanced than most imperative languages but it's also the often not mentioned that static typing gives a stronger check in a functional language than an imperative ones even in simple cases. This is because all input and output data flow is type checked in a function application, whereas imperative side effects might escape checking. For example, the type signature for a variable swapping procedure in C: void swap(int *a, int *b) This will still type check even if it modified only one of the argument references. However, if written functionally, it must return a pair: swap :: (Int,Int) - (Int,Int) Now the type checker will reject any implementation that fails to return a pair of results in every case. Of course for a trivial example like swap this is easy to ensure in any imperative language, but for more complex programs it is actually quite common to forget some update some component of the state. BTW, I found this and other interesting reflections on the avantages of FP vs. imperative in Martin Oderski's book Programming in Scala. Best regards, Pedro Vasconcelos ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Proving correctness
On Mon, 14 Feb 2011 12:54:55 +0100 Tillmann Rendel ren...@informatik.uni-marburg.de wrote: This benefit of explicit input and output values can interact nicely with parametric polymorphism: swap :: (a, b) - (b, a) This more general type signature makes sure we are not just returning the input values unswapped. Good point. This is a good reason why it's good practice to look for possiblity of writing more general functions and types even when they end up being used in a single instance. Pedro ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Proving correctness
On Mon, 14 Feb 2011 15:07:01 +0100 Gábor Lehel illiss...@gmail.com wrote: I'm not completely sure, but I suspect another part of it (or maybe I'm just rephrasing what you said?) has to do with the fact that in Haskell, basically everything is an expression. Yes, the fact that control statements (e.g. if-then-else) are expressions makes type checking much more effective. However, I think this is somewhat lost when programming imperative code in Haskell using a state or I/O monad (because a monadic type such as IO t does not discriminate what effects might take place, only the result type t). Of course one can use a more specialized monad (such ST for mutable references, etc.). I don't think that my imperative programs are automatically made more correct by writing them as monadic code in Haskell --- only that in Haskell I can opt for the functional style most of the time. BTW (slightly off topic) I found particularly annoying when teaching Python to be forced to use an imperative style (e.g. if-then-else are always statements). Scala is must better in this regard (altought it is not purely functional language); a statement is simply an expression whose return is unit. Pedro ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Haskell, C and Matrix Multiplication
On Mon, 17 Jan 2011 07:45:04 + Blake Rain blake.r...@gmail.com wrote: So, after drinking some coffee and having a smoke, I started the Haskell version: [foldl (+) 0 $ zipWith (*) x y | x - m1, y - transpose m2] I don't think this is correct; it's type is (Num a) = [[a]] - [[a]] - [a] rather than the expected (Num a) = [[a]] - [[a]] - [[a]] How about: mult m1 m2 = [[foldl (+) 0 $ zipWith (*) x y | y-transpose m2] | x-m1] Regarding performance: did you make sure you're forcing the evaluation of the result matrix? Regards, Pedro ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Haskell, C and Matrix Multiplication
On Mon, 17 Jan 2011 10:38:30 + Blake Rain blake.r...@gmail.com wrote: So sorry, I meant: mult :: (Num a) = [[a]] - [[a]] - [a] mult m1 m2 = [foldl (+) 0 $ zipWith (*) x y | x - m1, y - transpose m2] Shouldn't the result of multiplying an (n,k)-matrix by an (k,m)-matrix be an (n,m)-matrix? The way you've written it the result will be a list of length n*m. Regarding performance: did you make sure you're forcing the evaluation of the result matrix? I thought I was. It's printing the results as expected. Could be I'm just imagining I am and suffering a delusion or misunderstanding. Another issue to look out for is let-floating: if you perform the same matrix multiplication repeatedly, the compiler might very well lift the matrix calculation outside the loop. Pedro ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Not in scope: type constructor or class `Map'
On Thu, 30 Dec 2010 08:01:01 -0800 (PST) michael rice nowg...@yahoo.com wrote: Not sure what's going on here. Doesn't like line 5, the type statement. And what's with the semicolons in that line and in function main? import Control.Monad.Reader import qualified Data.Map as Map import Data.Maybe type Bindings = Map String Int; . The right hand side should be Map.Map String Int; alternatively add an unqualified import above for just the Map type: import Data.Map(Map) The semicolon is optional---the layout rule will insert it if you leave it out. Pedro ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
