- Haskell-Cafe - Haskell.org

Safe lists with GADT's
by Neil Mitchell 26 Feb '07

26 Feb '07

Hi I'm starting to play with GADT's, and I was trying to write a safe version of head and tail, on a safe version of lists. What I came up with is: > data ConsT a > data NilT > > data List a t where > Cons :: a -> List a b -> List a (ConsT b) > Nil :: List a NilT > > instance Show a => Show (List a t) where > show (Cons a b) = show a ++ ":" ++ show b > show Nil = "[]" > > safeHead :: List a (ConsT t) -> a > safeHead (Cons a b) = a > > safeTail :: List a (ConsT t) -> List a t > safeTail (Cons a b) = b > > safeMap :: (a -> b) -> List a t -> List b t > safeMap f Nil = Nil > safeMap f (Cons a b) = Cons (f a) (safeMap f b) Defining safeMap was trivial, but one thing I couldn't figure out was how to write something like fromList: fromList [] = Nil fromList (a:as) = Cons a (fromList as) fromList could obviously be anything such as reading from a file etc, where the input is not known in advance. Are there any techniques for this? In addition I was expecting to find some example like this in one of the papers/examples on GADT's, but I didn't. The Haskell wikibook has a slight example along these lines, but not with the recursive field in ConsT. [http://en.wikibooks.org/wiki/Haskell/GADT] Any help or hints would be appreciated, Thanks Neil

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Re: [Haskell] [Fwd: Re: Computer Language Shootout]
by Kirsten Chevalier 26 Feb '07

26 Feb '07

[redirecting to haskell-cafe, since this is getting to be a long discussion] On 2/26/07, Andrzej Jaworski <himself(a)poczta.nom.pl> wrote: > The examples I pointed to seem to share strong and relatively consistent > logic of a program. In case of large GA (e.g. Royal Road Problem) and IFP > (e.g. ADATE) SML was exhaustively proved to predict this logic much better. > Can you clarify what you mean by this? How do you formally prove that a programming language (rather than a specific implementation of one) performs better for a given problem? Or do you mean that a specific implementation of this problem in SML performed better than a specific implementation of it in Haskell? > In case of Algebraic Dynamic Programming C compiler seems to address > specific properties of a program more adequately which leads to substantial > improvements in optimization. It is important to stress that we deal in ADP > with strong logic of the domain application. Handcrafting C code for regular > applications does not show that kind of advantage, so it wouldn't leave > Haskell in the dust. In general declarative nature has the edge over C > craftsmanship (see: > http://www.clip.dia.fi.upm.es/papers/carro06:stream-interpreter-TR.pdf) > I've read a few of your posts and I still don't understand what you mean by a compiler "address[ing] specific properties of a program". Can you give an example of the kinds of program properties you're talking about, and explain how C compilers exploit them in a way that Haskell compilers currently fail to do? Cheers, Kirsten -- Kirsten Chevalier* chevalier(a)alum.wellesley.edu *Often in error, never in doubt "Religion is just a fancy word for the Stockholm Syndrome." -- lj user="pure_agnostic"

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Links in Haskell code in HaskellWiki
by Henk-Jan van Tuyl 26 Feb '07

26 Feb '07

L.S., Links in the Haskell code in the wiki pages point to wrong labels; for example, if 'print' is used, it is linked to: http://haskell.org/ghc/docs/latest/html/libraries/base/Prelude.html#v:print the colon should be "%3A": http://haskell.org/ghc/docs/latest/html/libraries/base/Prelude.html#v%3Apri… For '==', the link is: http://haskell.org/ghc/docs/latest/html/libraries/base/Prelude.html#v:== it should be: http://haskell.org/ghc/docs/latest/html/libraries/base/Prelude.html#v%3A%3D… (see for example http://haskell.org/haskellwiki/OOP_vs_type_classes ) Can someone correct this? -- Met vriendelijke groet, Henk-Jan van Tuyl -- http://Van.Tuyl.eu/ -- Using Opera's revolutionary e-mail client: https://secure.bmtmicro.com/opera/buy-opera.html?AID=789433

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reed-solomon or other ECC codes
by Bulat Ziganshin 26 Feb '07

26 Feb '07

Hello haskell-cafe, is there a haskell library for any ECC codes? -- Best regards, Bulat mailto:Bulat.Ziganshin@gmail.com

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Did quickchekc get dropped from ghc from 6.4 to 6.6?
by Thomas Hartman 26 Feb '07

26 Feb '07

According to http://www.cs.chalmers.se/~rjmh/QuickCheck/ Quickcheck is distributed with ghc. I seem to recall this came with ghc 6.4. After upgrading to ghc 6.6, however, I don't seem to have it anymore. Do I need to install it from cabal? If so, I assume this would start by wgetting http://hackage.haskell.org/packages/archive/QuickCheck/QuickCheck-1.0.tar.gz and building that. thomas. thartman@linodewhyou:~/learning/haskell/lists$ ghci ___ ___ _ / _ \ /\ /\/ __(_) / /_\// /_/ / / | | GHC Interactive, version 6.6, for Haskell 98. / /_\\/ __ / /___| | http://www.haskell.org/ghc/ \____/\/ /_/\____/|_| Type :? for help. Loading package base ... linking ... done. Prelude> :m Control.Monad Prelude Control.Monad> :m Test.QuickCheck Could not find module `Test.QuickCheck': Use -v to see a list of the files searched for. Prelude Control.Monad> :m Quickcheck Could not find module `Quickcheck': Use -v to see a list of the files searched for. Prelude Control.Monad>

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R wrapper in haskell
by Peng Zhang 26 Feb '07

26 Feb '07

Hi folks, My primary language is R, which is an imperative functional language. I start to learn haskell and try to use it instead when a project is time-consuming and it takes months in R. I like the language very much so far, but I do miss some important functions in R which can generate random numbers from all kinds of distributions. R provides a standalone library in c which contains all these functions and I try to write a wrapper for haskell. Since I am very new to haskell, I want to hear some suggestions to confirm that I am on the right track. The functions are the following: void set_seed(unsigned int, unsigned int); void get_seed(unsigned int *, unsigned int *); they can set or get seed for random number generator. For any distribution, we have four functions for it -- Normal distribution double dnorm4(double x, double mu, double sigma, int give_log) double pnorm5(double x, double mu, double sigma, int lower_tail,int log_p) double qnorm5(double p, double mu, double sigma, int lower_tail, int log_p) double rnorm(double mu, double sigma) where dnorm calculates density, pnorm calculates p-value, qnorm calculates quantile, and rnorm generates normal random variables. dnorm, pnorm, qnorm are easy since they don't have side-effect. I think I can just use the following: foreign import ccall "dnorm4" dnorm :: Double -> Double -> Double-> Int -> Double foreign import ccall "pnorm5" pnorm :: Double -> Double -> Double-> Int -> Int -> Double foreign import ccall "qnorm5" qnorm :: Double -> Double -> Double-> Int -> Int -> Double (Should I use CDouble or CInt here?) But for rnorm, if I use "foreign import ccall rnorm :: Double -> Double -> IO Double", then my main function should carry IO monad all the time. Maybe the better way is that I should "encapsulate" it into a state monad rng -> (rng, randomnumber). Therefore for rnorm, I should first write a wrapper in c like seed2 rnorm_wrapper(seed1, parameters){ set_seed(seed1); rnorm(parameters); get_seed(seed2); } and then write another wrapper for rnorm_wrapper in haskell. Can somebody tell me if this is the right approach? Thank you! Best, Peng

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haskell-art mailing list
by alex 26 Feb '07

26 Feb '07

Hi all, After a bit of discussion with Rohan Drape I've made a mailing list for inclusive discussion of 'artistic' uses of Haskell. The initial thought for it to be about Haskell sound and music in particular, but we decided to broaden it to include visual, robotic and related work as well. [Right now I'm seeing Haskell itself and all code written in it as high art, but I suppose this list is for discussion of Haskell code that outputs art as well.] If you'd like to join the list please visit this site: http://lists.lurk.org/mailman/listinfo/haskell-art Let me know if you have any problems subscribing. If it proves a popular and/or useful list, perhaps it could be moved to haskell.org in the future. alex

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How you can help improve Haskell implementations!
by dons＠cse.unsw.edu.au 26 Feb '07

26 Feb '07

Just a quick note to say that the Haskell implementation shootout is progressing, now supporting jhc, fixing a range of bugs, and providing more benchmark programs. Nice average numbers are also reported for the relative performance of each compiler or interpreter. On x86: http://www.cse.unsw.edu.au/~dons/nobench/i686/results.html On amd64: http://www.cse.unsw.edu.au/~dons/nobench/x86_64/results.html Several bugs in our beloved Haskell compilers have been spotted, and most of them already fixed! ** I'd like to invite contributions of benchmark programs**. If you have a nice benchmarky program, that's reasonably portable (mostly h98 + base libs is ok), and freely licensed, let me know, and we can add it to the benchmark suite. Programs that show up interesting results between systems or that were difficult to get to perform well are particuarly welcome. 3 kinds of programs are welcome: * small artificial programs that illustrate particular issues e.g. (prime sieves, FFT, great language shootout programs) * 'kernels' from larger programs, that illustrate some functionality e.g. a mandelbrot generator, or some other core algorithm * larger real programs. These are particularly welcome. Limited to around 25 modules maximum, and capable of being run with simple stdin/args, producing simple output. Raytracers, chess programs, and small theorem provers are existing examples. So send me your unwashed code. Whatever code doesn't kill them will only make our compilers stronger! :-) The darcs repo for nobench/nofib is at: http://www.cse.unsw.edu.au/~dons/nobench.html Happy hacking! Don

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A "real" Haskell Cookbook
by Martin Bishop 26 Feb '07

26 Feb '07

I made a preliminary page, and fleshed out some of the headers/sub-headers on the wiki page for a good Haskell Cookbook (aka NOT a PLEAC clone). Please contribute and/or fix the examples and explanations so we can make a really nice Cookbook for newbies. :) http://haskell.org/haskellwiki/Cookbook

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Memoisation
by Tony Morris 25 Feb '07

25 Feb '07

I have a backtracking algorithm that I need to memoise with. Rather than go into the intricacies of the algorithm, I figure (and hope) the factorial function is trivial enough to point out my problem. Simply, suppose I wish to calculate the factorial of 10, then "later" the factorial of 5. I have already calculated the factorial of 5, but now I must do it again. I have thought of various ways of preventing this; perhaps passing an Array in a state monad. I'm wondering if there is a general solution for this kind of problem. Thanks for any tips. -- Tony Morris http://tmorris.net/

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