March 2010 - Haskell-Cafe

swf flv decoder in haskell
by Louis Plissonneau 29 Mar '10

29 Mar '10

Hi, I have to add some monitoring functions to a shockwave/flash decoder: to precisely evaluate the play buffer (the bar showing download information when you watch a YouTube video for example). I wanted to go for either swfdec or gnash, but as I enjoy Haskell so much, I wonder if there is some open source project on this subject? (even if not all features are supported) Thanks in advance for your advices. Louis Plissonneau

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Re: [Haskell-cafe] Are there any female Haskellers?
by Alberto G. Corona 29 Mar '10

29 Mar '10

> IQ tests, for example. google it. > > 2010/3/28 Jochem Berndsen <jochem(a)functor.nl> > > Alberto G. Corona wrote: >> > The reasons for the sexual differences in mathematical abilities are >> > different, because math abilities are not a -primary- reason for >> > survival. Tools engineering and mastering is. If this is politically >> > incorrect I beg you pardon, but this is my honest theory about that. My >> > other hobby is evolution and evolutionary psichology. I really >> > recommend to learn about it. >> >> Could you point us to any evidence that supports your assumption that >> there are "sexual differences in mathematical abilities"? >> >> Thanks, Jochem >> >> -- >> Jochem Berndsen | jochem(a)functor.nl >> > >

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Re: [Haskell-cafe] Re: Are there any female Haskellers?
by Steve Schafer 29 Mar '10

29 Mar '10

On Mon, 29 Mar 2010 17:32:57 +0200, you wrote: >What's evil in being different? The point is that people use _generic_ differences as a rationale for discrimination against _individuals_. For example, in the US, it has, until recently, been used as an argument against female firefighters, because women, in general, have less upper-body strength than men, and are therefore less able to manage the equipment used. But there are, of course, plenty of women who have upper-body strength significantly above average, as well as men whose upper-body strength is well below average. So the appropriate basis for discrimination is, "Do you have the strength to manage the equipment?" rather than "Are you a man or a woman?" So yes, there are generic differences between As and Bs (whatever categories A and B may represent), but that should not be used as a rationale for discrimination against individual As or Bs, because there is nearly always substantial overlap between the categories in whatever criterion it is that you're measuring. -Steve Schafer

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Fwd: [Haskell-cafe] Re: Are there any female Haskellers?
by Alberto G. Corona 29 Mar '10

29 Mar '10

What's evil in being different? I only see here a form of discrimination that i can label as inherentely sexist and/elitist/or racist: the assumption that certain habilities, the normally associated historically whith men are worth to have and good, and whoever hasn´t them is worth to be discriminated. while the traditionally associated with women, some of the unique, are for slaves. 2010/3/29 Steve Schafer <steve(a)fenestra.com> On Sun, 28 Mar 2010 20:38:49 -0700, you wrote: > > > * The difference between genders is smaller than the difference between > >individuals > > If only people would understand and accept the near-universality of > this: > > "The difference between <any group you want to discriminate against> and > <any group you want to discriminate in favor of> is smaller than the > difference between individuals." > > -Steve Schafer > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe >

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ANNOUNCE: Haskelloids
by mrehayden＠googlemail.com 29 Mar '10

29 Mar '10

Haskelloids 0.1.0 has just been uploaded to HackageDB, It's a reasonably faithful (but incomplete) reproduction of the Atari classic, written mostly in the Yampa framework. See the wiki for screenshots. Wikipage: http://www.haskell.org/haskellwiki/Haskelloids HackageBD: http://hackage.haskell.org/package/Haskelloids Enjoy, Matthew Hayden

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Fwd: [Haskell-cafe] Graphical representation of Haskell code
by Miguel Vilaca 29 Mar '10

29 Mar '10

Hi all, Concerning INblobs, it's again online; a fire damaged the cable that links the university to the world!! I don't update the tool for a long time... but I'll take a look on that. Concerning visual functional programming, see this small chapter of my thesis about some of the existing languages. There are more subtleties on the visual side than those expected!! If you also consider debugging tools, take a look on Ghood http://hackage.haskell.org/package/GHood best regards Miguel Vilaça A 2010/03/23, às 05:31, Ronald Guida escreveu: > On Mon, Mar 22, 2010 at 7:02 PM, Dupont Corentin > <corentin.dupont(a)gmail.com> wrote: >> Hello, I’m relatively new to Haskell. >> I’m wondering if it exist a tool to graphically represent Haskell code. > ... >> Let’s try to do it on a simple example, as an exercise: >> f = Map (+1) > > Your graphic for "f = map (+1)" seems much more complex than the > corresponding code. I would agree with Ivan Miljenovic: >> I'm of the opinion that unless you just use it on small snippets, >> the generated images will be too large and unwieldy. > > The first question I would ask is /why/ would you like to visualize > some Haskell code? If you want to see the high-level structure of > a complex program, try SourceGraph. (I have never used it though.) > > On the other hand, if you are trying to visualize Haskell as part of > your efforts to learn the language, then I believe it would be best to > draw diagrams by hand, rather than relying on an automated tool. > The kinds of things that you'll want to depict are probably going to > vary considerably, depending on what you're trying to understand. > > Consider a few different implementations of the "map" function: > > -- version 1: recursion > map1 :: (a -> b) -> [a] -> [b] > map1 f [] = [] > map1 f (x:xs) = (f x) : map1 f xs > > -- version 2: fold > map2 :: (a -> b) -> [a] -> [b] > map2 f = foldr ((:) . f) [] > > -- version 3: continuation passing style > map3 :: (a -> b) -> [a] -> [b] > map3 f xs = map' (\x y -> f x : y) xs > where > map' k [] = [] > map' k (y:ys) = k y (map' k ys) > > -- version 4: list comprehension > map4 :: (a -> b) -> [a] -> [b] > map4 f xs = [f x | x <- xs] > > -- version 5: list monad > map5 :: (a -> b) -> [a] -> [b] > map5 f xs = xs >>= (return . f) > > These all do exactly the same thing, but each one uses different > techniques. If I'm trying to learn (or teach) Haskell, I would > probably need a slightly different visual "language" for each one > in order to capture the most relevant concepts in a useful way. > How would you visualize them? > > @Mihai Maruseac: > I think a visual debugger would be a wonderful idea. You may want > to consider how a visual debugger would work with each of these > versions of map. > > :-) You might also consider several versions of factorial :-) > http://www.willamette.edu/~fruehr/haskell/evolution.html > > -- Ron > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe

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Domains and Co-Domains
by Günther Schmidt 29 Mar '10

29 Mar '10

Hi, I can easily see how one identifies the domain and co-domain of a unary function. How would the domain of a function be expressed that takes more than one argument and arguments of different type? Günther

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GHC vs GCC
by Rafael Cunha de Almeida 29 Mar '10

29 Mar '10

Hello, During a talk with a friend I came up with two programs, one written in C and another in haskell. Haskell main :: IO () main = print $ rangeI 0 0 rangeK :: Int -> Int -> Int -> Int -> Int rangeK i j k acc | k < 1000 = if i * i + j * j + k * k `mod` 7 == 0 then rangeK i j (k+1) (acc+1) else rangeK i j (k+1) acc | otherwise = acc rangeJ :: Int -> Int -> Int -> Int rangeJ i j acc | j < 1000 = rangeJ i (j+1) (acc + rangeK i j 0 0) | otherwise = acc rangeI :: Int -> Int -> Int rangeI i acc | i < 1000 = rangeI (i+1) (acc + (rangeJ i 0 0)) | otherwise = acc C main() { printf("%d\n", rangeI(0, 0)); return 0; } rangeK(i, j, k, acc) { if (k < 1000) { if (i * i + j * j + k * k % 7 == 0) return rangeK(i, j, k+1, acc+1); else return rangeK(i, j, k+1, acc); } else return acc; } rangeJ(i, j, acc) { if (j < 1000) return rangeJ(i, j+1, acc + rangeK(i, j, 0, 0)); else return acc; } rangeI(i, acc) { if (i < 1000) return rangeI(i+1, acc + rangeJ(i, 0, 0)); else return acc; } I tried to keep both programs as similar as possible. I'm pretty confident that the algorithm being executed are exactly the same. That is, there's no lists being used vs. arrays or anything like that. I even used Int instead of Integer to make sure an equivalent version of +, * and mod are called (perhaps that was what I did wrong?) I compiled the haskell code with ghc -O3 and I compiled the C code with gcc -O3. The execution time of the programs were dramatically different. You can test it yourselves, but here is the time I've got in my system: The Haskell version: real 0m45.335s user 0m45.275s sys 0m0.004s against the C version: real 0m6.021s user 0m6.004s sys 0m0.004s Also, I found it interesting that a iterative version of that algorithm in C ran in the same time as the recursive version. So it seems like gcc was successfuly able to make those tail recursive functions into iterations. It strikes me as surprising that Haskell would perform so much slower in that example. I was expecting maybe a few seconds due to the garbage collector or something like that, but not more than 7 times slower.

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parsec and multiple simultaneous token types
by Jeremy Shaw 29 Mar '10

29 Mar '10

Hello, I have some input which is divided up into segments like this: ["foo", "hi", "there", "world"] And I want to use parsec to parse the segments. I am looking for a way to be able to use Char parsers on each segment, but also parse the list of segments as a whole. I have an implementation which works, but I am not feeling that satisfied with it. An example parser looks like this: testp :: GenParser String () (Char, String, String) testp = do segment "foo" st <- p2u (char 'h' >> char 'o') sg <- anySegment sg' <- anySegment return (st,sg, sg') If you use it to parse ["foo", "hi", "there", "world"] you get: *Main> test ["foo","hi","there","world"] (segment 2 character 2): unexpected "i" expecting "o" This is good -- the error tells you the segment and the offset. The combinators, segment and anySegment match on String tokens. The p2u function converts a Char parser into a String parser. The primary issue is that I can not figure out how to implement p2u so that it works under Parsec 2 and 3 with out changes. The secondary issue is that I feel like the DSL is not that great. I would rather write the above parser like this: testp :: GenParser String () (Char, String, String) testp = do string "foo" nextSegment char 'h' >> char 'o' nextSegment sg <- many anyChar nextSegment sg' <- many anyChar return (st,sg, sg') so, the combinators like 'many' would only see to the end of the current segment. nextSegment would bring the next segment into context (or possibly fail if the current segment was not completely consumed). I could also, perhaps, reimplement anySegment as: anySegment = do s <- many anyChar nextSegment return s and write: testp :: GenParser String () (Char, String, String) testp = do string "foo" nextSegment char 'h' >> char 'o' nextSegment sg <- anySegment sg' <- anySegment return (st,sg, sg') I don't see a way to implement this on top of parsec though. Am I missing some clever ideas here? I have attached my code that implements the first method. Any ideas how to make it parsec 2/3 agnostic ? The primary issue is that if I use runParser on the inner Char parser, I need some way to stick a ParseError back into the parent context (with out just converting it to a String and calling fail). I can see how to get/set the inputState, etc, in a portable way, but I don't see how to set the ParseError in a portable way.The attached code requires 3.1.0 because it uses mkPT / runParsecT :( thanks! - jeremy

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How can I parameterize the Candidate?
by Duane Johnson 28 Mar '10

28 Mar '10

Hello, How can I parameterize the type of the following data class so that any type can be a Candidate? type Candidate = String data Poll = Poll [Candidate] [Ballot] My initial thought was to simply put a type variable in place of Candidate, but that clearly won't work: data Poll = Poll [a] [Ballot] >> Not in scope: type variable `a' For context: I am building a voting library that addresses the issue of polarized American politics by implementing the Virtual Round Robin electoral method with a Maximum Majority Voting algorithm for breaking cycles. The beginnings of the library are available at http://github.com/canadaduane/votelib/blob/master/vote.hs What is a good approach to the parameterized type issue? Thank you, Duane Johnson

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