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Re: [Haskell-cafe] [Haskell] A riddle...
by Vo Minh Thu 16 Jul '12

16 Jul '12

Oh the mailing list was haskell@ instead of café. Sorry. (Now cc'ing café.) Andreas, please post this kind of mail to the haskell-cafe instead of the haskell mailing list. 2012/7/16 Vo Minh Thu <noteed(a)gmail.com>: > It seems like the infered type (and thus bounds) is different when you > force the result to be a Color or not. Just give explicit type > signatures and conversion functions. > > Cheers, > Thu > > 2012/7/16 Andreas Abel <andreas.abel(a)ifi.lmu.de>: >> Today a student came to me with a piece of code that worked it executed by >> itself, but produced different result in the context of his larger problem. >> We cut down the example to the following: >> >>> import Graphics.UI.Gtk >>> >>> -- should produce [(26471,0,65535),... >>> rgbliste = >>> (map (\ i -> >>> let rb = 5 * (mod (mod 181935629 (4534+i)) 100)-250+128 in >>> let gb = 5 * (mod (mod 128872693 (5148+i)) 100)-250+128 in >>> let bb = 5 * (mod (mod 140302469 (7578+i)) 100)-250+128 in >>> let r = min 255 $ max 0 rb in >>> let g = min 255 $ max 0 gb in >>> let b = min 255 $ max 0 bb in >>> (r*257,g*257,b*257)) [0..]) >>> >>> --farbliste = map (\ (r,g,b) -> Color r g b) rgbliste >>> >>> main :: IO () >>> main = do >>> print $ head rgbliste >> >> >> If you run it, it prints (26471,0,65535). >> If you uncomment the def. of farbliste, it prints (44461,65535,65535). >> >> I was surprised. What is going on? >> >> Cheers, >> Andreas >> >> -- >> Andreas Abel <>< Du bist der geliebte Mensch. >> >> Theoretical Computer Science, University of Munich >> Oettingenstr. 67, D-80538 Munich, GERMANY >> >> andreas.abel(a)ifi.lmu.de >> http://www2.tcs.ifi.lmu.de/~abel/ >> >> >> _______________________________________________ >> Haskell mailing list >> Haskell(a)haskell.org >> http://www.haskell.org/mailman/listinfo/haskell

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lazy boxed array and builder?
by Evan Laforge 16 Jul '12

16 Jul '12

he recent discussion of whether storablevector should be deprecated in favor of vector reminded me: vector supports storable arrays, but it doesn't support lazy arrays. While storablevector has lazy arrays and a builder, it doesn't support boxed types (it would be become misnamed if it did!). So it seems the niche of boxed lazy arrays is unfilled. And if vector grew a lazy variant it could subsume storablevector and we could consolidate array libraries a little more. It seems a lot of Writer monad type stuff (logging etc.) could be made more efficient with a boxed lazy array builder, at the cost of rougher grained laziness. Does such a thing already exist? I'd check hackage, but it's down... http://www.downforeveryoneorjustme.com/http://hackage.haskell.org/

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Getting a segmentation fault when starting/stopping the RTS, from C, several times.
by Captain Freako 15 Jul '12

15 Jul '12

Hi experts, Should I expect the following C code to run to completion, or am I trying to do something that was never intended? Thanks, -db C code: 1 #include <stdio.h> 2 #include <stdlib.h> 3 #include "HsFFI.h" 4 5 int main() 6 { 7 int argc = 1, i; 8 char* argv[] = {"ghcDll", NULL}; // argv must end with NULL 9 char** args = argv; 10 11 for(i=0; i<10; i++) { 12 fprintf(stderr, "Starting up the RTS...\n"); 13 hs_init(&argc, &args); 14 printf("Shutting down the RTS...\n"); 15 hs_exit(); 16 } 17 } This is what I get, when I compile and run it: dbanas@dbanas-eeepc:~/prj/haskell/amitool$ make test_rts rm -f test_rts ghc -o test_rts -lHSrts -lm -lffi -lrt test_rts.o dbanas@dbanas-eeepc:~/prj/haskell/amitool$ ./test_rts Starting up the RTS... Shutting down the RTS... Starting up the RTS... Shutting down the RTS... Segmentation fault

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Plain lambda inside banana brackets in the arrow notation
by Tsuyoshi Ito 15 Jul '12

15 Jul '12

Hello, In a program, I have an arrow MyArr and a combinator called repeat of the following type: repeat :: Int -> (Int -> MyArr e a) -> MyArr e a My problem is that the code becomes messy when I use this combinator inside the arrow notation, and I am looking for a way to write the code in a more readable way. To explain the problem, first consider the following combinator repeat', which is less general than repeat: repeat' :: Int -> MyArr (e, Int) a -> MyArr e a repeat' n f = repeat n g where g i = arr (\e -> (e, i)) >>> f Combinator repeat' is nice to use in the arrow notation, thanks to banana brackets and the interpretation of lambda: test1 :: MyArr [Double] String test1 = proc xs -> do let y = func1 xs z <- job1 -< xs (|(repeat' 100) (\i -> job2 -< xs !! i + y + z)|) -- func1 :: [Double] -> Double -- job1 :: MyArr [Double] Double -- job2 :: MyArr Double String However, in my program, I often have to use repeat instead of repeat' like: test2 :: MyArr [Double] String test2 = proc xs -> do let y = func1 xs z <- job1 -< xs repeat 100 (\i -> proc (xs, y, z) -> job3 (i * 2) -< xs !! i + y + z) -< (xs, y, z) -- job3 :: Int -> MyArr Double String Note that variable i is used as an argument to function job3 outside MyArr, and this cannot be done with repeat'. The code for test2 looks messy to me because I have to write “(xs, y, z)”, that is, the list of variables used inside the subcomputation explicitly (and twice). It does not seem possible to use banana brackets here because the type of the subcomputation does not meet the requirements stated in http://www.haskell.org/ghc/docs/7.4.2/html/users_guide/arrow-notation.html#…. How can I use combinators like repeat, which takes a plain function as an argument, in the arrow notation in a more readable way? Or am I trying to do an impossible thing? Best regards, Tsuyoshi

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I can see lists "working" for matrix algorithms if the access patterns are mostly from the start or end of rows and columns.
by KC 15 Jul '12

15 Jul '12

Are there any such matrix algorithms that operate mainly from the start or end or rows and columns? Note: If the access pattern starts from the end of a row then one stores the row in reverse order in a list. -- -- Regards, KC

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Re: [Haskell-cafe] [Haskell-beginners] Graph Coloring Library?
by KC 14 Jul '12

14 Jul '12

This may help: - use hMatrix to find the eigenvalues of the adjacency matrix Then by the following theorems: "An upper bound on the chromatic number in terms of eigenvalues was discovered by Wilf(1967). Chromatic Number(G) <= 1 + lambda_1(G) Hoffman (1977) discovered a lower bound on the chromatic number in terms of the smallest and largest eigenvalue. Chromatic Number(G) >= 1 - (lambda_1/lambda_n) for n>=2." >From "The Mutually Beneficial Relationship of Graphs and Matrices", Richard A. Brualdi,CBMS Series,Number 115, 2011. On Fri, Jul 13, 2012 at 1:43 PM, Alec Story <avs38(a)cornell.edu> wrote: > I have a problem where I need to find the smallest k-coloring for a graph > that represents conflicts between objects. Is there a Haskell library that > will do this for me? I'm not particularly concerned about speed, and it's > unlikely that I'll generate really bad edge cases, but I'd prefer to do > something other than write the really bad try-every-case algorithm. > > -- > Alec Story > Cornell University > Biological Sciences, Computer Science 2012 > > _______________________________________________ > Beginners mailing list > Beginners(a)haskell.org > http://www.haskell.org/mailman/listinfo/beginners > > -- -- Regards, KC

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ANNOUNCE: hspec-1.3.0
by Simon Hengel 14 Jul '12

14 Jul '12

Hi, hspec-1.3.0 is out [1]. This release comes with two major new features: - BDD-style combinators to make assertions [2] - Automatic test discovery [3] Test.Hspec now exports parts of the monadic API and the new BDD-style combinators. You can use Test.Hspec.Core as a drop-in replacement for legacy code. The full release notes are here at [4]. A basic introduction to Hspec is at [5]. Feedback is very welcome! Cheers, Simon [1] http://hackage.haskell.org/package/hspec [2] https://github.com/sol/hspec-expectations#readme [3] https://github.com/hspec/hspec/tree/master/hspec-discover#readme [4] https://github.com/hspec/hspec/blob/master/CHANGES.markdown [5] http://hspec.github.com/

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Re: [Haskell-cafe] [Haskell-beginners] Graph Coloring Library?
by KC 14 Jul '12

14 Jul '12

On Hackage there is the Graphalyze package but it doesn't seem to do vertex coloring. Are you talking about undirected graphs? On Sat, Jul 14, 2012 at 12:23 PM, Alec Story <avs38(a)cornell.edu> wrote: > I'd prefer an exact solution, but it doesn't have to be particularly > performant (this is research code). > > The graphs are pretty small in general, all my test cases will have < 10 > nodes, but have high connectivity. They don't have any special properties. > On Jul 14, 2012 2:50 PM, "KC" <kc1956(a)gmail.com> wrote: > >> Are you looking for an exact or approximate solution. >> >> How many nodes and edges? >> >> Any special properties that the graph has? >> >> >> On Fri, Jul 13, 2012 at 1:43 PM, Alec Story <avs38(a)cornell.edu> wrote: >> >>> I have a problem where I need to find the smallest k-coloring for a >>> graph that represents conflicts between objects. Is there a Haskell >>> library that will do this for me? I'm not particularly concerned about >>> speed, and it's unlikely that I'll generate really bad edge cases, but I'd >>> prefer to do something other than write the really bad try-every-case >>> algorithm. >>> >>> -- >>> Alec Story >>> Cornell University >>> Biological Sciences, Computer Science 2012 >>> >>> _______________________________________________ >>> Beginners mailing list >>> Beginners(a)haskell.org >>> http://www.haskell.org/mailman/listinfo/beginners >>> >>> >> >> >> -- >> -- >> Regards, >> KC >> > -- -- Regards, KC

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RFCQ: The Classy Prelude: a more fully polymophic, drop-in alternative prelude http://www.yesodweb.com/blog/2012/07/classy-prelude
by Max Cantor 13 Jul '12

13 Jul '12

I'd like to share Michael Snoyman's Classy Prelude: http://www.yesodweb.com/blog/2012/07/classy-prelude . The basic gist of the work is that many of the standard prelude functions are made for concrete types such as [a], [(a,b)], and [Char] but can be much more polymorphic. While this complicates error messages and makes learning Haskell more difficult, we believe experienced Haskellers will benefit from the added polymorphism. Note that we aren't saying that the standard Prelude should be replaced, rather we are offering this as a drop-in alternative. Note that it reexports most of the standard Prelude types so it should be very low impact to switch over. We love to hear your comments. Note however, that this is a RFCQ with the 'C' standing for constructive.. Thanks, Max

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Monads with "The" contexts?
by Takayuki Muranushi 13 Jul '12

13 Jul '12

tl;dr: Is there any way to pass the Supercombinator level names (the names in haskell source codes with zero indent) as context to a Monad so that it will read from "The" context? Hello, everyone. I'm thinking of representing our knowledge about our life, the universe and everything as Haskell values. I'd also like to address the uncertainties in our knowledge. The uncertainties are usually continuous (probabilistic distributions) but that's another story. Please forget about it for a while. We learned that List is for nondeterministic context. > earthMass, sunMass, marsMass :: [Double] Let's pretend that there are large uncertainty in our knowledge of the Earth mass. > earthMass = [5.96e24, 5.97e24, 5.98e24] Let's also pretend that we can measure the other bodys' masses only by their ratio to the earth mass, and the measurements have large uncertainties. > sunMass = (*) <$> [2.5e5, 3e5, 4e5] <*> earthMass > marsMass = (*) <$> [0.01, 0.1, 1.0] <*> earthMass Then, how many Mars mass object can we create by taking the sun apart? > sunPerMars :: [Double] > sunPerMars = (/) <$> sunMass <*> marsMass Sadly, this gives too many answers, and some of them are wrong because they assume different Earth mass in calculating Sun and Mars masses, which led to inconsistent calculation. *Main> length $ sunPerMars 81 We had to do this way; > sunMass' e = map (e*) [2.5e5, 3e5, 4e5] > marsMass' e = map (e*) [0.01, 0.1, 1.0] > sunPerMars' :: [Double] > sunPerMars' = do > e <- earthMass > (/) <$> sunMass' e <*> marsMass' e to have correct candidates (with duplicates.) *Main> length $ sunPerMars' 27 The use of (e <- earthMass) seems inevitable for representing that the two Earth masses are taken from the same source of nondeterminism. However, as the chain of the reasoning grows, we can easily imagine the function arguments will grow impractically large. To get the Higgs mass, we will need to feed them all the history of research that led to the measurement of it. There is "the" source of nondeterminism for Earth mass we will always use. Is there a way to represent this? For example, can we define earthMass'' , sunMass'' , marsMass'' all in separate modules, and yet have (length $ sunPerMars'' == 27) ? By the way, *Main> length $ nub $ sort sunPerMars' 16 is not 9. That's another story, I said! Thanks in advance. -- Takayuki MURANUSHI The Hakubi Center for Advanced Research, Kyoto University http://www.hakubi.kyoto-u.ac.jp/02_mem/h22/muranushi.html

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