March 2009 - Haskell-Cafe

generalized shuffle
by Manlio Perillo 23 Mar '09

23 Mar '09

Hi. I have implemented a generalized shuffle function, for the random-shuffle package http://hackage.haskell.org/cgi-bin/hackage-scripts/package/random-shuffle I have not yet commited the change, before that I would like to receive some feedbacks (especially by the original author of the shuffle function, in Cc) The new function is defined in this example: http://hpaste.org/fastcgi/hpaste.fcgi/view?id=2808 Note that it make use of functions not exported by the System.Random.Shuffle module. I have generalized the original shuffle function in two ways: 1) It is now possible to obtain a random "sample" from a sequence, by doing, as an example: shuffles [1..10] [2, 3, 4, 2, 1] Note that this is equivalent at taking the first 5 elements of the full shuffled sequence, and the performance are pratically the same. 2) It is now possible to pass an infinite "r-sequence" to the shuffle function, as an example: shuffles [1..10] $ cycle [9, 8 .. 1] The result is an infinite list, with elements cyclically extracted from the "r-sequence". When the "r-sequence" contains random numbers, we get an infinite sequence containing all possible random "choices" of elements in the original sequence. Why I'm doing this? The reason is that in a project I need to extract random elements from a list (and I need to extract a lot of them), and using "normal" methods [1] seems to be rather inefficient. Using the `shuffles` function should be much more efficient, since it uses a tree, and this tree is built only once. [1] by normal method I mean: extract a random number, in the range [0, n] (where n is the length of the sequence), and get the element indexed by that number. Indexing for a list if very expensive. And it is not very fast, even using arrays (unless you use non portable unsafeRead). Thanks Manlio Perillo

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HSTringTemplate and syb-with-class
by Kemps-Benedix Torsten 23 Mar '09

23 Mar '09

Hello all, I'm trying to use the generic capabilities of HSTringTemplate. The documentation claims that the package is able to automatically generate instances of ToSElem if syb-with-class is installed but gives no further details. I installed syb-with-class and then installed HSTringTemplate with additional configure parameter syb-with-class=True. But when I import Text.StringTemplate.GenericWithClass and then try deriving ToSElem or $(derive ToSElem), I just get an error like "Can't make a derived instance of `ToSElem ...". Any suggestions or pointer to further docs? Kind regards Torsten

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Function to cast types
by Anonymous Anonymous 23 Mar '09

23 Mar '09

Hello, I'm new to haskell, I'm wondering how can you write a function that will do the following: fromIntToString :: Int -> String this is a cast function to cast an Int to a String. I know such function exist, however let's assume it didn't exist. How would I write such function? Cause I have no idea, because there are infinity possibilities if I do it like: fromIntToString x | x == 1 = "1" | x == 2 = "2" -- And so on... I've also thinked about defining the defining the data types Int and String (I know you cannot redefine them, at least I think so), however I've no succes. Thank you in advance for answering my question! PS: if possible please do not use any casting functions that are predefined.

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System.Random.Shuffle fix
by friggin friggin 23 Mar '09

23 Mar '09

I was looking for a shuffling algorithm to shuffle mp3-playlists so was very happy to see System.Random.Shuffle: http://hackage.haskell.org/cgi-bin/hackage-scripts/package/random-shuffle-0… However I get errors,non-exhaustive patterns in function shufleTree or extractTree depending how I call it. Errors are at the bottom. I fixed it but I don't have the math skills to see if I perhaps broke it statistically ... Here is my fix, someone (don't remember who, helped me a little): http://hpaste.org/fastcgi/hpaste.fcgi/view?id=2789#a2789 the shuffle at the end is with the fix. *Freet> S.shuffle [1..10] [1..3] Loading package syb ... linking ... done. Loading package base-3.0.3.0 ... linking ... done. Loading package old-locale-1.0.0.1 ... linking ... done. Loading package old-time-1.0.0.1 ... linking ... done. Loading package random-1.0.0.1 ... linking ... done. Loading package random-shuffle-0.0.2 ... linking ... done. [2,4,6*** Exception: src\System\Random\Shuffle.hs:(52,6)-(55,30): Non-exhaustive patterns in function shuffleTree *Freet> S.shuffle [1..3] [1..10] [2,*** Exception: src\System\Random\Shuffle.hs:(66,6)-(79,27): Non-exhaustive patterns in function extractTree *Freet> :load "c:/ghc/ghc-6.10.1/progs/Mp3Player/Shuffle.hs" [1 of 1] Compiling Shuffle ( C:\ghc\ghc-6.10.1\progs\Mp3Player\Shuffle.hs, interpreted ) Ok, modules loaded: Shuffle. *Shuffle> shuffle [1..3] [1..10] [2,1,3] *Shuffle> shuffle [1..10] [1..3] [2,4,6*** Exception: C:\ghc\ghc-6.10.1\progs\Mp3Player\Shuffle.hs:(27,13)-(31,30): Non-exhaustive patterns in function shuffle' *Shuffle> shuffle [1..10] [1..10] [2,4,6,8,10,3,9,7,5,1] *Shuffle>

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A small display problem using Helium
by Anonymous Anonymous 23 Mar '09

23 Mar '09

Hello, I've written something simple: main :: IO () main = do lijn <- getLine putStrLn lijn Now if I import it in Helium it will do the following: Test> main test -- (here I'm typing test) test it will be displayed two times, why? Because it displays my input and the output of the function. Now I want to write a function that does NOT display my input, any suggestions how I can achieve this? Thanks in advance!

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Hackage download and popularity statistics
by Don Stewart 23 Mar '09

23 Mar '09

For the first time, we've got download and popularity statistics from Hackage: http://www.galois.com/blog/2009/03/23/one-million-haskell-downloads/ Find out if your package made the top 100, and when we reach our 1 millionth hackage download! -- Don

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Hackage upload problems?
by Sebastiaan Visser 23 Mar '09

23 Mar '09

Hello, Currently I'm trying to upload a minor update of Salvia to Hackage to fix some dependency issues but Hackage times out all the time? Both the CLI tool and the web-interface do not react to my upload request. Any known problems here? Gr, Sebastiaan.

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Sometimes I wish there was a global variable
by Rafael Cunha de Almeida 23 Mar '09

23 Mar '09

Hello, I am writing a OpenGL program in haskell, it can be found in: http://github.com/aflag/galo/tree/master But I hope this e-mail will be self-contained :). My main function goes like this: (...) rotX <- newIORef (0.0::GLfloat) rotY <- newIORef (0.0::GLfloat) pos <- newIORef (0.0::GLfloat, 0.0, 0.0) displayCallback $= display (map f range) rotX rotY pos keyboardMouseCallback $= Just (keyboardMouse rotX rotY pos) (...) Notice that rotX, rotY and pos are meant to be used as comunication between the keyboardMouse and display functions. They need to be set as 0 first, so display won't do anything. Only when they user press a few buttons that those values change, so display behaves accordanly. In a state-based language I would place display and keyboardMouse in one module and let them communcate to each other like they want. In haskell, I'm not quite sure how to do it except by that parameter passing style. I thought about how state-monad may help with that. But I'm not sure how I'd make the state variable to be contained inside a display/keyboardMouse module. []'s Rafael

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Re: [Haskell-cafe] least fixed points above something
by Jens Blanck 23 Mar '09

23 Mar '09

> > > On Friday 20 March 2009 5:23:37 am Ryan Ingram wrote: > > On Fri, Mar 20, 2009 at 1:01 AM, Dan Doel <dan.doel(a)gmail.com> wrote: > > > However, to answer Luke's wonder, I don't think fixAbove always finds > > > fixed points, even when its preconditions are met. Consider: > > > > > > f [] = [] > > > f (x:xs) = x:x:xs > > > > > > twos = 2:twos > > > > How about > > > > > fixAbove f x = x `lub` fixAbove f (f x) > > > > Probably doesn't work (or give good performance) with the current > > implementation of "lub" :) > > > > But if "lub" did work properly, it should give the correct answer for > > fixAbove f (2:undefined). > > This looked good to me at first, too (assuming it works), and it handles my > first example. But alas, we can defeat it, too: > > f (x:y:zs) = x:y:x:y:zs > f zs = zs > > Now: > > f (2:_|_) = _|_ > f _|_ = _|_ > > fix f = _|_ > > fixAbove f (2:_|_) = 2:_|_ `lub` _|_ `lub` _|_ ... > = 2:_|_ > > Which is not a fixed point of f. But there are actually multiple choices of > fixed points above 2:_|_ > > f (2:[]) = 2:[] > forall n. f (cycle [2,n]) = cycle [2,n] > > I suppose the important distinction here is that we can't arrive at the > fixed > point simply by iterating our function on our initial value (possibly > infinitely many times). I suspect this example won't be doable without an > oracle of some kind. > > Ah well. > -- Dan > > Thanks for all comments on my question, especially those bashing my poor code. The above approach does not apply to my case. What I have is a monotone function f on a partial order satisfying f x >= x, for all x. Given that the partial order is in fact a cpo this is enough to guarantee that a least fixed point can be found above any point in the partial order simply by iterating f, although not necessarily in finite time. Taking the lub of x and the fixed point of f (over bottom) need not give a fixed point even if one exists. Think of reachability in a graph from a starting set. Let S be some fixed set, and let f return all points reachable in 0 or 1 step from the union of S and the argument to f. Then fix f is the set of points reachable from S, which is a fixed point. But adding some point x outside fix f will in general not give me a fixed point, even though a unique fixed point exists (the set reachable from the union of {x} and fix f). Jens

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SoC idea: interactive profiling
by Patai Gergely 23 Mar '09

23 Mar '09

Hello all, I entered a little proposal in the issue tracker: http://hackage.haskell.org/trac/summer-of-code/ticket/1570 As I see it, the graphical part could be a warm-up exercise with an already useful product, and interaction with the rts would be the actual challenge. Reasoning about laziness is far from trivial (at least I often feel lost when trying to grok all the interaction within Reactive ;), and being able to browse profiler output on the spot is something I felt the need for a few times already. Eventually, this could be a contribution to an IDE project, even though it could as well remain a standalone application for all the emacs/vi folk. Profiling tools for parallel programs are in the make already, but I haven't seen anything more convenient than hp2ps and little add hoc solutions for analysing resource usage patterns. How do you all go about profiling? Gergely -- http://www.fastmail.fm - Does exactly what it says on the tin

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