April 2007 - Haskell-Cafe

darcs error
by Ruben Zilibowitz 15 Apr '07

15 Apr '07

What does it mean if I'm trying to check out a darcs repository and I get the following error? darcs: ./.DS_Store: openBinaryFile: does not exist (No such file or directory) .DS_Store files ought to be filtered out by the default boring file. I don't know how one got into the repository and started causing this error. Is there some way to repair the repository? darcs repair doesn't seem to help. Regards, Ruben

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Weaving fun
by Bas van Dijk 15 Apr '07

15 Apr '07

Hello, For my own exercise I'm writing a function 'weave' that "weaves" a list of lists together. For example: weave [[1,1,1], [2,2,2], [3,3]] ==> [1,2,3,1,2,3,1,2] weave [[1,1,1], [2,2], [3,3,3]] ==> [1,2,3,1,2,3,1] Note that 'weave' stops when a list is empty. Right now I have: weave :: [[a]] -> [a] weave ll = work ll [] [] where work ll = foldr f (\rst acc -> work (reverse rst) [] acc) ll f [] g = \_ acc -> reverse acc f (x:xs) g = \rst acc -> g (xs:rst) (x:acc) However I find this definition hard to read and I'm questioning its efficiency especially due to the 'reverse' parts (how do they impact performance and can they be removed?) So I'm wondering if 'weave' can be defined more "elegantly" (better readable, shorter, more efficient, etc.)? happy hacking, Bas van Dijk

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Calling C function in Mac OS X
by Sergey Perminov 15 Apr '07

15 Apr '07

I wish to optimize Haskell code using ByteString, direct reading Doubles form it, direct writing Doubles to it. I've tried Don Stewart's code http://hpaste.org/26 that uses calling to C functions to implement necessary readDouble & showDouble. readDouble works ok. showDouble always return "nan" in Mac OS X (but works well in Windows). I'm using GHC 6.6 in Mac (http://www.haskell.org/ghc/download_ghc_66.html#macosxppc) and in Windows (http://www.haskell.org/ghc/download_ghc_66.html#windows) I've made showInt based on showDouble: http://hpaste.org/1390 It works well in Mac OS X. It seems that problem happens only when I send CDouble (or CLDouble) to C function. My conf: PowerBook G4, Mac OS X 10.4.9, powerpc-apple-darwin8-gcc-4.0.1 What may cause this? How to fix it?

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k-minima in Haskell
by raghu vardhan 15 Apr '07

15 Apr '07

What's the best way to implement the following function in haskell: Given a list and an integer k as input return the indices of the least k elements in the list. The code should be elegant and also, more importantly, must not make more than the minimum O(k*length(list)) number of operations. R M Send a FREE SMS to your friend's mobile from Yahoo! Messenger. Get it now at http://in.messenger.yahoo.com/

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Do monads imply laziness?
by Brian Hurt 14 Apr '07

14 Apr '07

This is probably an off-topic question, but I can't think of a better forum to ask it: does the existance of monads imply laziness in a language, at least at the monadic level? Consider the following: a purely functional, eagerly evaluated programming language, that uses monads to encapsulate the awkward squad. In this programming language, a program generates an IO monad whose encapsulating computation performs side effecting affections- it writes to stdout, say. But this generated monad never has it's value evaluated- the monad is tossed away uninspected. Does the side effect happen? If the answer is at least potientially "no", then monads are lazily evaluated, and thus monads imply laziness (at least at the monadic level). On the other hand, if the answer is "yes", then monads do not imply laziness. Thanks, Brian

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Left-factoring with Parsec
by Joel Reymont 14 Apr '07

14 Apr '07

Suppose I have expr = expr ":" expr ":" expr. Can the above be left-factored to fail on empty input so that my parser doesn't go into a loop? Thanks, Joel -- http://wagerlabs.com/

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Translating perl -> haskell, string "fill ins" with an error on invalid input seems awfully complex. Is there a way to simplify?
by Thomas Hartman 14 Apr '07

14 Apr '07

I was translating some perl code to haskell as a learning exercise and wound up with the following. (below) Simple code that accepts some string arguments, and prints a string -- so, of type String -> String -> String -> String -> IO (). I like to be concise, but I get the feeling something went awry. What seems to be costing me the most is checking whether the various arguments are legitimate, and printing a helpful error message if not. I was able to achieve this by using Maybe and error on failed pattern match, but as said, seems kind of overly complicated. Is there a simpler way to do the following, eg for function gen_gnuplot_financial_script :: String -> String -> String -> String -> IO () ? By the way this is being used in http://code.google.com/p/gnuplotwebinterface/ ************** module Common where gnuplot_png_settings = "set terminal png transparent nocrop enhanced size 600,400\n" ++ "set pm3d implicit at s" gnuplot_math_settings = gnuplot_png_settings ++ "\n" ++ "set border 4095 \n\ \ set xlabel \"x\" \n\ \ set ylabel \"y\"" gnuplot_timeseries_settings = gnuplot_png_settings ++ "\n" ++ "set xdata time # The x axis data is time \n" ++ "set timefmt \"%d-%b-%y\" # The dates in the file look like 10-Jun-04 \n" ++ "set format x \"%b %d\" #On the x-axis, we want tics like Jun 10" gen_gnuplot_math_script :: String -> String -> IO () gen_gnuplot_math_script style function = let maybePlotCmd = lookup style style_to_plotcmd style_to_plotcmd = [("math-2d","plot"),("math-3d","splot")] in case maybePlotCmd of Just plotcmd -> putStrLn $ gnuplot_math_settings ++ "\n" ++ plotcmd ++ " " ++ function _ -> error $ "bad style: " ++ style gen_gnuplot_financial_script :: String -> String -> String -> String -> IO () gen_gnuplot_financial_script company displaymode startDate endDate = let maybeCompanyFile = lookup company company_to_companyfile maybeModeString = lookup displaymode displaymode_to_modestring maybeTitleEnd = lookup displaymode displaymode_to_titleend company_to_companyfile = [("ibm","data/ibm.dat"),("cisco","data/cisco.dat")] displaymode_to_modestring = [("points", "using 1:2 with linespoints"), ("candles","using 1:($2+$3+$4+$5)/4:4:3 with yerrorbars")] displaymode_to_titleend = [("points","daily prices"),("candles","opening prices")] in case ( maybeCompanyFile, maybeModeString, maybeTitleEnd ) of ( Just companyfile, Just modestring, Just titleEnd) -> putStrLn $ gnuplot_timeseries_settings ++ "\n" ++ "plot [\"" ++ startDate ++ "\":\"" ++ endDate ++ "\"]" ++ " '" ++ companyfile ++ "'" ++ modestring ++ " title \"" ++ company ++ " " ++ titleEnd ++ "\"" _ -> error $ "bad lookup. " ++ company ++ " -> company file: " ++ ( show maybeCompanyFile ) ++ "\n" ++ " " ++ displaymode ++ " -> displaymode: " ++ ( show maybeModeString ) ++ "\n" ++ " " ++ displaymode ++ " -> titleEnd: " ++ ( show maybeTitleEnd)

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replacement
by hossein rohani 14 Apr '07

14 Apr '07

Write a function with three parameters, two atoms and a list, (say p1, p2 and L) that returns the list, L, with all occurrences of the first given atom, p1, replaced by the second one, p2. If P2 be nil, the given atom should be deleted and the returned list cannot contain anything in place of it, no matter how deep it occurred in the list. Note: Remember it says that "no matter how deep it occurred in the list.".It means thatwe may have nested list example: replace 1 2 [1 2 3 4] => [2 2 3 4] replace 1 2 [[1 2 3][3 4 5 1]] => [[2 2 3][3 4 5 1]] --------------------------------- Ahhh...imagining that irresistible "new car" smell? Check outnew cars at Yahoo! Autos.

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Type checking with Haskell
by Joel Reymont 13 Apr '07

13 Apr '07

Folks, The ghc/compiler/typecheck directory holds a rather large body of code and quick browsing through did not produce any insight. How do you implement type checking in haskell? Assume I have an Expr type with a constructor per type and functions can take lists of expressions. Do I create a function taking an Expr, pattern-matching on appropriate constructor and returning True on a match and False otherwise? Thanks, Joel -- http://wagerlabs.com/

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generate Haskell code from model
by Steffen Mazanek 13 Apr '07

13 Apr '07

Hello everybody, I would like to start a discussion on how to generate best-practice Haskell code from a model, e.g. from EMF. It seems to be very difficult and unconvenient to simulate OOP with Haskell (as described in the paper "Object-oriented style overloading for Haskell"). However, I think it might be possible to generate "something useful" out of a model to not start from scratch an implementation. For example, I could imagine to generate for every class a module, import and export lists and a type class (with some restrictions) at the very least. In the EMF book there is an example, PurchaseOrder, look at http://www.awprofessional.com/content/images/0131425420/samplechapter/budin… at page 11. How would you like generated code from this model to look like? The most obvious way would be something like this: data PurchaseOrder = PurchaseOrder {shipTo::String, billTo:: String, items::[Item]} data Item = Item {productName::String, quantity::Int, price::Float} This does not work well if function names are reused or inheritance comes into play, however, even this might be a useful starting point (better than nothing). The benefits of the model+generate approach are well known. Best practices in programming are propagated, for Haskell e.g. use different modules for different things, use the tedious import/export lists, Haddock your code... What are your ideas? Best regards, Steffen -- Dipl.-Inform. Steffen Mazanek Institut für Softwaretechnologie Fakultät Informatik Universität der Bundeswehr München 85577 Neubiberg Tel: +49 (0)89 6004-2505 Fax: +49 (0)89 6004-4447 E-Mail: steffen.mazanek(a)unibw.de

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