March 2011 - Beginners

Writer and mappend
by Britt Anderson 15 Mar '11

15 Mar '11

I have a question related to using Writer with a user defined datatype where I want some of the fields to append and others to just take the last value as a reduced example, if I had a data type like this: data Mydat = Mydat {list1 :: [String], item1 :: Float} and I wanted a Writer MyDat a where the strings keep on appending, but the float value just takes the last value. would I have to define Float as an instance of Monoid like instance Monoid Float where mempty = 0.0 mappend a b = a ?or maybe mappend a b = b before defining an instance of MyDat Or can I do something like instance Monoid MyDat where mempty = Mydat [] 0 mappend a b = (Mydat (mappend (list1 a) (list1 b)) (item1 a)) without defining an instance for Float? Thank you, Britt

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Which package is loaded?
by Obscaenvs 14 Mar '11

14 Mar '11

How do I find out which module a package is loaded from? I have xmonad-contrib-0.9.1 and xmonad-contrib-0.9.2 installed. I did $ sudo ghc-pkg hide xmonad-contrib-0.9.1 but it seems the module used is still xmonad-contrib-0.9.1 (I *think*). I have in my xmonad.hs the line "import qualified XMonad.Util.Dzen as Dzen" which works - but only with the legacy functions (e.g. dzen , but *not* dzenConfig), hence my hobby conclusion that it is xmonad-contrib-0.9.1 that is used -- despite the fact that it is "hidden", but what do I know? So how *does* GHC find a package, and within it, a module? I have been using "the Google" and the documentation for a while now, but I've come up emtpy handed. Is the module system really so badly documented? /Fredrik Carlén

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mustache in Haskell
by Nicholas Wieland 14 Mar '11

14 Mar '11

Hi *, I'm studying Haskell and having a lot of fun in the process. I would like to start a puppy project just to improve my studying, and considering I'm mainly a web guy I think that implementing something more or less simple like mustache (http://mustache.github.com/) without pretending to release something stable any time soon can be a good start. My first question is: should I use Happy as a parser or there are simpler/better options, or maybe even parse it myself ? ngw

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Cabal install woes
by Tom Murphy 12 Mar '11

12 Mar '11

Hi everyone, I'm having Cabal problems. Any help would really be appreciated. When I try: "cabal install soegtk" or "cabal install gtk" or "cabal install glib" I get the error (abridged): " Cannot find gtk2hsC2hs Please install `gtk2hs-buildtools` first and check that the install directory is in your PATH (e.g. HOME/.cabal/bin). [...] gio-0.12.0 depends on glib-0.12.0 which failed to install. glib-0.12.0 failed during the configure step. The exception was: exit: ExitFailure 1 [...] " But: gtk2hsC2hs and gtk2hs-buildtools ARE where they're supposed to be (gtk2hsC2hs is in ~/.cabal/bin, and gtk2hs-buildtools is in ~/.cabal/share). A possibly related issue is that when I run "cabal update" it tells me my Cabal isn't the newest version. I successfully run "cabal install cabal-install", then do "cabal update" and get the same message again. As a side question, is there a good resource about Cabal in greater detail than a how-to? I'm looking for, say, what ARE the executables in .cabal/bin? How do they relate to packages loaded in ghci? Thanks for your time! Tom

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gcNew with simple draw example haskell gtk
by Berlin Brown 12 Mar '11

12 Mar '11

I am trying to draw a simple pixel with gtk/haskell. This is the code, but I get an error. GHC 6.12.3? with the most recent gtk library. Error: # ghc --make SimpleCells.hs [1 of 1] Compiling Main ( SimpleCells.hs, SimpleCells.o ) Graphics\UI\Gtk\Misc\DrawingArea.chs:22:10: Not in scope: `gcNew' ----- import Graphics.UI.Gtk renderScene d ev = do dw <- widgetGetDrawWindow d (w,h) <- widgetGetSize d gc <- gcNew dw return True main = do initGUI window <- windowNew drawing <- drawingAreaNew onExpose drawing (renderScene drawing) window `onDestroy` mainQuit windowSetDefaultSize window 800 600 windowSetPosition window WinPosCenter widgetShowAll window mainGUI -- Berlin Brown (berlin dot brown at gmail.com) http://botnode.com http://berlinbrowndev.blogspot.com/

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Is this an appropriate use of the writer monad?
by Alex Rozenshteyn 11 Mar '11

11 Mar '11

I need to (for class) write a minimax implementation. I'm currently doing this by functionally defining the entire tree and later pruning it down to the size I want. My assignment requires that I output the number of nodes I evaluated. I am considering wrapping the tree generation with the Writer monad (I'm not yet certain how I'd do this). Does this sound reasonable to you haskellers who know more than I? Would anyone suggest a better way? -- Alex R

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Which haskell array implementation to use? AKA what are the pros and cons of each
by Rohit Garg 10 Mar '11

10 Mar '11

I am reposting the question I posted here http://stackoverflow.com/questions/5196394/which-haskell-array-implementati… I am somewhat surprised at the answers. It could be that I didn't pose it very well, but I was surprised nonetheless to know that the present array library in Platform is considered horrendous, the suggested version (Data.Vector) can't parallelize operations over boxed arrays and the repa one (which actually does boxed arrays) doesn't work with any stable build as of today. I only need -> very easy parallelization -> map/filter type operations -> support for matrices Any help would be much appreciated. -- Rohit Garg http://rpg-314.blogspot.com/

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Beginners issue with 'Why Haskell matters' example code
by Michael Anckaert 10 Mar '11

10 Mar '11

Hey everyone I'm new on the list here and just started out learning Haskell and functional programming. I have over 12 years experience programming in various languages, from C to Python. I started out with the Why Haskell matters paper ( http://haskell.org/haskellwiki/Why_Haskell_Matters) and ran into a problem with the code below. It's discussed in the paper but when I save it to test.hs and load it into ghci, I get the following output: ------------------------ [1 of 1] Compiling Main ( test.hs, interpreted ) test.hs:6:22: parse error on input `=' Failed, modules loaded: none. ------------------------ Could someone give a pointer on where my error lies? qsort [] = [] qsort (x:xs) = qsort less ++ [x] ++ qsort more where less = filter (<x) xs more = filter (>=x) xs -- Kind regards Michael Anckaert <michael.anckaert(a)sinax.be> http://www.sinax.be

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Help!!
by Jessica Baker 10 Mar '11

10 Mar '11

3.1 Step 1 Create a Haskell script le called crypt.hs and enter the code for he Caesar cipher from Section 5.5 of the text book (Hutton). Make sure you understand how all the functions work. 3.2 Step 2 Transform your script le (from Step 1) into a Haskell program that reads text from the standard input, encrypts the text using the Caesar cipher, and writes the result to the standard output. Your program should take a command-line argument that species the shift factor to be used. So, for example: cat poem.txt | ./crypt 3 Eqvdqjxlqlqj wkh vnb Hrz khdylob lw glhv Iqwr wkh Whvw dzdb Pdvw wrxfk dqg vljkw dqg vrxqg Nr orqjhu wr eh irxqg Hrz krshohvv xqghujurxqg Fdoov wkh uhpruvhixo gdb If the command-line argument is 0 (zero) then the program should crack the code and output the deciphered text. So, the following command pipeline should echo the original message: cat poem.txt | ./crypt 3 | ./crypt 0 Ensanguining the sky How heavily it dies Into the West away Past touch and sight and sound No longer to be found How hopeless underground Falls the remorseful day

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Nested data types and bisimilarity
by dan portin 08 Mar '11

08 Mar '11

I asked this question in another location, but perhaps this is a better palce. It's unclear to me how conditions for a relation to be a bisimulation on nested data types can be generated. I understand, by analogy with streams and binary trees, how to define such conditions for regular types. However, consider the types T a = T a [T a] N a = N (T [N a]) and a relation R over then. What conditions must be fulfilled for R to a bisimulation? In particular, how is the nesting of a list (or tree) dealt with?

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