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gtk2hs and threadDelay
by gabre＠caesar.elte.hu 26 Dec '11

26 Dec '11

Thanks for the quick answers. I dont really know, what timer functions there are, I googled "timing, scheduling, delaying in haskell", but the only function I found, was threaddelay. How did you mean "adding a timer"? On the other side, I checked Graphics.UI.Gtk.Gdk.drawWindowProcessUpdates. I guess, I should use this in the function, where I draw something, and wait, and draw something else, and I should put it between the two drawing steps to get an immediate update. If I do so, the drawing gets blocked again, and nothing happens for some second, and then everything is drawn at the same time. Sorry, I m a very beginner at this topic.

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gtk2hs and threadDelay
by gabre＠caesar.elte.hu 26 Dec '11

26 Dec '11

Hello! I ve some questions in connection with GTK2HS and threadDelay. GTK2HS is a well known GUI tool for Haskell, I wanted to draw with it (in Cairo). The threadDelay is the function from Control.Concurrent. I wanted nothing else but make my unthreaded program wait for X microseconds. (one-threaded) Consider the following parts from a code: -- onClick event for a button onClicked (btn gui) (drawProgram gui preprocPrg) -- code for drawProgram drawProgram gui preprocPrg = do let drArea = (drawWin gui) drw <- widgetGetDrawWindow drArea renderWithDrawable drw (myDraw 200 200) threadDelay 1800000 renderWithDrawable drw (myDraw2 200 200) I expect this code to do the following: when somebody clicks the button "btn" from record gui, the program draws something, and then waits for 1800000 microsecs (I hope threadDelay is measured in microsecs), and then draws something else. In fact, the code doesnt do this, but waits 1800000+ microsecs, and draws everything at the same time. Is there an error somewhere? MyDraw functions are: myDraw w h = do setSourceRGB 255 0 0 setLineWidth 2 moveTo (0.5 * w) (0.43 * h) lineTo (0.33 * w) (0.71 * h) stroke myDraw2 w h = do moveTo (0.33 * w) (0.71 * h) setSourceRGB 1 2 0 setLineWidth 15 lineTo (0.66 * w) (0.71 * h) lineTo (0.5 * w) (0.43 * h) stroke If it is needed, I ll give more information and code. Thank you for your help! Gabor Hosszu

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Prelude Implementation of Last overlapping patterns?
by Mark Stoehr 25 Dec '11

25 Dec '11

The standard Prelude implementation of 'last' is as follows \begin{code} last :: [a] -> a last [x] = x last (_:xs) = last xs last [] = error "Prelude.last: empty list" \end{code} Isn't [x] equivalent to (x:[]) hence wouldn't [1] match both [x] and (_:xs)? If that's the case then we would have last [1] == 1 and last [1] == last [] but that doesn't happen. Why?

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construct list
by Andy 24 Dec '11

24 Dec '11

I have been trying to simulate "Russian peasant multiplication" in Haskell , but I can't produce a list of tuples after I enter 2 integers. I have tried constructing many accumulator functions to do so, but keep getting IO errors. This is NOT a homework question. I am a senior learning Haskell, and finding it very enjoyable, although frustrating at times. ---------------------------------------------------------------------------------- main1 = do putStrLn "Russian Peasant Multiplication \n" putStrLn "Enter 2 integers : " a <- getInt b <- getInt -- at this point I would like a list of tuples of the form [ (a `div` 2 , b*2 )] , starting with the 2 initial integers, down to -- where the first value is 1 in the last tuple: ie if the initial numbers were 121 and 3 say , I'd like the list -- [ (121,3) , (60,6) , (30,12) , (15,24) , (7,48) , (3,96) , (1,192) ] produced and available in main for further manipulation. ---------------------------------------- -- to read in an Int , I used : getInt :: IO Int getInt = do line <- getLine return( read line :: Int)

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Looking for a Mentor
by heathmatlock 24 Dec '11

24 Dec '11

Hi, I'm wanting to get started writing some Haskell code and would find it helpful if someone were to meet with me regularly to work on some problems; even if it's just going through Project Euler stuff. Just need to speak with someone weekly checking in my progress and discussing some of the methods used or which should have been used. I certainly don't mind working on bugs for some Haskell related project either. Any takers? -- Heath Matlock +1 256 274 4225

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error monad and Map lookup
by Dennis Raddle 22 Dec '11

22 Dec '11

Is there a more elegant way to write 'test' in the following? import qualified Data.Map as M import Data.Map(Map) import Control.Monad.Error testMap :: Map Int String testMap = M.fromList [(1, "whatever"), (2, "dude")] test :: Int -> Either String String test x = do y <- case M.lookup x testMap of Nothing -> throwError "not in map" Just z -> return z return y

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error logging
by Dennis Raddle 20 Dec '11

20 Dec '11

Okay, I want to write code in the Either monad in order to throw errors with logging. I did this a while ago but can't find my code and I forget what I did. Note that I'm using ghc 6.12.3, which I installed via the haskell platform. First question is: do I use Control.Monad.Error or Control.Monad.Either? Second question is: I want to have each monadic function within the stack of functions that is currently running catch the error and add some context to the log. For instance I need to have a function called, let's say, "catchAndAnnotate" that functions like the following. I'm just making this up-- how is it really done? func1 :: SomeType -> Either String SomeType func1 x = ("running in func1 with argument " ++ show x) `catchAndAnnotate` (do y <- somework x when (isBad y) (throwError "oops") return y) somework :: SomeType -> Either String SomeType somework x = ("running in somework with argument ++ show x) `catchAndAnnotate` (whatever x) whatever :: SomeType -> Either String SomeType whatever = ...

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llvm haskell bindings
by Philippe Sismondi 19 Dec '11

19 Dec '11

I am running OS X 10.6.8 with Xcode 4. I have the Haskell Platform installed, ghc v. 7.0.3. I cannot get the llvm haskell bindings to install correctly using cabal. I get this at the tail end of messages from cabal: checking llvm-c/Core.h usability... yes checking llvm-c/Core.h presence... yes checking for llvm-c/Core.h... yes checking llvm/Support/DynamicLibrary.h usability... yes checking llvm/Support/DynamicLibrary.h presence... yes checking for llvm/Support/DynamicLibrary.h... yes checking for LLVMModuleCreateWithName in -lLLVMCore... no checking for LLVMModuleCreateWithName in -lLLVMCore... no configure: error: could not find LLVM C bindings cabal: Error: some packages failed to install: llvm-0.10.0.1 failed during the configure step. The exception was: ExitFailure 1 Has anyone had any luck getting this set up? Googling leads me to some rather terse comments that the Apple llvm installation does not include the required llvm libraries. I hate to install a separate set of llvm stuff with macports, or the llvm repository, because I fear that I might run into all kinds of puzzling conflicts with the Apple stuff. Ideas? - P -

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FFI export lazy list of string
by Alexander.Vladislav.Popov 19 Dec '11

19 Dec '11

Hi, Haskellers. Advise me please, how I can export lazy and potentially infinite list of string from Haskell program. I think I must call it iteratively: the first call initiate some structure and other calls iterate over it, something like pair of function `find_first' and `find_next'. And how to marshall this structure between programs. Or think in a wrong way? Does any example exist how I can make it? Alexander Popov

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Map instances in the list-tries package
by Matthew Moppett 19 Dec '11

19 Dec '11

I want to use the list-tries package for a little hobby project, but my naive idea about how it should work is obviously wrong: module TryPat where import Data.ListTrie.Patricia.Map a = singleton "harry" 99 yields the error: No instance for (Data.ListTrie.Base.Map.Map map0 Char) arising from a use of `singleton' Possible fix: add an instance declaration for (Data.ListTrie.Base.Map.Map map0 Char) In the expression: singleton "harry" 99 In an equation for `a': a = singleton "harry" 99 Now, there is some stuff about this in the docs: The data types are parametrized over the map type they use internally to store the child nodes: this allows extending them to support different kinds of key types or increasing efficiency. Child maps are required to be instances of the Map class in Data.ListTrie.Base.Map<http://hackage.haskell.org/packages/archive/list-tries/0.4.1/doc/html/Data-…>. Some operations additionally require an OrdMap instance. But frankly, I don't understand it, especially the type signatures. Any hints on how I might go about making an instance for Map map0 Char?

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