December 2011 - Beginners

question about any
by Manfred Lotz 29 Dec '11

29 Dec '11

Hi there, might be trivial but anyway. I have a usage of 'any' and 'all' but I only have a predicate p :: a -> IO Bool. I wrote my own functons for this: mor :: Monad m => [m Bool] -> m Bool mor = liftM or . sequence mand :: Monad m => [m Bool] -> m Bool mand = liftM and . sequence or' :: Monad m => ( a -> m Bool) -> [a] -> [m Bool] or' _ [] = [] or' p (x:xs) = p x : or' p xs and' :: Monad m => ( a -> m Bool) -> [a] -> [m Bool] and' _ [] = [] and' p (x:xs) = p x : and' p xs myany :: Monad m => (a -> m Bool) -> [a] -> m Bool myany p = mor . or' p myall :: Monad m => (a -> m Bool) -> [a] -> m Bool myall p = mand . and' p which seems to do what I want. Question: Is there any libray function I could use to do this? -- Thanks, Manfred

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list range
by Stanisław Findeisen 28 Dec '11

28 Dec '11

Hi Is list range (for example: [1..10]) a language construct or a function? What type does it have? -- This e-mail address is invalid, see: http://people.eisenbits.com/~stf/public-email-note.html . OpenPGP: E3D9 C030 88F5 D254 434C 6683 17DD 22A0 8A3B 5CC0

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Getting the current system date outside the IO Monad
by AbdulSattar Mohammed 27 Dec '11

27 Dec '11

I'm still learning Haskell and I've started learning Yesod. I pretty much built a blog before I came this silly roadblock. Whenever I create a new post, I want it take the current system time as its creation time. I understand it's an impure operation and it must go into the IO Monad to be performed. But, in Yesod, your handler is of the type Handler RepHtml. I don't get how you could get the impure value into a pure function without main playing any part. Here's the link for my blog: http://hpaste.org/55757#line85 -- Warm Regards, AbdulSattar Mohammed

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

26 Dec '11

Hello! I managed to do it with timeoutAdd: --code below is the onClicked function: drawProgram gui preprocPrg = do let drArea = (drawWin gui) drw <- widgetGetDrawWindow drArea -- let list = compileToList preprocPrg -- renderWithDrawable drw (drawFromList list) renderWithDrawable drw (myDraw 200 200) -- drawWindowProcessUpdates drw True timeoutAdd ((\_ -> do renderWithDrawable drw (myDraw2 200 200) return False) 5) 500 return () But "sometimes" it gives the following error: (<interactive>:23929): Gdk-WARNING **: Using Cairo rendering requires the drawable argument to have a specified colormap. All windows have a colormap, however, pixmaps only have colormap by default if they were created with a non-NULL window argument. Otherwise a colormap must be set on them with gdk_drawable_set_colormap (<interactive>:23929): Gdk-CRITICAL **: IA__gdk_cairo_region: assertion `region != NULL' failed <interactive>: user error (NULL pointer) <interactive>: interrupted <interactive>: warning: too many hs_exit()s What can cause this? I think the other parts of my program are correct, and I cant understand, how can ghci "randomly give" this error. (every 5. try) To be honest, my goal is to make a "turtle graphics" program, so I ll have to write a function, which draws a line on the GUI in slow motion: there will be even more problems with timing, because I ll have to draw the line pixel by pixel.

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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 24 Dec '11

24 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 23 Dec '11

23 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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