Code from Haskell School of Expression hanging.

29 Jan 2011

      I got Hudak's Haskell School of Expression today and just tried a couple of the graphics programs from the software I downloaded at the book site. Both seem to hang on opening a window. Is there a fix for this problem? I'm on a Fedora 14 Linux (32-bit) box running GHC version 6.12.3.

Michael

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*SimpleGraphics> :l Snowflake.lhs
[1 of 2] Compiling SOE              ( SOE.hs, interpreted )
[2 of 2] Compiling Snowflake        ( Snowflake.lhs, interpreted )
Ok, modules loaded: Snowflake, SOE.
*Snowflake> main

============

This code was automatically extracted from a .lhs file that
uses the following convention:

-- lines beginning with ">" are executable
-- lines beginning with "<" are in the text,
     but not necessarily executable
-- lines beginning with "|" are also in the text,
     but are often just expressions or code fragments.

...
module Snowflake where
import SOE

m = 81  :: Int -- multiple of 3 for triangle size
x = 250 :: Int -- x and y coordinates of
y = 250 :: Int --         center of snowflake
colors = [ Magenta, Blue, Green, Red, Yellow ]

snowflake :: Window -> IO ()
snowflake w = do
   drawTri w x y m 0 False -- draw first triangle w/flat top
   flake   w x y m 0 True  -- begin recursion to complete job

flake :: Window -> Int -> Int -> Int -> Int -> Bool -> IO ()
flake w x y m c o = do
   drawTri w x y m c o  -- draw second triangle
   let c1 = (c+1)`mod`5 -- get next color
   if (m<=3) then return ()  -- if too small, we're done
      else do
        flake w (x-2*m) (y-m) (m`div`3) c1 True  -- NW
        flake w (x+2*m) (y-m) (m`div`3) c1 True  -- NE
        flake w  x    (y+2*m) (m`div`3) c1 True  -- S
        flake w (x-2*m) (y+m) (m`div`3) c1 False -- SW
        flake w (x+2*m) (y+m) (m`div`3) c1 False -- SE
        flake w  x    (y-2*m) (m`div`3) c1 False -- N

drawTri :: Window -> Int -> Int -> Int -> Int -> Bool -> IO ()
drawTri w x y m c o =
   let d =  (3*m) `div` 2
       ps = if o
            then [(x,y-3*m),  (x-3*m,y+d), (x+3*m,y+d)] -- side at bottom
            else [ (x,y+3*m), (x-3*m,y-d), (x+3*m,y-d)] -- side at top
   in drawInWindow w
        (withColor (colors !! c)
           (polygon ps))

main
   = runGraphics (
     do w <- openWindow "Snowflake Fractal" (500,500)
        drawInWindow w (withColor White
          (polygon [(0,0),(499,0),(499,499),(0,499)]))
        snowflake w
        spaceClose w
     )

spaceClose :: Window -> IO ()
spaceClose w
   = do k <- getKey w
        if k==' ' || k == '\x0'
           then closeWindow w
           else spaceClose w

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