[SOLVED] haskell & Gtk2hs : no point is displayed in a canvas (parallel drawing)

I wrote a little program which has to display the mandelbrot set, in haskell, using Gtk2Hs.

thought there is no compilation error, nothing is displayed in the canvas (the component in which the points are colored)...

could you help me to debug this logical error?

my code:

module Main where

import           Control.Monad            (when)
import           Graphics.Rendering.Cairo as C
import           Graphics.UI.Gtk
import           Graphics.UI.Gtk.Builder  ()


main :: IO()
main = do
    _ <- initGUI
    builder <- builderNew
    builderAddFromFile builder "09-mandelbrot.ui"

    window   <- builderGetObject builder castToWindow "Figure de Mandelbrot"
    canvas <- builderGetObject builder castToDrawingArea "drawingarea1"
    _ <- onExpose canvas $ const (updateCanvas canvas)

    widgetShowAll window
    mainGUI


updateCanvas :: DrawingArea -> IO Bool
updateCanvas canvas = do
  win <- widgetGetDrawWindow canvas
  (width, height) <- widgetGetSize canvas
  _ <- mapM_ (affiche win)  (points (fromIntegral width) (fromIntegral height))

  return True

k :: Int
k=100

mandelbrot :: Double -> Double -> Bool
mandelbrot a b =
  let
    mandelrec :: Double -> Double -> Int -> Bool
    mandelrec x y i
      | (x * x + y * y > 4) = False
      | (i==k) && (x * x + y * y <= 4) = True
      | otherwise = mandelrec x' y' (i+1)
            where x' = x * x - y * y + a
                  y' = 2 * x * y + b
  in mandelrec 0 0 0

affiche2 :: DrawWindow -> Double -> Double -> IO()
affiche2 win a b = do
  renderWithDrawable win $ setSourceRGB 0 1 0
  renderWithDrawable win $ setLineWidth 1
  renderWithDrawable win $ C.rectangle a b 1 1 
  renderWithDrawable win stroke


affiche :: DrawWindow -> ((Double,Double), (Double,Double)) -> IO ()
affiche win ((a0,a), (b0,b)) = when (mandelbrot a b) $ postGUIAsync (affiche2 win a0 b0)

colonnes :: Double -> [(Double, Double)]
colonnes w = [ (t,t/w*4-2) | t<-[0..(w-1)] ]

lignes :: Double -> [(Double, Double)]
lignes h = [ (t,t/h*4-2) | t<-[0..(h-1)] ]

points :: Double -> Double -> [((Double, Double), (Double, Double))]
points w h = [ (colonne,ligne)| colonne <- colonnes w,ligne <- lignes h]

main() is not interesting, it works, I am sure. update_canvas grab some values (width, height, win) and call the side-effect function, affiche, providing it the values in "points" (points contains the good values, namely the coordinates of the points between [-2..2] for the 2 axes. mandelbrot is good, since I succeeded in drawing the mandelbrot set (yet all points were drawed together). I think if there is a problem, it could come from affiche or affiche2, but I'm new to Gtk programming.

thank you.

EDIT

Well, it works with your change, but why? and I have another question : if I raise the parameter k (say to 1000), the set is displayed only 17 seconds after having launched the program, and it is displayed very quickly; but that's not what I would like : I want the points be drawed as soon as they are computed. DO you know what change I must do?

EDIT 2

here is a code which - works : it displays the image in <10s - doesn't use any UI or GLADE file - draws points after all are computed

module Main where

import           Control.Monad            (when)
import           Graphics.Rendering.Cairo as C
import           Graphics.UI.Gtk
import           Graphics.UI.Gtk.Builder  ()


main :: IO()
main = do
    _ <- initGUI

    window <- windowNew
    windowSetPosition window WinPosCenter
    windowSetDefaultSize window 500 350
    set window [windowTitle := "Ensemble de Mandelbrot"]
    on window objectDestroy mainQuit

    canvas <- drawingAreaNew
    canvas `on` sizeRequest $ return (Requisition 450 300)
    window `containerAdd` canvas

    _ <- onExpose canvas $ const (updateCanvas canvas)

    widgetShowAll window
    mainGUI


updateCanvas :: DrawingArea -> IO Bool
updateCanvas canvas = do
  win <- widgetGetDrawWindow canvas
  (width, height) <- widgetGetSize canvas
  _ <- mapM_ (affiche win)  (points (fromIntegral width) (fromIntegral height))

  return True

k :: Int
k=100 -- 100 : after launching, u must wait less than 10s

mandelbrot :: Double -> Double -> Bool
mandelbrot a b =
  let
    mandelrec :: Double -> Double -> Int -> Bool
    mandelrec x y i
      | (x * x + y * y > 4) = False
      | (i==k) && (x * x + y * y <= 4) = True
      | otherwise = mandelrec x' y' (i+1)
            where x' = x * x - y * y + a
                  y' = 2 * x * y + b
  in mandelrec 0 0 0

affiche2 :: DrawWindow -> Double -> Double -> IO()
affiche2 win a b = renderWithDrawable win $ do
    setSourceRGB 0 0 0
    setLineWidth 1
    C.rectangle a b 1 1 
    stroke


affiche :: DrawWindow -> ((Double,Double), (Double,Double)) -> IO ()
affiche win ((a0,a), (b0,b)) = when (mandelbrot a b) $ postGUIAsync (affiche2 win a0 b0)

colonnes :: Double -> [(Double, Double)]
colonnes w = [ (t,t/w*4-2) | t<-[0..(w-1)] ]

lignes :: Double -> [(Double, Double)]
lignes h = [ (t,t/h*4-2) | t<-[0..(h-1)] ]

points :: Double -> Double -> [((Double, Double), (Double, Double))]
points w h = [ (colonne,ligne)| colonne <- colonnes w,ligne <- lignes h]

olivier