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Task/Animate-a-pendulum/Haskell/animate-a-pendulum.hs

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+import Graphics.HGL.Draw.Monad (Graphic, )
+import Graphics.HGL.Draw.Picture
+import Graphics.HGL.Utils
+import Graphics.HGL.Window
+import Graphics.HGL.Run
+
+import Control.Exception (bracket, )
+import Control.Arrow
+
+toInt = fromIntegral.round
+
+pendulum = runGraphics $
+  bracket
+    (openWindowEx "Pendulum animation task" Nothing (600,400) DoubleBuffered (Just 30))
+    closeWindow
+    (\w -> mapM_ ((\ g -> setGraphic w g >> getWindowTick w).
+		    (\ (x, y) -> overGraphic (line (300, 0) (x, y))
+				  (ellipse (x - 12, y + 12) (x + 12, y - 12)) )) pts)
+ where
+    dt = 1/30
+    t = - pi/4
+    l = 1
+    g = 9.812
+    nextAVT (a,v,t) = (a', v', t + v' * dt) where
+	a' = - (g / l) * sin t
+	v' = v + a' * dt
+    pts = map (\(_,t,_) -> (toInt.(300+).(300*).cos &&& toInt. (300*).sin) (pi/2+0.6*t) )
+	$ iterate nextAVT (- (g / l) * sin t, t, 0)