Learning Haskell and OpenGL at the same time.

hello-world.hs

import Graphics.Rendering.OpenGL
import Graphics.UI.GLUT
import Data.IORef

main :: IO ()
main = do 
  (progname, _) <- getArgsAndInitialize
  createWindow "Hello World"
  angle <- newIORef 0.0
  displayCallback $= display angle
  repeatedly 10 $ do
    increase angle
    forceDisplay
  mainLoop

display angle = do
  clear [ ColorBuffer ]
  rotationAngle <- get angle
  preservingMatrix $ do 
    rotate rotationAngle zAxis
    renderSquare red 0.2
  sayHello rotationAngle
  flush

repeatedly milliseconds callback = do
  addTimerCallback milliseconds $ again milliseconds callback

again milliseconds callback = do
  callback
  addTimerCallback milliseconds $ again milliseconds callback

increase angle = do
  rotationAngle <- get angle
  angle $=! (rotationAngle + 0.1)

forceDisplay = postRedisplay Nothing

renderSquare c size = renderPrimitive Quads $ do
  color c
  square size

sayHello angle = do
  color white
  currentRasterPosition $= bottomLeftCorner
  renderString TimesRoman24 $ "Hello World! (" ++ (show angle) ++ ")"

square size = 
  let delta = size / 2
  in do
    vertex (vertexAt (0 + delta) (0 - delta) 0)
    vertex (vertexAt (0 + delta) (0 + delta) 0)
    vertex (vertexAt (0 - delta) (0 + delta) 0)
    vertex (vertexAt (0 - delta) (0 - delta) 0)

bottomLeftCorner = Vertex4 (-1.0) (-1.0) 0.0 1.0

red = rgb 1 0 0
white = rgb 1 1 1

zAxis = vectorTo 0 0 1

rgb :: Float -> Float -> Float -> Color3 Float
rgb = Color3

vertexAt :: Float -> Float -> Float -> Vertex3 Float
vertexAt x y z = Vertex3 x y z

vectorTo :: Float -> Float -> Float -> Vector3 Float
vectorTo x y z = Vector3 x y z