{-
   Hello.hs (adapted from hello.c which is (c) Silicon Graphics, Inc.)
   Copyright (c) Sven Panne 2002-2018 <svenpanne@gmail.com>
   This file is part of HOpenGL and distributed under a BSD-style license
   See the file libraries/GLUT/LICENSE

   This is a simple, introductory OpenGL program.
-}

import Graphics.UI.GLUT

display :: DisplayCallback
display = do
   -- clear all pixels
   clear [ ColorBuffer ]

   -- draw white polygon (rectangle) with corners at
   -- (0.25, 0.25, 0.0) and (0.75, 0.75, 0.0)
   color (Color3 1.0 1.0 (1.0 :: GLfloat))
   -- resolve overloading, not needed in "real" programs
   let vertex3f = vertex :: Vertex3 GLfloat -> IO ()
   renderPrimitive Polygon $ mapM_ vertex3f [
      Vertex3 0.25 0.25 0.0,
      Vertex3 0.75 0.25 0.0,
      Vertex3 0.75 0.75 0.0,
      Vertex3 0.25 0.75 0.0]

   -- don't wait!
   -- start processing buffered OpenGL routines
   flush

myInit :: IO ()
myInit = do
   -- select clearing color
   clearColor $= Color4 0 0 0 0

   -- initialize viewing values
   matrixMode $= Projection
   loadIdentity
   ortho 0 1 0 1 (-1) 1

-- Declare initial window size, position, and display mode (single buffer and
-- RGBA). Open window with "hello" in its title bar. Call initialization
-- routines. Register callback function to display graphics. Enter main loop and
-- process events.
main :: IO ()
main = do
   _ <- getArgsAndInitialize
   initialDisplayMode $= [ SingleBuffered, RGBMode ]
   initialWindowSize $= Size 250 250
   initialWindowPosition $= Position 100 100
   _ <- createWindow "hello"
   myInit
   displayCallback $= display
   mainLoop
