move uniform write into loop

src/Main.hs

@@ -141,10 +141,14 @@ main = do
 
   -- -- UNIFORMS
 
-  -- get the uniform's location out of the shader program
+  -- -- get the uniform's location out of the shader program
-  uniLoc <- get $ GL.uniformLocation sp "u_color"
+  -- uniLoc <- get $ GL.uniformLocation sp "u_color"
-  -- write data to the uniform
+  -- -- write data to the uniform
-  GL.uniform uniLoc $= (GL.Color4 1 0.5 0 1 :: GL.Color4 GL.GLfloat)
+  -- GL.uniform uniLoc $= (GL.Color4 1 0.5 0 1 :: GL.Color4 GL.GLfloat)
+
+  -- create an MVar for pulsating red channel
+  red <- newMVar 0
+  increment <- newMVar 0.05
 
   err <- get GL.errors
   print $ "pre-loop errors: " <> show err
@@ -171,6 +175,17 @@ main = do
       -- clear buffers before drawing
       GL.clear [GL.ColorBuffer]
       
+      -- throw away previous errors
+      -- void $ get GL.errors
+
+      -- get the uniform's location out of the shader program
+      uniLoc <- get $ GL.uniformLocation sp "u_color"
+      -- write data to the uniform
+      redValue <- takeMVar red
+      incrementValue <- takeMVar increment
+      let newRed = redValue + incrementValue
+      GL.uniform uniLoc $= (GL.Color4 newRed 0.5 0 1 :: GL.Color4 GL.GLfloat)
+
       -- the actual drawing happens here
       void $ get GL.errors
       GL.drawElements GL.Triangles 6 GL.UnsignedInt nullPtr