This, wireframe :: Double -> Double -> Double -> IO () wireframe wx wy wz = do -- yz plane renderPrimitive LineLoop $ do vertex $ Vertex3 0.0 0.0 0.0 vertex $ Vertex3 0.0 wy 0.0 vertex $ Vertex3 0.0 wy wz vertex $ Vertex3 0.0 0.0 wz produces this: No instance for (VertexComponent Double) arising from a use of `vertex' Possible fix: add an instance declaration for (VertexComponent Double) In the expression: vertex In a stmt of a 'do' block: vertex $ Vertex3 0.0 wy 0.0 In the second argument of `($)', namely `do { vertex $ Vertex3 0.0 0.0 0.0; vertex $ Vertex3 0.0 wy 0.0; vertex $ Vertex3 0.0 wy wz; vertex $ Vertex3 0.0 0.0 wz }' and thusly this :-( Changing the declaration to GLdouble -> GLdouble -> GLdouble -> IO() and using (0.0::GLdouble) fixes it, and I'm not clear on why it's not automagic. There are many times I see the compiler doing type conversion an numerican arguments although sometimes the occasional fracSomethingIntegralorOther is required. I was hoping for some enlightenment. Thank you. Brian