Your error is in those last 2 lines.
position.x clamp(position.x, 0, screen_size.x)
position.y clamp(position.y, 0, screen_size.y)
You're missing =
signs there. So, this:
position.x = clamp(position.x, 0, screen_size.x)
position.y = clamp(position.y, 0, screen_size.y)