However, I do not really understand the cause of the original problem. Why do I need the functional dependency to make this work ?
Suppose you have an instance: instance Pixel MyPixel where... instance Image MyImage MyPixel where width i = countPixels i ... And somebody (not necessarily you) makes another instance: instance Pixel CustomPixel where... instance Image MyImage CustomPixel where width i = 0 ... Now, take any image im :: MyImage Then you'd expect that width im :: Int But now there are TWO implementations of "width :: MyImage -> Int" and no way for compiler to figure out which one to use. GHC sort of foresees this problem. By adding a functional dependency you instruct it to forbid the second declaration, when the first one is present, thus removing the problem.