[Haskell-cafe] Functional Dependencies and multiparameter typeclasses

[Harald ROTTER]

Dear Haskellers,

I am using multi parameter typeclasses to represent images consisting of
pixels (e.g. bitmap images).

{-# OPTIONS_GHC -XMultiParamTypeClasses #-}
module Bitmap where

      -- | a pixel could be a Word8 (e.g. a graysclale image)
      -- | or a 3-tuple for RGB images
      class Pixel p where
            .... <pixel related functions go here>


      -- | an image could be a UArray or a list of lists of pixels
      class Pixel p => Image a p where
            width  :: a -> Int
            height :: a -> Int
            dims   :: a -> (Int, Int)
            dims img = (height img, width img)


If I try to load this module into ghci I get:

      Could not deduce (Image a p) from the context (Image a p2)
      arising from a use of 'height' at .....

      and

      Could not deduce (Image a p1) from the context (Image a p2)
      arising from a use of 'width' at ......

where both errors originate from the 'dims' function.
Eventually I figured out that I could remedy the situattion by using
functional dependencies like this:

      class Pixel p => Image a p | a -> p where
            ...

However, I do not really understand the cause of the original problem. Why
do I need the functional dependency to make this work ?
Any help is appreciated.

Thanks

Harald.



" Ce courriel et les documents qui y sont attaches peuvent contenir des informations confidentielles. Si vous n'etes  pas le destinataire escompte, merci d'en informer l'expediteur immediatement et de detruire ce courriel  ainsi que tous les documents attaches de votre systeme informatique. Toute divulgation, distribution ou copie du present courriel et des documents attaches sans autorisation prealable de son emetteur est interdite." 

" This e-mail and any attached documents may contain confidential or proprietary information. If you are not the intended recipient, please advise the sender immediately and delete this e-mail and all attached documents from your computer system. Any unauthorised disclosure, distribution or copying hereof is prohibited."