Thanks for the quick response. Your explanation is so much clearer than
ghc's error messages :-))
Harald.
|---------+------------------------------>
| | Miguel Mitrofanov |
| | <miguelim...@yandex|
| | .ru> |
| | |
| | |
| | 24.08.2009 13:29 |
| | Delivered date: |
| | 24.08.2009 13:31 |
| | |
|---------+------------------------------>
>------------------------------------------------------------------------------------------------------------|
|
|
| To: Harald ROTTER <harald.rot...@sagem.com>
|
| cc: haskell-cafe@haskell.org
|
| Subject: Re: [Haskell-cafe] Functional Dependencies and multiparameter
typeclasses |
>------------------------------------------------------------------------------------------------------------|
> 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.
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