[Haskell-cafe] Functional Dependencies and multiparametertypeclasses

Harald ROTTER harald.rotter at sagem.com
Mon Aug 24 07:51:03 EDT 2009

Thanks for the quick response. Your explanation is so much clearer than
ghc's error messages :-))


|         |           Miguel Mitrofanov  |
|         |           <miguelimo38 at yandex|
|         |           .ru>               |
|         |                              |
|         |                              |
|         |           24.08.2009 13:29   |
|         |           Delivered date:    |
|         |           24.08.2009 13:31   |
|         |                              |
  |                                                                                                            |
  |     To:       Harald ROTTER <harald.rotter at sagem.com>                                                      |
  |     cc:       haskell-cafe at haskell.org                                                                     |
  |     Subject:  Re: [Haskell-cafe] Functional Dependencies and multiparameter     typeclasses                |

> However, I do not really understand the cause of the original problem.
> 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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