Re: [Haskell-cafe] Factoring into type classes

Am Montag, den 19.01.2009, 14:47 +0100 schrieb Daniel Fischer:

Am Montag, 19. Januar 2009 14:31 schrieb Antoine Latter:

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2009/1/19 Luke Palmer :

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As a side curiosity, I would love to see an example of any data structure which has more than one Functor instance. Especially those which have more than one useful functor instance.

(,) ?

-Antoine

Wrong kind. And (,) a has only one useful instance.

What about instance Functor ((,) a) where fmap f (x,y) = (x, f y) and instance Functor (, a) where fmap f (x, y) = (f x, y) ? Of course, the latter is not legal Haskell. But if it was, then it might be useful. Is there any way to declare this Functor instance, possibly with some GHC extensions?