[Haskell-cafe] Re: Problem with fundeps.

Ashley Yakeley ashley at semantic.org
Sun Jan 2 17:04:31 EST 2005

In article <courier.41D863C1.00005081 at averell>, karczma at info.unicaen.fr 

GHCi is correct to complain:

> class Vspace a v | v -> a

OK, the first parameter ("a") depends on the second ("v").

> instance Vspace a a where

And this determines it: the first parameter must always be the same as 
the second.

> instance (Vspace a v) => Vspace a (c->v) where

This is incompatible with the previous instance declaration, since "a" 
is not the same as "c -> v". And any instance decl that were permitted 
by the fundep would in any case overlap.

Ashley Yakeley, Seattle WA

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