On Friday 04 March 2005 16:32, Keean Schupke wrote:
> robert dockins wrote:
> > Is that really how this is done? That doesn't seem like it can be right:
> >
> > instance X (a b) -- single parameter class where 'a' has an arrow kind
> >
> > is very different from:
> >
> > instance X a b -- multiple parameter class
> >
> > I would expect a type constructed with 'appT' to correspond to the
> > first declaration, and not to the second.
>
> Yup, thats how it is done, I have some complex working TH that generates
> multi parameter classes with fundeps
> instances etc... and I can say for definite it all works fine:
>
> For the above examples
>
> appT X (appT a b) -- X is applied once (to a applied to b)
>
> appT (appT X a) b -- X is applied twice first to a then to b
But this has nothing to do with the instance question. I agree with Robert
that
AppT a b
definitely sounds like type constructor application. How can this help with
multi parameter class instances? Consider:
class Bogus a b
instance Bogus Int Char
How do you express the /instance/ in TH? Using AppT? AppT would make sense for
instance Show a => Show [a] where ...
where one would express the '[a]' in TH as
AppT ListT (VarT mkName "a")
Ben
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