Benjamin Franksen wrote:
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 LiftT (VarT mkName "a")
That would be:
(using 6.4 syntax)
AppT (AppT (ConT (mkName "Bogus")) (ConT ''Int)) (ConT ''Char)
If the instance is using type variables:
instance Bogus Int a
you get:
AppT (AppT (ConT (mkName "Bogus")) (ConT ''Int)) (VarT (mkName "a"))
Keean.
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