On Tue, Aug 17, 2010 at 8:54 PM, Dan Knapp
Below, please find a snippet from a program I'm working on, and the error it produces. I was told in #haskell that this was "pretty suspect" and could conceivably be a ghc bug. So I'm reporting it here. I'd also be grateful for workarounds. [...]
I've just found a fairly simple one: make instance resolution happen where the equality constraints are not in scope. {-# LANGUAGE GADTs #-} data TemplateValue a where TemplateList :: [a] -> TemplateValue [a] instance Eq a => Eq (TemplateValue a) where (==) = eqBy (==) eqBy :: (a -> a -> Bool) -> TemplateValue a -> TemplateValue a -> Bool eqBy (==) (TemplateList xs) (TemplateList ys) = xs == ys I've also found that, surprisingly at this point, the following typechecks: eq :: Eq a => TemplateValue a -> Bool eq (TemplateList xs) = xs == xs