On Tue, Aug 17, 2010 at 8:54 PM, Dan Knapp <[email protected]> wrote:
> 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
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