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. This is on ghc 6.12.1.20100203, but if people can't reproduce it I'll install a newer one; I'm just not eager to do that because of course it means rebuilding quite a lot of things. {-# LANGUAGE GADTs #-} module Foo where data TemplateValue t where TemplateList :: [a] -> TemplateValue [a] instance (Eq a) => Eq (TemplateValue a) where (==) (TemplateList a) (TemplateList b) = (==) a b Foo.hs:7:45: Could not deduce (Eq a1) from the context (a ~ [a2]) arising from a use of `==' at Foo.hs:7:45-52 Possible fix: add (Eq a1) to the context of the constructor `TemplateList' In the expression: (==) a b In the definition of `==': == (TemplateList a) (TemplateList b) = (==) a b In the instance declaration for `Eq (TemplateValue a)' -- Dan Knapp "An infallible method of conciliating a tiger is to allow oneself to be devoured." (Konrad Adenauer)