| The following code (coming from Vertigo), which compiles OK with | ghc-6.4.2, is rejected by ghc in HEAD (from this week-end) : | | data Range = Range String Float Float Float | | class AsRange nrs ns | nrs -> ns where | asRange :: nrs -> (ns, [Range]) | | instance (AsRange rsa nsa, AsRange rsb nsb) | => AsRange (rsa, rsb) (nsa, nsb) where | asRange (a, b) = ((nsa, nsb), rsa++rsb) | where | (nsa, rsa) = asRange a | (nsb, rsb) = asRange b | | produces: | Illegal instance declaration for `AsRange (rsa, rsb) (nsa, nsb)' | (the Coverage Condition fails for one of the functional | dependencies) | In the instance declaration for `AsRange (rsa, rsb) (nsa, nsb)' | | Is it normal ? Yes, this is exactly as expected. (The fact that 6.4.2 allowed it was really a bug.) The behaviour is documented here: http://www.haskell.org/ghc/dist/current/docs/users_guide/type-extensions .html#instance-decls (in 7.4.4.1). You can find the background here http://research.microsoft.com/%7Esimonpj/papers/fd%2Dchr/ However, if you add -fallow-undecidable-instances it'll be allowed (see section 7.4.4.2). Simon