I have problems with upgrading some already working code for GHC 6.12. The following code: ----------------------------------------------------------- {-# LANGUAGE DeriveDataTypeable, GeneralizedNewtypeDeriving, TemplateHaskell #-} import Data.Data import Language.Haskell.TH.Syntax class Nat a class (Data a, Lift a) => ProcType a instance (Lift a, Nat s) => Lift (FSVec s a) where lift (FSVec xs) = undefined newtype Nat s => FSVec s a = FSVec {unFSVec :: [a]} deriving (Eq, Typeable, Data) -- The following line does not cause a problem in GHC 6.10 -- instance (Typeable s, Nat s, ProcType a) => ProcType (FSVec s a) ----------------------------------------------------------- works fine in ghc 6.10.4 and I get (extra stuff removed): // *Main> :i FSVec newtype (Nat s) => ... instance (Data a, Typeable s, Nat s) => Data (FSVec s a) -- Defined at ghctest.hs:16:26-29 ... // but in 6.12.2 what I get is: // *Main> :i FSVec newtype (Nat s) => ... instance (Data s, Data a, Nat s) => Data (FSVec s a) ... // So, although the last line in the above code compiles correctly in ghc 6.10.4, in 6.12.2 GHC complains with: // ghctest.hs:19:10: Could not deduce (Data s) from the context (Typeable s, Nat s, ProcType a) arising from the superclasses of an instance declaration at ghctest.hs:19:10-64 Possible fix: add (Data s) to the context of the instance declaration In the instance declaration for `ProcType (FSVec s a)' Failed, modules loaded: none. // Why are they different? Why 6.12 derives a stronger constraint? Is it a bug or it was not working correctly before? -- Hosein Attarzadeh