I've attached the simplest example of my code that used to compile in GHC 6.10 now gives the error in GHC 6.12.1: baddata.hs:33:14: No instances for (Data Const, Data Var) arising from the 'deriving' clause of a data type declaration at baddata.hs:33:14-17 Possible fix: add an instance declaration for (Data Const, Data Var) or use a standalone 'deriving instance' declaration instead, so you can specify the instance context yourself When deriving the instance for (Data (Domain e g)) If I replace:
data Domain e g = Domain (Expr (Const :+: Var)) deriving (Data, Typeable)
with:
data Domain e g = Domain (Expr (Const)) deriving (Data, Typeable)
then everything compiles. If nothing else, the error message is misleading. -Ron