Hi I get the error message: ERROR - *** The type checker has reached the cutoff limit while trying to *** determine whether: *** Typeable (Foo Expr) *** can be deduced from: *** () *** This may indicate that the problem is undecidable. However, *** you may still try to increase the cutoff limit using the -c *** option and then try again. (The current setting is -c100) I find this surprising because in the same file I have: typename_Expr = mkTyCon "Expr" instance Typeable Expr where typeOf _ = mkTyConApp typename_Expr [] typename_Foo = mkTyCon "Foo" instance Typeable1 Foo where { typeOf1 _ = mkTyConApp typename_Foo [] } instance Typeable a => Typeable (Foo a) where { typeOf = typeOfDefault } I then went even further, giving it an explicit instance with: instance Typeable (Foo Expr) where And still I get the same error message. I originally had -c40 (default), but bumping it up has no effect. Any idea what may be causing this? Thanks Neil