+ -- For now, I only allow tau-types (not polytypes) in
+ -- the head of an instance decl.
+ -- E.g. instance C (forall a. a->a) is rejected
+ -- One could imagine generalising that, but I'm not sure
+ -- what all the consequences might be
+ check_one ty = do { check_tau_type (Rank 0) UT_NotOk ty
+ ; checkTc (not (isUnLiftedType ty)) (unliftedArgErr ty) }
+