What about phantom types? {-# LANGUAGE KindSignatures, DataKinds #-} data IsNormalized = Normalized | NotNormalized data Sum (n :: IsNormalized) = Value Int | Sum (Sum n) (Sum n) You could still say instance MyClass (Sum n) where… but you could also write normalizeSum :: Sum NotNormalized -> Sum Normalized -- or even create a class with two inhabitants for this instance Eq (Sum NotNormalized) where (==) = (==) `on` normalizeSum instance Eq (Sum Normalized) where … -- real implementation It's not ideal. For example if you want to sort a list, it would still be better to normalize the whole list before sorting, but at least you could still use the other operators afterwards without a "denormalization" step. So maybe it would help reduce some boilerplate? Cheers, MarLinn