Dear Haskellers, I'm working on a small library for representing semigroup (or monoid) actions on a set http://hackage.haskell.org/package/semigroups-actions. The MultiParamTypeClasses extension seems to be best suited for the task, as a group can act on many sets, and a set can be acted on by different groups: -- | Represents an action of semigroup @g@ to set @a@. -- -- Laws: @'Endo' . 'act'@ must be a homomorphism of semigroups. class Semigroup g => SemigroupAct g a where act :: g -> (a -> a) But soon I realized that with MPTC the compiler has problems inferring types and I had to explicitly specify types when using `act` in many places. Because it seems that in most cases a set will have only a single group acting on it, I was thinking about using FDs: class Semigroup g => SemigroupAct g a | a -> g where But on the other hand, this can limit the generality of the type class. I cannot decide which one I should choose. What would you suggest? According to your experience, would you choose plain MPTC or FD? Best regards, Petr Pudlak