“…Consider for instance automata theory: deterministic automata can be conveniently regarded as certain kind of coalgebras on Set [33], nondeterministic automata as the same kind of coalgebras but on EM(P f ) [35], and weighted automata on EM(S) [4]. Here, P f is the finite powerset monad, modelling nondeterministic computations, while S is the monad of semimodules over a semiring S, modelling various sorts of quantitative aspects when varying the underlying semiring S. It is worth mentioning two facts: first, rather than taking coalgebras over EM(T ), the category of algebras for the monad T , one can also consider coalgebras over Kl(T ), the Kleisli category induced by T [20]; second, these two approaches based on monads have lead not only to a deeper understanding of the subject, but also to effective proof techniques [6,7,14], algorithms [1,8,22,36,39] and logics [19,21,27].…”