问题 It is widely known that Applicative generalises Arrows . In the Idioms are oblivious, arrows are meticulous, monads are promiscuous paper by Sam Lindley, Philip Wadler and Jeremy Yallop it is said that Applicative is equivalent to Static Arrows, that is arrows for which the following isomorphism holds: arr a b :<->: arr () (a -> b) As far as I can understand, it could be illustrated the following way: Note: newtype Identity a = Id { runId :: a } . Klesli Identity is a Static Arrow as it wraps