Haskell引入Applicative functor有什么意义？

Though rarer in computer science, one can use category theory directly, which defines a monad as a functor with two additional natural transformations. So to begin, a structure requires a higher-order function (or "functional") named map to qualify as a functor:

category of functors (from values to computations)

So if you don't yet understand functors or applicative functors, check out part 1 and part 2 of this series!