An Applicative functor is a type constructor which lets you lift functions of arbitrary numbers of arguments to functions whose arguments are wrapped with that

An Applicative functor is a type constructor which lets you lift functions of arbitrary numbers of arguments to functions whose arguments are wrapped with that type constructor. Concretely, if you think about promises in JavaScript, for example, if you have a function A -> B -> C -> D, say, and you have three promises of types Promise A, Promise B and Promise C, you can construct a Promise of type Promise D by running the three in parallel, and applying your three-argument function when they are all done. So you've turned a function of type A -> B -> C -> D into one of type Promise A -> Promise B -> Promise C -> Promise D. If you can do that sensibly for any number of function arguments, you have an Applicative. ("sensibly" here means that there are type class laws which have to hold) Every Monad is also an Applicative functor, since you could use do notation to compose your promises instead, but there are other interesting Applicatives which do not come from Monads.