Allows application of a function in an Applicative context to a value in an Applicative context.

This structure is an intermediate between a functor and a monad (technically, a strong lax monoidal functor). Compared with monads, this interface lacks the full power of the binding operation (chain), but it has more instances and it is sufficient for many uses.

References:

• The Essence of the Iterator Pattern
• Applicative programming with effects

Note that having an Applicative instance implies Functor and Apply implementations are also available, which is why the Applicative interface is a subtype of Functor and Apply.

Applicative implementations must obey the following laws:

1. Identity: A.ap(A.of(x => x), v) <-> v
2. Homomorphism: A.ap(A.of(f), A.of(x)) <-> A.of(f(x))
3. Interchange: A.ap(u, A.of(y)) <-> A.ap(A.of(f => f(y)), u)
4. Functor's map (can be derived): A.map(f, u) <-> A.ap(A.of(f), u)

Equivalent with the Chain type-class in the static-land specification.