Scala Programming Projects

cats.Apply

Apply is a subclass of Functor. It declares an additional ap function. Here is a simplified definition of cats.Apply:

trait Apply[F[_]] extends Functor[F] {
  def ap[A, B](ff: F[A => B])(fa: F[A]): F[B]
  /** Alias for [[ap]]. */
  @inline final def <*>[A, B](ff: F[A => B])(fa: F[A]): F[B] =
  ap(ff)(fa)

This signature means that for a given context F, if we have a function A => B inside of F, we can apply it to A inside of another F to obtain F[B]. We can also use ap alias operator, <*>. Let's try it out with different F contexts, as follows:

import cats.implicits._ 
Option[String => String]("Hello " + _).ap(Some("Apply"))
// res0: Option[String] = Some(Hello Apply)
Option[String => String]("Hello " + _) <*> None
// res1: Option[String] ...