Mastering Functional Programming

Implementation of the type class

Let's take a look at how the type class can be implemented for a data type. For example, let's look at Either:

implicit val applicative: Applicative[Fx] = new Applicative[Fx] {  override def ap[A, B](ff: Fx[A => B])(fa: Fx[A]): Fx[B] = (ff, fa)  match {    case (Right(f), Right(a)) => Right(f(a))    case (Left(e1), Left(e2)) => Left(e1 ++ e2)    case (Left(e), _) => Left(e)    case (_, Left(e)) => Left(e)  }  override def pure[A](a: A): Fx[A] = Right(a)}

You can see how the type class can be implemented for Either with Left being List[String]. So, as you can see, if two computations are successful, that is, they are Right, we simply combine them. However, if at least one of them is Left, we combine the Left side of both computations ...