Mastering Functional Programming

Applicative type class

We would like to define a new primitive for the preceding scenario. We can call this method zip:

type Fx[A] = Either[List[String], A]def zip[A, B](f1: Fx[A], f2: Fx[B]): Fx[(A, B)] = (f1, f2) match {  case (Right(r1), Right(r2)) => Right((r1, r2))  case (Left(e1), Left(e2)) => Left(e1 ++ e2)  case (Left(e), _) => Left(e)  case (_, Left(e)) => Left(e)}

The method will take two computations as its arguments, and it will output a combined result of two of its supplied inputs as a tuple under their common effect type.

Also notice that we are dealing with specific cases of Left being a list of strings. This is to combine multiple error strings for multiple failed computations into one error report.

The way it works is that if ...