Scala example source code file (EndoUsage.scala)

This example Scala source code file (EndoUsage.scala) is included in the alvinalexander.com "Java Source Code Warehouse" project. The intent of this project is to help you "Learn Scala by Example" ^TM.

Learn more about this Scala project at its project page.

Java - Scala tags/keywords

app, endo, endousage, ilist, int, o\'connor, person, stew, string

The EndoUsage.scala Scala example source code

package scalaz.example

import scalaz.{Endo,IList,Zip}
import scalaz.std.function._
import scalaz.std.anyVal._
import scalaz.syntax.monoid._
import scalaz.syntax.foldable._

object EndoUsage extends App {
  // The scala Endo class is a class which wraps functions from A ⇒ A
  // for some A. This class exists in order to supply some special
  // typeclass instances, since functions where the domain and the
  // codomain are the same type have some special properties.

  // there already exists a Monoid instance for any Function1 where
  // there exists a monoid for the codomain, the append function of
  // this monoid returns a function which for a given input, returns
  // the resut of applying each function to this input and append the
  // two results to each other:
  val f1: Int ⇒ Int = _ + 1
  val f2: Int ⇒ Int = _ * 10

  assert(f1(1) == 2)
  assert(f2(1) == 10)

  val f3 = f1 |+| f2

  assert(f3(1) == f1(1) + f2(1))
  assert(f3(10) == f1(10) + f2(10))

  // for an endofunctor, we can alternatively represent the monoid
  // append operation as function composition:

  val ef3 = Endo(f1) |+| Endo(f2)

  assert(ef3(1) == (f1 compose f2)(1))
  assert(ef3(10) == (f1 compose f2)(10))

  // having this monoid allows us to take a Foldable full of
  // endomorphisms for some type, and squash that down into a single
  // function:

  case class Person(first: String, last: String, age: Int)

  val lowercaseFirst: Endo[Person] = Endo(p ⇒ p.copy(first=p.first.toLowerCase))
  val lowercaseLast: Endo[Person] = Endo(p ⇒ p.copy(last=p.last.toLowerCase))
  val trimFirst: Endo[Person] = Endo(p ⇒ p.copy(first=p.first.trim))
  val trimLast: Endo[Person] = Endo(p ⇒ p.copy(last=p.last.trim))
  val birthday: Endo[Person] = Endo(p ⇒ p.copy(age = p.age+1))
  val endos: IList[Endo[Person]] = IList(lowercaseFirst, lowercaseLast, trimFirst, trimLast, birthday)

  val stewBefore = Person(" Stew ", "O'Connor", 70)
  assert(endos.suml.apply(stewBefore) == Person("stew", "o'connor", 71))

  // endofunctors can be zipped together to get back an endofunctor that operates on tuples:
  val ints = IList(1,2,3)
  val strings = IList("a","b","c")

  val plus1: Endo[Int] = Endo(_ + 1)
  val toUpper: Endo[String] = Endo(_.toUpperCase)

  val foo = (ints zip strings) map Zip[Endo].zip(plus1, toUpper).run
  assert(foo == IList((2,"A"),(3,"B"),(4,"C")))
}