ConvertingForToMapFlatMapWithFilter.scala

/**
  * Methods showing how to transform for/yield based expression into pure map/flatMap/withFilter expression.
  * The idea is - this transformations is possible using simple substitutions.
  *
  * Exapmples based on the coursera "Functional Program Design in Scala"
  * Lecture 1.2 "Translation of For"
  * https://www.coursera.org/learn/progfun2/lecture/zJcCG/lecture-1-2-translation-of-for
  *
  * @author Piotr Reszke
  */


case class Book(title: String, authors: List[String])

object ConvertingForToMapFlatMapWithFilter {

  val books = List(
    Book("title1", List("author1", "author2")),
    Book("title2", List("author2", "author3")),
    Book("title3", List("author1", "author2", "author3")),
    Book("title4", List("author3"))
  )

  /**
    * Base method constructed with for/yield. We want to transform this into map/flatMap/withFilter.
    */
  def listBooksByAuthor(author: String) = {
    for(b <- books; a <- b.authors if a == author) yield b.title
  }

  /**
    * First step: changing "b <- books" generator from original method into "books.flatMap"
    */
  def listBooksByAuthor1(author: String) = {
    books.flatMap(b => for (a <- b.authors if a == author) yield b.title)
  }

  /**
    * Second step: change for condition if a == author into b.authors.withFilter
    */
  def listBooksByAuthor2(author: String) = {
    books.flatMap(b => for (_ <- b.authors.withFilter(a => a == author)) yield b.title)
  }

  /**
    * Third step: change simple for yield into single map call
    */
  def listBooksByAuthor3(author: String) = {
    books.flatMap (b => b.authors.withFilter(a => a == author) map (_ => b.title))
  }

  def main(args: Array[String]): Unit = {
    println(listBooksByAuthor("author1"))
    println(listBooksByAuthor1("author1"))
    println(listBooksByAuthor2("author1"))
    println(listBooksByAuthor3("author1"))
  }
}