Scala的for表达式进阶

for表达式强大而简洁的表现力

case class Person(name: String, isMale: Boolean, children: Person*)
object For_Expressive {
    def main(args: Array[String]) {
        val lauren = Person("Lauren", false)
        val rocky = Person("Rocky", true)
        val vivian = Person("Vivian", false, lauren, rocky)
        val persons = List(lauren, rocky, vivian)
        
        val result = persons filter (person => !person.isMale) flatMap(person => 
        (person.children map (child => (person.name, child.name))))
        println(result)
        
        val forResult = for (person <- persons; if !person.isMale; child <- person.children) //scala实质上也会将for循环变为map的方式
        yield (person.name, child.name)
        println(forResult)
    }
}

Scala中For表达式的生成器、定义和过滤器

object ForInAction {
    def main(args: Array[String]) {
        val lauren = Person("Lauren", false)
        val rocky = Person("Rocky", true)
        val vivian = Person("Vivian", false, lauren, rocky)
        val persons = List(lauren, rocky, vivian)
    
        val forResult = for {person <- persons; name = person.name; if !person.isMale; child <- person.children} //生成器、定义、过滤器
        yield (person.name, child.name)
        println(forResult)
    
        val content =for(x <- List(1,2,3); y <- List("Hadoop","Spark","Flink")) yield(x,y) //两个生成器
        println(content)
    }
}

使用For表达式做查询

case class Book(title : String , authors : List[String])
object For_Query {
    def main(args: Array[String]) {
        val books: List[Book] = List(
        Book("Structure and Interpretation ", List("Abelson , Harold", "Sussman")),
        Book("Principles of Compiler Design",
        List("Aho, Alfred", "Ullman, Jeffrey")),
        Book("Programming in Modula-2", List("Wirth, Niklaus")),
        Book("Introduction to Functional Programming", List("Bird, Richard")),
        Book("The Java Language Specification",
        List("Gosling, James", "Joy, Bill", "Steele, Guy", "Bracha, Gilad")))
        
        //    val result = for(b <- books ; a <- b.authors if a startsWith "Gosling") yield b.title
        val result = for(b <- books if (b.title indexOf "Programming") >= 0 ) yield b.title
        println(result)
    }
}

使用For表达式实现map、flatMap、filter

object For_Advanced {
    def main(args: Array[String]) {}
    def map[A, B](list: List[A], f: A => B): List[B] =
        for(element <- list) yield f(element)
    def flatMap[A, B](list: List[A], f: A => List[B]): List[B] =
        for(x <- list; y <- f(x)) yield y
    def filter[A](list: List[A], f: A => Boolean): List[A] =
        for(elem <- list if f(elem)) yield elem
}