Map and filter implemented in for expressions

// scala code
object Test {

  object Demo {
    def map[A, B](xs: List[A], f: A => B): List[B] = {
      for(x <- xs) yield f(x)
    }

    def flatMap[A, B](xs: List[A], f: A => List[B]): List[B] = {
      for(x <- xs; y <- f(x)) yield y
    }

    def filter[A](xs: List[A], f: A => Boolean): List[A] = {
      for(x <- xs; if f(x)) yield x
    }
  }

}