map/flatmap/filter implemented as for-expression

Demo.sc

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], p: A => Boolean): List[A] =
            for (x <- xs if p(x)) yield x
}