fold.scala

import scala.language.higherKinds
import scala.collection.generic.CanBuildFrom
import scala.collection.GenTraversableOnce

def map[A, This[A] <: Traversable[A], B, That](o: This[A])(f: (A) => B)(implicit bf: CanBuildFrom[This[A], B, That]): That = {
      (o.foldLeft (bf.apply()) ({(a, b) => a += f(b)})).result()
}

def filter[A, This[A] <: Traversable[A]](o: This[A])(f: (A) => Boolean)(implicit bf: CanBuildFrom[This[A], A, This[A]]): This[A] = {
      (o.foldLeft (bf.apply()) ({(a, b) => if (f(b)) a += b else a})).result()
}

def flatMap[A, This[A] <: Traversable[A], B, That](o: This[A])(f: (A) => GenTraversableOnce[B])(implicit bf: CanBuildFrom[This[A], B, That]): That = {
      (o.foldLeft (bf.apply()) ({(a, b) => f(b).foldLeft (a) (_ += _)})).result()
}