from Scalaz Video

Fp.scala

package org.songpp.fp
/*
 * scala
 */
trait FoldLeft[F[_]] {
  def foldLeft[A, B](xs: F[A], init: B, func: (B, A) => B): B
}

object FoldLeft {
  implicit object FoldLeftList extends FoldLeft[List] {
    def foldLeft[A, B](xs: List[A], init: B, func: (B, A) => B): B = xs.foldLeft(init)(func)
  }
}

trait Monoid[T] {
  def mappend(a: T, b: T): T
  def mzero: T
}

object Monoid {

  implicit object IntMonoid extends Monoid[Int] {
    def mappend(a: Int, b: Int): Int = a + b
    def mzero = 0
  }

  implicit object StringMonoid extends Monoid[String] {
    def mappend(a: String, b: String): String = a + b
    def mzero = ""
  }

}

object Main {

  def multmonoid = new Monoid[Int] {
    def mappend(a: Int, b: Int): Int = a * b
    def mzero = 1
  }

  def sum[M[_], T](xs: M[T])(implicit monoid: Monoid[T], fl: FoldLeft[M]) = fl.foldLeft(xs, monoid.mzero, monoid.mappend)

  def p(a: Any) = println("===> " + a.toString())

  def main(args: Array[String]): Unit = {
    println()

    p(sum(List(1, 2, 3, 4)))
    p(sum(List(1, 2, 3, 4))(multmonoid, FoldLeft.FoldLeftList))
    p(sum(List("a", "b", "c")))

    println()
  }

}