しゅごいぃぃぃいいい

MonoidFoldLeft.scala

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

trait FoldLeft[F[_]] {
  def foldLeft[A, B](xs: F[A], b: B, f: (B, A) => B): B
}

object Monoid {
  implicit val intMonoid = new Monoid[Int] {
    def mzero = 0
    def mappend(a: Int, b: Int): Int = a + b
  }
  implicit val stringMonoid = new Monoid[String] {
    def mzero = ""
    def mappend(a: String, b: String): String = a + b
  }
}

object FoldLeft {
  implicit val foldLeftList = new FoldLeft[List] {
    def foldLeft[A, B](xs: List[A], b: B, f: (B, A) => B): B = xs.foldLeft(b)(f)
  }
  implicit val foldLeftVector = new FoldLeft[Vector] {
    def foldLeft[A, B](xs: Vector[A], b: B, f: (B, A) => B): B = xs.foldLeft(b)(f)
  }
}

def sum[M[_]: FoldLeft, A: Monoid](xs: M[A]): A = {
  val m = implicitly[Monoid[A]]
  val fl = implicitly[FoldLeft[M]]
  fl.foldLeft(xs, m.mzero, m.mappend)
}



import Monoid._
import FoldLeft._

sum(List(1,2,3))
//=> 6

sum(List("a", "b", "c"))
//=> "abc"

sum(Vector(1,2,3))
//=> 6

sum(Vector("a", "b", "c"))
//=> "abc"

//sum(List('a', 'b', 'c'))
//=> error

//sum(Array(1, 2, 3))
//=> error