scala collection monad

build.sbt

scalaVersion := "2.10.2"

scala_collection_monad.scala

import scala.collection.generic.{GenericTraversableTemplate, GenericCompanion, CanBuildFrom}
import collection.{GenTraversableLike, GenTraversable}

// https://github.com/scalaz/scalaz/blob/v7.1.0-M2/core/src/main/scala/scalaz/std/IndexedSeq.scala

trait Functor[F[_]]{
  def map[A, B](fa: F[A])(f: A => B): F[B]
}

trait Monad[F[_]] extends Functor[F]{
  def unit[A](a: A): F[A]
  override def map[A, B](fa: F[A])(f: A => B): F[B] = flatMap(fa)(a => unit(f(a)))
  def flatMap[A, B](fa: F[A])(f: A => F[B]): F[B]
}

trait GenTraversableMonad{

  type CC[+A] <: GenTraversable[A] with GenericTraversableTemplate[A, CC] with GenTraversableLike[A, CC[A]]
  protected def companion: GenericCompanion[CC]
  protected implicit def canBuildFrom[A, B]: CanBuildFrom[CC[A], B, CC[B]]

  implicit val instance: Monad[CC] =
    new Monad[CC]{
      def flatMap[A, B](fa: CC[A])(f: A => CC[B]) = fa flatMap f
      def unit[A](a: A) = companion.apply(a)
      override def map[A, B](fa: CC[A])(f: A => B) = fa map f
    }
}

object VectorMonad extends GenTraversableMonad{
  override type CC[+A] = Vector[A]
  protected def companion = Vector
  protected implicit def canBuildFrom[A, B] = Vector.canBuildFrom
}

object ListMonad extends GenTraversableMonad{
  override type CC[+A] = List[A]
  protected def companion = List
  protected implicit def canBuildFrom[A, B] = List.canBuildFrom
}

object StreamMonad extends GenTraversableMonad{
  override type CC[+A] = Stream[A]
  protected def companion = Stream
  protected implicit def canBuildFrom[A, B] = Stream.canBuildFrom
}

object Main extends App {
  import ListMonad._
  implicitly[Monad[List]].flatMap(List(1)){a => List(a)} 
}