トランポリン化でStackOverflowの回避 ref: http://qiita.com/yyu/items/f1601749097ba3ee03db

file2.txt

Cons(1,Cons(2,Cons(3,Cons(4,Nil))))

file4.scala

def apply[A](as : A*) : List[A] = {
  def loop(k : List[A] => List[A], xs : Seq[A]) : List[A] =
    if (xs.isEmpty) k(Nil)
    else loop(x => k(Cons(xs.head, x)), xs.tail)

  loop(x => x, as)
}

file6.scala

def apply[A] (as : A*) : List[A] = {
  def loop(k : List[A] => TailRec[List[A]], xs : Seq[A]) : TailRec[List[A]] =
    if (xs.isEmpty) k(Nil)
    else loop( x => Suspend(() => k(Cons(xs.head, x))), xs.tail)

  loop( x => Return(x), as ).run
}

List.scala

package fpinscala.datastructures

sealed trait List[+A]
case object Nil  extends List[Nothing]
case class Cons[+A](head : A, tail : List[A]) extends List[A]

object List {
  def apply[A](as : A*) : List[A] =
    if (as.isEmpty) Nil
    else Cons(as.head, apply(as.tail : _*))
}

TailRec.scala

package fpinscala.tailrec

sealed trait TailRec[A] {
  final def run : A = this match {
    case Return(v)  => v
    case Suspend(k) => k().run
  }
}

case class Return[A](v : A) extends TailRec[A]
case class Suspend[A](resume : () => TailRec[A]) extends TailRec[A]

TestList.scala

import fpinscala.datastructures.List

object TestList {
  def main(args : Array[String]) : Unit = {
    val xs = List(1, 2, 3, 4)
    println(xs)
  }
}

TestListWithStackOverflow.scala

import fpinscala.datastructures.List

object TestListWithStackOverflow {
  def main(args : Array[String]) : Unit = {
    val xs = List( (1 to 10000).toSeq : _* )
    println(xs)
  }
}