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import Stream._
trait Stream[+A]
 
case object Empty extends Stream[Nothing]
case class Cons[+A](h: () => A, t: () => Stream[A]) extends Stream[A]
 
object Stream {
 
  def cons[A](hd: => A, tl: => Stream[A]): Stream[A] = { 
    lazy val head = hd 
    lazy val tail = tl Cons(() => head, () => tail) 
  } 
 
  def empty[A]: Stream[A] = Empty 
 
  def apply[A](as: A*): Stream[A] = 
    if (as.isEmpty) empty 
    else cons(as.head, apply(as.tail: _*))
 
  def toListFast: List[A] = {
    val buf = new collection.mutable.ListBuffer[A] 
 
    @annotation.tailrec 
    def go(s: Stream[A]): List[A] = s match { 
      case Cons(h,t) => buf += h() go(t()) 
      case _ => buf.toList 
    } 
    go(this) 
  }
 
  def take(n: Int): Stream[A] = 
    if (n > 0) this match { 
      case Cons(h, t) if n == 1 => cons(h(), Stream.empty) // we can say Stream.empty 
      case Cons(h, t) => cons(h(), t().take(n-1)) 
      case _ => Stream.empty
    } else Stream() 
 
  def constant[A](a: A): Stream[A] = { 
    lazy val tail: Stream[A] = Cons(() => a, () => tail)
    tail
  }
}
 
object Main extends App {
  println(constant(5).take(3).toList)
}

/opt/scala/bin/scalac: line 50: /dev/null: Permission denied
Main.scala:11: error: value Cons is not a member of Stream[A]
    lazy val tail = tl Cons(() => head, () => tail) 
                       ^
Main.scala:12: error: type mismatch;
 found   : Unit
 required: Stream[A]
  } 
  ^
Main.scala:20: error: not found: type A
  def toListFast: List[A] = {
                       ^
Main.scala:21: error: not found: type A
    val buf = new collection.mutable.ListBuffer[A] 
                                                ^
Main.scala:24: error: not found: type A
    def go(s: Stream[A]): List[A] = s match { 
                               ^
Main.scala:24: error: not found: type A
    def go(s: Stream[A]): List[A] = s match { 
                     ^
Main.scala:31: error: not found: type A
  def take(n: Int): Stream[A] = 
                           ^
Main.scala:33: error: constructor cannot be instantiated to expected type;
 found   : Cons[A]
 required: Stream.type
      case Cons(h, t) if n == 1 => cons(h(), Stream.empty) // we can say Stream.empty 
           ^
Main.scala:33: error: not found: value h
      case Cons(h, t) if n == 1 => cons(h(), Stream.empty) // we can say Stream.empty 
                                        ^
Main.scala:34: error: constructor cannot be instantiated to expected type;
 found   : Cons[A]
 required: Stream.type
      case Cons(h, t) => cons(h(), t().take(n-1)) 
           ^
Main.scala:34: error: not found: value h
      case Cons(h, t) => cons(h(), t().take(n-1)) 
                              ^
Main.scala:34: error: not found: value t
      case Cons(h, t) => cons(h(), t().take(n-1)) 
                                   ^
Main.scala:45: error: value take is not a member of Stream[Int]
  println(constant(5).take(3).toList)
                      ^
13 errors found
spoj: The program compiled successfully, but Main.class was not found.
      Class Main should contain method: def main(args: Array[String]).

Standard output is empty