Purely Functional Data Structure ch02.scala

import cats._
import cats.data._
import cats.implicits._

sealed trait StreamCell[T]
case class StreamNil[T]() extends StreamCell[T]
case class StreamValue[T](value: T, next: Eval[StreamCell[T]]) extends StreamCell[T]

object Main {

    def take[T](n: Int, s: Eval[StreamCell[T]]): Eval[StreamCell[T]] = {
        Eval.later {
            n match {
                case 0 =>
                    StreamNil[T]()
                case _ =>
                    s.value match {
                        case StreamNil() =>
                            StreamNil[T]()

                        case StreamValue(value, next) =>
                            StreamValue[T](value, take(n - 1, next))
                    }
            }
        }
    }

    def toList[T](s: Eval[StreamCell[T]]):List[T] = {
        s.value match {
            case StreamValue(value, next) =>
                value +: toList(next)
            case _ =>
                Nil
        }
    }

    def main(args: Array[String]): Unit = {
        val values = Eval.later {
            StreamValue[Char]('H', Eval.later {
                StreamValue[Char]('I', Eval.later {
                    StreamNil()
                })
            })
        }
        println(toList(take[Char](1, values)))
    }
}