P04 (*) Find the number of elements of a list.

P04.scala

package com.learner.s99

/**
 * (*) Find the number of elements of a list.
 * Example:
 * scala> length(List(1, 1, 2, 3, 5, 8))
 * res0: Int = 6
 */

object P04 {
  def length(l: List[Any]): Int = length(l, 0)

  private def length(l: List[Any], lengthSoFar: Int): Int = l match {
    case List() => lengthSoFar
    case head::tail => length(tail, lengthSoFar + 1)
  }
}

P04Spec.scala

package com.learner.s99

import org.scalatest.{MustMatchers, FlatSpec}

class P04Spec extends FlatSpec
with MustMatchers {
  behavior of "A List"
  
  it must "return 4 for List(1,2,3,4) as length" in {
    P04.length(List(1,2,3,4)) mustBe 4
  }
  it must "return 0 empty List as length" in {
    P04.length(List()) mustBe 0
  }
}