Scala 编译器不识别尾递归

Question

我正在尝试使用尾递归创建毕达哥拉斯三重级数，但编译器似乎无法识别它，即使我没有在 return 语句中进行任何计算。

错误代码是这样的：Cannot rewrite recursive call: it is not in tail position

这里是尾递归的代码：

  def tailPythagorean(positiveNumber: Int): List[(Int, Int, Int)] = {
  return tailRunner(positiveNumber, positiveNumber, List())
    .take(positiveNumber);
  }

  @tailrec def tailRunner(biggerNumber: Int, smallerNumber: Int, list: List[(Int, Int, Int)]): List[(Int, Int, Int)] = {
    if (biggerNumber == 0 || smallerNumber == 0) {
      return list;
    }

    val smallNumber = if (smallerNumber == 1) biggerNumber - 1 else smallerNumber - 1;
    
    return tailRunner(biggerNumber, smallNumber, List((((biggerNumber + 1) * (biggerNumber + 1) - (smallerNumber * smallerNumber)), (2 * (biggerNumber + 1) * smallerNumber), ((biggerNumber + 1) * (biggerNumber + 1) + (smallerNumber * smallerNumber)))) ::: list);
  }

下面是未针对尾递归优化的递归解决方案的代码（供参考）：

def recursivePythagorean(positiveNumber: Int): List[(Int, Int, Int)] = {
  return recursiveRunner(positiveNumber, positiveNumber)
    .take(positiveNumber);
  }

  def recursiveRunner(biggerNumber: Int, smallerNumber: Int): List[(Int, Int, Int)] = {
    if (biggerNumber == 0 || smallerNumber == 0) {
      return List();
    }
    if (smallerNumber == 1) {
      return recursiveRunner(biggerNumber - 1, biggerNumber - 1) ::: List((((biggerNumber + 1) * (biggerNumber + 1) - (smallerNumber * smallerNumber)), (2 * (biggerNumber + 1) * smallerNumber), ((biggerNumber + 1) * (biggerNumber + 1) + (smallerNumber * smallerNumber))));
    }
    else {
      return recursiveRunner(biggerNumber, smallerNumber - 1) ::: List((((biggerNumber + 1) * (biggerNumber + 1) - (smallerNumber * smallerNumber)), (2 * (biggerNumber + 1) * smallerNumber), ((biggerNumber + 1) * (biggerNumber + 1) + (smallerNumber * smallerNumber))));
    }
  }

编辑：根据@AminMal 和@Jörg W Mittag 的评论，确实是 return 关键字的存在导致编译器抱怨。删除它们为我修复了它：

  def tailPythagorean(positiveNumber: Int): List[(Int, Int, Int)] = {
    tailRunner(positiveNumber, positiveNumber, List())
      .take(positiveNumber);
  }

  @tailrec def tailRunner(biggerNumber: Int, smallerNumber: Int, list: List[(Int, Int, Int)]): List[(Int, Int, Int)] = {
    if (biggerNumber == 0 || smallerNumber == 0) {
      return list;
    }
    if (smallerNumber == 1) {
      tailRunner(biggerNumber - 1, biggerNumber - 1, List((((biggerNumber + 1) * (biggerNumber + 1) - (smallerNumber * smallerNumber)), (2 * (biggerNumber + 1) * smallerNumber), ((biggerNumber + 1) * (biggerNumber + 1) + (smallerNumber * smallerNumber)))) ::: list);
    }
    else {
      tailRunner(biggerNumber, smallerNumber - 1, List((((biggerNumber + 1) * (biggerNumber + 1) - (smallerNumber * smallerNumber)), (2 * (biggerNumber + 1) * smallerNumber), ((biggerNumber + 1) * (biggerNumber + 1) + (smallerNumber * smallerNumber)))) ::: list);
    }
  }

Answer 1

尾递归方法和局部函数不允许使用return进行尾递归调用。只需从最后一行删除 return 关键字。

Scala 编译器不识别尾递归

Scala compiler doesn't recognize tail recursion

recursion

scala

tail-recursion

pythagorean

scala-3