@tailrec为什么这个方法不能用'包含一个不在尾部位置的递归调用'编译？ [英] @tailrec why does this method not compile with 'contains a recursive call not in tail position'?

问题描述

  @tailrec
  private def loop[V](key: String): V = {
    key match {
      case _ => loop(key)
    }
  }

这种方法不能编译和抱怨说它'包含一个不在尾部位置的递归调用'。有人可以向我解释发生了什么事吗？这个错误信息对我来说没有意义。

This method doesn't compile and complains that it 'contains a recursive call not in tail position'. Can someone explain to me what's going on? This error message doesn't make sense to me.

推荐答案

如果指定了泛型类型，它就编译好：

It compiles ok if the generic type is specified:

import scala.annotation.tailrec

 @tailrec
  private def loop[V](key: String): V = {
    key match {
      case _ => loop[V](key)
    }
  }

我认为错误消息在这种情况下是误导性的。

I think the error message is misleading in this case.

一个简化的版本给出了一个更好的提示：

A simplified version gives a better hint on what's going on:

scala>  @tailrec
     |   private def loop[V](key: String): V = {
     |     loop(key)
     |   }
<console>:14: error: could not optimize @tailrec annotated method loop: it is called recursively with different type arguments
           loop(key)
           ^

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