斯卡拉tailrec标注错误 [英] Scala tailrec annotation error

问题描述

我 ImmutableEntity 调用的Java抽象类和含有称为类级别注解几个子类 @DBTable 。我试图用尾递归方法斯卡拉搜索注释的类层次结构：

I have a Java abstract class called ImmutableEntity and several subclasses that contain a class-level annotation called @DBTable. I am trying to search a class hierarchy for the annotation using a tail-recursive Scala method:

  def getDbTableForClass[A <: ImmutableEntity](cls: Class[A]): String = {
    @tailrec
    def getDbTableAnnotation[B >: A](cls: Class[B]): DBTable = {
      if (cls == null) {
        null
      } else {
        val dbTable = cls.getAnnotation(classOf[DBTable])
        if (dbTable != null) {
          dbTable
        } else {
          getDbTableAnnotation(cls.getSuperclass)
        }
      }
    }

    val dbTable = getDbTableAnnotation(cls)
    if (dbTable == null) {
      throw new
              IllegalArgumentException("No DBTable annotation on class " + cls.getName)
    } else {
      val value = dbTable.value
      if (value != null) {
        value
      } else {
        throw new
                IllegalArgumentException("No DBTable.value annotation on class " + cls.getName)
      }
    }
  }

当我编译这个code，我得到的错误：无法优化@tailrec标注的方法：它是递归与不同类型的参数调用。什么是错的我内心的方法是什么？

When I compile this code, I am getting the error: "could not optimize @tailrec annotated method: it is called recursively with different type arguments". What is wrong with my inner method?

感谢。

推荐答案

这是因为编译器实现由环尾递归的方式。这是作为转换的链条，从斯卡拉Java字节codeS一步完成。每个转换必须产生一个程序，再次的Type-正确的。但是，你不能改变的变量类型中旬循环执行，这就是为什么编译器不能扩展到一个类型正确的循环。

It's because of the way the compiler implements tail-recursion by loops. This is done as one step in a chain of transformations from Scala to Java bytecodes. Each transformation must produce a program that's again type-correct. However, it you can't change the type of variables in mid-loop execution, that's why the compiler could not expand into a type-correct loop.

这篇关于斯卡拉tailrec标注错误的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！