当在对象上定义但不在类上定义时，方法是尾递归的

Question

在对象上定义递归方法：

object Recursive {
    def recurse(maxDepth: Int = 10): Unit = {
        if (maxDepth == 0) throw new Exception
        recurse(maxDepth - 1)
    }
}

给出：

scala> Recursive.recurse(10)
java.lang.Exception
        at Recursive$.recurse(<console>:7)
        at .<init>(<console>:7)
        at .<clinit>(<console>)
        at RequestResult$.<init>(<console>:9)
        at RequestResult$.<clinit>(<console>)
        at RequestResult$scala_repl_result(<console>)
        at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
        at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
        at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
        at java.lang.reflect.Method.invoke(Method.java:597)
        at scala.tools.nsc.Interpreter$Request$$anonfun$loadAndRun$1$$anonfun$apply$17.apply(Interpreter.scala:988)
        at scala.tools.nsc.Interpreter$Request$$anonfun$loadAndRun$1$$anonfun$apply$17.apply(Interpreter.scala:988)
        at scala.util.control.Exception$Catch.apply(Exception.scal...

但在类上定义递归方法：

class Recursive {
    def recurse(maxDepth: Int = 10): Unit = {
        if (maxDepth == 0) throw new Exception
        recurse(maxDepth - 1)
    }
}

给出：

scala> new Recursive recurse(10)
java.lang.Exception
        at Recursive.recurse(<console>:7)
        at Recursive.recurse(<console>:8)
        at Recursive.recurse(<console>:8)
        at Recursive.recurse(<console>:8)
        at Recursive.recurse(<console>:8)
        at Recursive.recurse(<console>:8)
        at Recursive.recurse(<console>:8)
        at Recursive.recurse(<console>:8)
        at Recursive.recurse(<console>:8)
        at Recursive.recurse(<console>:8)
        at Recursive.recurse(<console>:8)
        at .<init>(<console>:7)
        at .<clinit>(<console>)
        at RequestResult$.<init>(<console>:9)
        at RequestResult$.<clinit>(<console>)
        at RequestResult$scala_repl_result(<console>)
        at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
        at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
        at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAcc..

方法是相同的。为什么在类上定义时不是尾递归？

Answer 1

如果您希望执行尾递归，则不能覆盖recurse。如果 recurse 是可重写的，如在 class 声明中那样，则其中的任何递归都必须使用动态方法调用（因为它可能是多态的），这无法优化为goto 风格的语句。

object 单例声明静态地确保对递归的明确调用，并让编译器继续进行尾递归优化。

Answer 2

如果您希望某个方法是尾递归的，则应该使用 @tailrec 对其进行注释。如果编译器无法应用 TCO，则会出错。

scala> import annotation._                           
import annotation._

scala> class Recursive {                                     
     |     @tailrec def recurse(maxDepth: Int = 10): Unit = {
     |         if (maxDepth == 0) throw new Exception        
     |         recurse(maxDepth - 1)                         
     |     }                                                 
     | }
<console>:12: error: could not optimize @tailrec annotated method: it is neither private nor final so can be overridden
           @tailrec def recurse(maxDepth: Int = 10): Unit = {
                        ^