虽然经过@tailrec

Question

我期待到斯卡拉TCO并写了下面的代码

import scala.annotation.tailrec 
final def tailReccursionEx(str:String):List[String]={ 

    @tailrec 
    def doTailRecursionEx(str:String,pos:Int,accu:List[String]):List[String]={ 
    if(pos==str.length) return accu 
    else{ 
     doTailRecursionEx(str,pos+1,accu++accu.foldLeft(List[String](str(`pos`).toString)){ 
              (l,ch)=>l:+ch+str(`pos`)}) 
    } 
} 

    doTailRecursionEx(str,0,List[String]()) 
} 

我已经通过了@tailrec测试，我相信我的功能就是我的Scala代码没有得到TCO'ed自递归尾部呼叫。然而，当我看着到Java字节代码

javap -c -private RecursionEx\$\$anonfun\$doTailRecursionEx\$1\$1 

我看不到所承诺的转到的TCO自我递归函数。这是字节码。

public RecursionEx$$anonfun$doTailRecursionEx$1$1(java.lang.String, int); 
    Code: 
    0: aload_0 
    1: aload_1 
    2: putfield #35; //Field str$2:Ljava/lang/String; 
    5: aload_0 
    6: iload_2 
    7: putfield #41; //Field pos$1:I 
    10: aload_0 
    11: invokespecial #93; //Method scala/runtime/AbstractFunction2."<init>":()V 
    14: return 

} 

Answer 1

我想你需要在不同的生成的类文件上运行javap。您目前正在检查的文件对应于您使用的作为foldLeft的一部分的封闭文件。如果您尝试查看“RecursionEx $ .class”文件，您应该看到尾部调用递归。当我编译代码：

import scala.annotation.tailrec 

object RecursionEx { 
    @tailrec 
    final def doTailRecursionEx(str: String, pos: Int, accu: List[String]): List[String] = { 
     if (pos == str.length) return accu 
     doTailRecursionEx(str, pos + 1 , accu ++ accu.foldLeft(List[String](str(`pos`).toString)) { 
          (l, ch) => l :+ ch + str(`pos`) 
         }) 
    } 
    def main(args: Array[String]) { 
     doTailRecursionEx("mew",0,List[String]()) 
    } 
} 

，然后运行javap -c -private RecursionEx$我看到了对代码的相关部分如下：末

public final scala.collection.immutable.List doTailRecursionEx(java.lang.String, int, scala.collection.immutable.List); 

    Code: 
    0: iload_2 
    1: aload_1 
    2: invokevirtual #21; //Method java/lang/String.length:()I 
    5: if_icmpne 10 
    8: aload_3 
    9: areturn 
    10: iload_2 
    11: iconst_1 
    12: iadd 
    13: aload_3 
    14: aload_3 
    15: getstatic #26; //Field scala/collection/immutable/List$.MODULE$:Lscala/collection/immutable/List$; 
    18: getstatic #31; //Field scala/Predef$.MODULE$:Lscala/Predef$; 
    21: iconst_1 
    22: anewarray #17; //class java/lang/String 
    25: dup 
    26: iconst_0 
    27: getstatic #31; //Field scala/Predef$.MODULE$:Lscala/Predef$; 
    30: aload_1 
    31: invokevirtual #35; //Method scala/Predef$.augmentString:(Ljava/lang/String;)Lscala/collection/immutable/StringOps; 
    34: iload_2 
    35: invokeinterface #41, 2; //InterfaceMethod scala/collection/immutable/StringLike.apply:(I)C 
    40: invokestatic #47; //Method scala/runtime/BoxesRunTime.boxToCharacter:(C)Ljava/lang/Character; 
    43: invokevirtual #53; //Method java/lang/Object.toString:()Ljava/lang/String; 
    46: aastore 
    47: checkcast #55; //class "[Ljava/lang/Object;" 
    50: invokevirtual #59; //Method scala/Predef$.wrapRefArray:([Ljava/lang/Object;)Lscala/collection/mutable/WrappedArray; 
    53: invokevirtual #62; //Method scala/collection/immutable/List$.apply:(Lscala/collection/Seq;)Lscala/collection/immutable/List; 
    56: new #64; //class RecursionEx$$anonfun$doTailRecursionEx$1 
    59: dup 
    60: aload_1 
    61: iload_2 
    62: invokespecial #67; //Method RecursionEx$$anonfun$doTailRecursionEx$1."<init>":(Ljava/lang/String;I)V 
    65: invokeinterface #73, 3; //InterfaceMethod scala/collection/LinearSeqOptimized.foldLeft:(Ljava/lang/Object;Lscala/Function2;)Ljava/lang/Object; 
    70: checkcast #75; //class scala/collection/TraversableOnce 
    73: getstatic #26; //Field scala/collection/immutable/List$.MODULE$:Lscala/collection/immutable/List$; 
    76: invokevirtual #79; //Method scala/collection/immutable/List$.canBuildFrom:()Lscala/collection/generic/CanBuildFrom; 
    79: invokevirtual #85; //Method scala/collection/immutable/List.$plus$plus:(Lscala/collection/TraversableOnce;Lscala/collection/generic/CanBuildFrom;)Ljava/lang/Object; 
    82: checkcast #81; //class scala/collection/immutable/List 
    85: astore_3 
    86: istore_2 
    87: goto 0 

与goto，正如你所期望的。