type-conversion - index() 错误消息对于第三个参数不正确？

Question

sub count-a {
    my $word = "banana";
    my $count = 0;
    my $foo;      #  Source of error: $foo intentionally not given a value.
                 #  The value ought to be zero.

    while True {
      $foo = index $word, "a", $foo;
      last unless $foo.defined;
      $foo++;
      $count++
  }
  return $count;
}

say count-a;

这个错误信息是错误的吗？

Cannot resolve caller index(Str: Str, Any); none of these signatures match:
  (Str:D $: Cool:D $needle, *%_)
  (Str:D $: Str:D $needle, *%_)
  (Str:D $: Cool:D $needle, Cool:D $pos, *%_)
  (Str:D $: Str:D $needle, Int:D $pos, *%_)
in sub count-a at scrap.p6 line 11
in block <unit> at scrap.p6 line 18

错误消息说 index() 将接受“Any”的第三个参数，这是我用 $foo.WHAT 验证时给出的参数。

Answer 1

third parameter of index is defined as an Int that is actually coerced to Cool ( Cool 是 class that can represent either a number or a string in Perl 6 )。但是你定义它的方式my $foo (其类型将为“任何”)它不能被识别为其中任何一个。一旦你给它一个值，要么"0"或 0 ， 那可行。

my $foo = 0;  

因为类型将被 index 正确识别的签名。