问题
In reading about Perl 6, I see a feature being trumpeted about, where you no longer have to do:
return "0 but true";
...but can instead do:
return 0 but True;
If that's the case, how does truth work in Perl 6? In Perl 5, it was pretty simple: 0, "", and undef are false, everything else is true.
What are the rules in Perl 6 when it comes to boolean context?
回答1:
Perl 6 evaluates truth now by asking the object a question instead of looking at its value. The value is not the object. It's something I've liked about other object languages and will be glad to have in Perl: I get to decide how the object responds and can mutate that. As ysth said, you could do that in Perl 5 with overload, but I always feel like I have to wash my hands after doing it that way. :)
If you don't do anything to change that, Perl 6 behaves in the same way as Perl 5 so you get the least amount of surprise.
回答2:
Truthness test just calls the .true method on an object, so the "mix in" operation $stuff but True just (among other things) overrides that method.
This is specified in S02, generally enum types (of which Bool is one) are described in S12.
回答3:
See Synopsis 12: Roles.
The rules are the same, but the "but" copies the 0 and applies a role to the copy that causes it to be true in boolean context.
You can do the same thing with overload in Perl 5.
回答4:
According to O'Reilly's Perl 6 and Parrot Essentials, false is 0, undef, the empty string, and values flagged as false. true is everything else.
Also, Perl 6 has both a primitive boolean type and by having True and False roles that any value can mix in (so you can have a "0 but True" value or a "1 but False" one for example, or a false list containing elements, or a true list that's empty).
See http://www.mail-archive.com/macosx@perl.org/msg09930.html
回答5:
So to combine what I think to be the best of everyone's answers:
When you evaluate a variable in boolean context, its .true() method gets called. The default .true() method used by an object does a Perl 5-style <0, "", undef> check of the object's value, but when you say "but True" or "but False", this method is overridden with one that doesn't look at the value just returns a constant.
One could conceivable write a true() method which, say, returned true when the value was even and false when it was odd.
回答6:
One false value that gets neglected nearly everywhere is "0". I recently made this painful discovery that "0" is false in PERL 5. Gee. A non-empty string that's false. I was really hoping that would change in PERL6, but I guess not.
> if ( "0" ) { say "True" } else { say "False" }
False
The ||= idiom clobbered some strings I really wasn't expecting:
$ perl -e '$x = "0"; $x ||= ""; print ">>$x<<\n";'
>><<
来源：https://stackoverflow.com/questions/125265/how-does-perl-6-evaluate-truthiness