How does Perl 6 evaluate truthiness?

Question

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?

Answer 1

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.

Answer 2

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.

Answer 3

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.

Answer 4

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.

Answer 5

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

Answer 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";'
>><<