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?

Participate

in Perl 5 it is common to use '0E0' as a 'zero but true' return value – Omor

I'll give you an upvote for use of the worth "truthiness" in a technical question :) – Cap

@PaulDixon: The term is also used in the Ruby community. – Honegger

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.

Participate

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.

Royce

That'd be more object oriented than Ruby - only false and nil lack truthiness, and there's nothing you can do about it, unless you monkeypatch ! and do !!foo. – Honegger

Answer 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.

Distributive

Er, don't you mean "true in a boolean context"? – Baughman

Answer 4

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.

Guimond

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/[email protected]/msg09930.html

Diphenylhydantoin

That book is ancient and does not represent the current state of the language. – Royce

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";'
>><<

Domiciliary

That is why defined-or // is so nice. It's enabled (starting in Perl 5.10) by use 5.010. – Apnea

Oops, I just remembered that since defined-or did not conflict with previous syntax, it is enabled by default. – Apnea

This answer was written about P6 prior to its first official release in 2015. if ( "0" ) { say "True" } else { say "False" } yields True. The P6 compiler rejects the other line at compile-time. With two changes it compiles and produces what I assume @Domiciliary would consider the correct result: my $x = "0"; $x ||= ""; print ">>$x\<<\n"; # >>0<<. – Leclair

@Domiciliary you got your wish. In the Raku REPL: put "0".Bool; returns True, while put "".Bool; returns False. At the command-line, raku -e 'my $x = "0"; $x ||= ""; put ">>"~$x~"<<";' returns >>0<<. HTH. – Wsan