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?

in Perl 5 it is common to use '0E0' as a 'zero but true' return value — EvdB, Sep 24 '08 at 12:33
I'll give you an upvote for use of the worth "truthiness" in a technical question :) — Paul Dixon, Oct 08 '08 at 07:13

score 18 · Answer 1 · edited Dec 09 '11 at 05:59

18

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.

edited Dec 09 '11 at 05:59

Andrew Grimm

78,473
57
200
338

answered Sep 24 '08 at 07:38

brian d foy

129,424
31
207
592

1

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`. – Andrew Grimm Dec 09 '11 at 06:01

score 11 · Answer 2 · edited Oct 09 '08 at 13:41

11

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.

edited Oct 09 '08 at 13:41

brian d foy

129,424
31
207
592

answered Sep 24 '08 at 03:47

ysth

96,171
6
121
214

1

Er, don't you mean "true in a boolean context"? – Ovid Sep 24 '08 at 12:18

score 11 · Answer 3 · answered Sep 24 '08 at 07:44

11

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.

answered Sep 24 '08 at 07:44

moritz

12,710
1
41
63

score 6 · Answer 4 · edited Oct 08 '08 at 07:11

6

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

edited Oct 08 '08 at 07:11

brian d foy

129,424
31
207
592

answered Sep 24 '08 at 03:42

Mark Cidade

98,437
31
224
236

2

That book is ancient and does not represent the current state of the language. – brian d foy Oct 08 '08 at 07:11

score 3 · Accepted Answer · edited Oct 09 '08 at 13:58

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.

kovacsbv · Answer 6 · 2014-05-02T17:42:24.323

1

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";'
>><<

edited May 02 '14 at 17:42

answered May 02 '14 at 17:31

kovacsbv

351
4
11

That is why [defined-or](http://perltraining.com.au/tips/2008-03-03.html) `//` is so nice. It's enabled (starting in Perl 5.10) by `use 5.010`. – Christopher Bottoms May 15 '15 at 16:06
Oops, I just remembered that since defined-or did not conflict with previous syntax, it is enabled by default. – Christopher Bottoms May 15 '15 at 22:22
1

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 @kovacsbv would consider the correct result: `my $x = "0"; $x ||= ""; print ">>$x\<<\n"; # >>0<<`. – raiph May 16 '19 at 16:10

How does Perl 6 evaluate truthiness?

6 Answers6

Linked