From @mauke

Created by @mauke

perldoc perlsyn includes this section​:

| Truth and Falsehood
| The number 0, the strings '0' and "", the empty list "()", and "undef"
| are all false in a boolean context. All other values are true.
| Negation of a true value by "!" or "not" returns a special false value.
| When evaluated as a string it is treated as "", but as a number, it is
| treated as 0. Most Perl operators that return true or false behave this
| way.

I don't think this belongs in perlsyn. Truthiness is a property of values, not
of syntactic constructs, and indeed the paragraph is talking about "all other
values" -- which makes it internally inconsistent because 'the empty list "()"'
is not a distinct value. It's a syntactic construct that evaluates to either
undef or no values at all (an empty list) in scalar/list context, respectively.

That is, '()' and 'undef' are simply two particular spellings of the value
undef, but there are many more. Similarly, '0', '0x0', '0b0000_0000', '1 - 1',
'2 ^ 2', ... would be different spellings of the value 0. Therefore I'd remove
"()" from the list and move that part to perldata.

The part about negation yielding a dualvar for false isn't really a syntactic
property either. I'd put it either in perldata (because it's about a special
value returned by some operators) or perlop (because it's about the behavior of
several operators).

Thoughts?

Flags:
    category=docs
    severity=low

Site configuration information for perl 5.16.2:

Configured by mauke at Thu Nov  1 15:23:47 CET 2012.

Summary of my perl5 (revision 5 version 16 subversion 2) configuration:
   
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Locally applied patches:
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@INC for perl 5.16.2:
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From bmb@Mail.Libs.UGA.EDU

On 11/9/12, l.mai@​web.de (via RT) <perlbug-followup@​perl.org> wrote​:

perldoc perlsyn includes this section​:

| Truth and Falsehood
| The number 0, the strings '0' and "", the empty list "()", and
"undef"
| are all false in a boolean context. All other values are true.
| Negation of a true value by "!" or "not" returns a special false
value.
| When evaluated as a string it is treated as "", but as a number, it
is
| treated as 0. Most Perl operators that return true or false behave
this
| way.

I don't think this belongs in perlsyn. Truthiness is a property of values,
not
of syntactic constructs, and indeed the paragraph is talking about "all
other
values" -- which makes it internally inconsistent because 'the empty list
"()"'
is not a distinct value. It's a syntactic construct that evaluates to
either
undef or no values at all (an empty list) in scalar/list context,
respectively.

It think this is a helpful location for folks coming from other
languages where there are 'true' and 'false' syntactic elements.

That is, '()' and 'undef' are simply two particular spellings of the value
undef

Huh? :-)

, but there are many more. Similarly, '0', '0x0', '0b0000_0000', '1 -
1',
'2 ^ 2', ... would be different spellings of the value 0. Therefore I'd
remove
"()" from the list and move that part to perldata.

The part about negation yielding a dualvar for false isn't really a
syntactic
property either. I'd put it either in perldata (because it's about a
special
value returned by some operators) or perlop (because it's about the behavior
of
several operators).

Thoughts?

I think the comprehensive statements, 'The number 0, the strings '0'
and "" , the empty list () , and undef are all false in a boolean
context. All other values are true.' are helpful as a compact
reference unit.

--
Brad

The RT System itself - Status changed from 'new' to 'open'

From @ikegami

On Fri, Nov 9, 2012 at 2​:18 PM, Brad Baxter <bmb@​mail.libs.uga.edu> wrote​:

I don't think this belongs in perlsyn. Truthiness is a property of
values,
not of syntactic constructs

It think this is a helpful location for folks coming from other

languages where there are 'true' and 'false' syntactic elements.

That doesn't change anything. Syntactic elements relating to variables are
in perldata too.

That is, '()' and 'undef' are simply two particular spellings of the
value
undef

Huh? :-)

If you by "()" you mean the operator that returns undef or an empty list​:
Operators aren't true or false.
If you by "()" you mean an empty list​: Lists aren't true or false.

Either way you look at it, "()" doesn't belong there.

From Eirik-Berg.Hanssen@allverden.no

On Fri, Nov 9, 2012 at 9​:03 PM, Eric Brine <ikegami@​adaelis.com> wrote​:

That is, '()' and 'undef' are simply two particular spellings of the
value

undef

  ... in a scalar context, yes, of which boolean is one, right?

Huh? :-)

If you by "()" you mean the operator that returns undef or an empty list​:
Operators aren't true or false.
If you by "()" you mean an empty list​: Lists aren't true or false.

Either way you look at it, "()" doesn't belong there.

  I think I'm most annoyed that it calls is "the empty list". It
propagates the myth of lists in scalar contexts.

  Either way, yes, please do away with it.

$ perl -MData​::Dump=pp -e 'sub nil { +() } pp do { nil }; pp do { if (nil)
{ die } }'
()
undef
$ echo "Myth busted?"

Eirik

From zefram@fysh.org

Done in commit 77fae43.

-zefram

@xsawyerx - Status changed from 'open' to 'pending release'

From @khwilliamson

Thank you for filing this report. You have helped make Perl better.

With the release yesterday of Perl 5.28.0, this and 185 other issues have been
resolved.

Perl 5.28.0 may be downloaded via​:
https://metacpan.org/release/XSAWYERX/perl-5.28.0

If you find that the problem persists, feel free to reopen this ticket.

@khwilliamson - Status changed from 'pending release' to 'resolved'