[doc] join with undef values

[hexcoder-]

Where

https://perldoc.perl.org/blead/functions/join as of 05/10/2024.

Description

join EXPR,LIST

1. The documentation does not state how the return value is being built, when LIST contains 'undef' values.

F.e.:
my $rec = join(':', undef, undef);
does return ':' (with warnings).

What is missing in the documentation is that LIST items are stringified so that undefined values will be transformed to empty strings yielding the result of two empty string LIST items.

(In my first reading I overlooked the statement that join ALWAYS returns a string. So I assumed a single LIST item of undef would result in an undef value).

[Grinnz]

All string functions interpret their arguments as strings, with the exception of length which special-cases undef inputs.

[hexcoder-]

Thanks!
My main investigation was #22207, which had me look up join´s documentation.

[mauke]

The very first sentence:

Joins the separate strings of LIST into a single string with fields separated by the value of EXPR, and returns that new string.

Mentions both that list elements are treated as strings and that the return value is a string.

[hexcoder-]

I agree. The implication is that undefs are not preserved, which I would have liked to be told explicitly.

Ok, if I am the only person to find this description very terse and incomplete regarding "undefined", fell free to reject this issue.

[guest20]

@hexcoder- The counter-question is, how could undefs be preserved in a string when all strings are defined?

[hexcoder-]

@guest20: when all strings are defined, there are obviously no undefs to be preserved.
I would modify the question like this "what should join do when there are undefined arguments?"

The motivation for this issue/question came from my "code review" of

perl5/lib/perl5db.pl

Line 2494 in 8cacb84

my ($val) = join( "', '", DB::eval(@_) );

and the following line

perl5/lib/perl5db.pl

Line 2495 in 8cacb84

$val = ( ( defined $val ) ? "'$val'" : 'undef' );

where the defined operator is applied to the result of a join.

[Grinnz]

I think you missed the point of the question: strings are by definition defined, not in the argument list but in all of Perl. So when undef is passed to a function that takes strings, it is always interpreted as the empty string with a warning.