DreamT has asked for the wisdom of the Perl Monks concerning the following question:

Hi,
I want to set a value to undef in a TT template:
[% hashref.value1 = undef %]
, but this only produces an empty string. 
$VAR1 = {
...
'value1' => ''
...
}

[download]

Is there a way around this? From what I've read it seems to be a "feature" in Template Toolkit that undefs are treated as empty strings.

Replies are listed 'Best First'.
Re: Undef value in Template Toolkit
by Fletch (Bishop) on Aug 20, 2019 at 14:32 UTC

    What exactly "way around this" do you want? Perl itself treats undef in a string context as an empty string. What do you expect TT to do instead?

    The cake is a lie.
    The cake is a lie.
    The cake is a lie.

[reply]
      The goal is to set the mentioned variable to undef, rather than an empty string.
[reply]

        This sounds like an XY problem(?). I would argue that manipulating data within templates is generally a mistake. Caring about what form unviewable data is within a view also seems like a mismatch to the context.

[reply]
        This seems to be some intermediate or side effect goal to me. TT's output is a string, so for that purpose undef and the empty string are equivalent. Maybe you have something like [% IF hashref.defined('value1') %]...[% END %] later on, or you want to pass a value back to the Perl code which called TT's process? For these situations you can [% hashref.delete('value1') %] which should do the trick unless you also need to distinguish between keys which exist from keys with an undefined value.
[reply]

        It is undef. However you're displaying it is stringifying it most likely.

        Update: Oops, I stand corrected. Assigning undef in a template does look like it gets an empty string. Interesting. Amended code sample setting baz directly in the template.

        Another update: Did some poking and I'm most of the way to convincing myself to agreeing that this might be a bug. It's prossibly worth at least pinging the mailing list with a sample script like this and seeing what the maintainers say.

        Update the third: ++ and agreement with those below that relying on results from logic in the display template for anything other than displaying things just raises the hackles. 

        #!/usr/bin/env perl
use strict;
use warnings;

use Template ();

my $t = Template->new( {} );

my $tmpl_source = <<"EOT";
This is my template.
[% timestamp %]
[%- IF foo.defined %]
foo is defined and is "[% foo %]"
[%- ELSE %]
foo is undef
[%- END %]
[%- IF bar.defined %]
bar is defined and is "[% bar %]"
[%- ELSE %]
bar is undef
[%- END %]
[%- baz = undef -%]
[%- IF baz.defined %]
baz is defined and is "[% baz %]"
[%- ELSE %]
baz is undef
[%- END %]
Done.
EOT

$t->process( \$tmpl_source, { foo => undef,
                             bar => q{},
                             timestamp => scalar localtime() } )
  or die $t->error, qq{\n};

exit 0;

__END__
$ perl tt_test.plx
This is my template.
Tue Aug 20 11:07:30 2019
foo is undef
bar is defined and is ""
baz is defined and is ""
Done.

        [download]

        The cake is a lie.
        The cake is a lie.
        The cake is a lie.

[reply]
[d/l]
[select]

Back to Seekers of Perl Wisdom