Question

In my experience of languages that support the feature, programs that call functions with named parameters rather than positional parameters are easier to read and maintain.

I think Perl has this feature, but it’s not working for me.

Is it a quirk of the package I’m using, or am I doing it wrong?

Setting up the function call

My fist Perl project is to use the HTML::TableExtract package to extract table data from HTML markup and display it as text.

The following code sets up the parser:

use strict;
use warnings;
use HTML::TableExtract;

my $markup = <<MARKUP;
<table>
  <tr> <th>a</th> <th>b</th> <th>c</th> </tr>
  <tr> <td>1</td> <td>2</td> <td>3</td> </tr>
  <tr> <td>4</td> <td>5</td> <td>6</td> </tr>
</table>
MARKUP

my $parser = HTML::TableExtract->new() ;

$parser->parse($markup) ;

The documentation says that I can dump output to the Command Prompt using the tables_dump method and use parameters $show_content and $col_sep to control the output format:

tables_report([$show_content, $col_sep])

Return a string summarizing extracted tables, along with their depth and count. Optionally takes a $show_content flag which will dump the extracted contents of each table as well with columns separated by $col_sep. Default $col_sep is ‘:’.

tables_dump([$show_content, $col_sep])

Same as tables_report() except dump the information to STDOUT.

Calling with positional and with named parameters

If I pass positional parameters in documentation order, I get the output I expect:

$parser->tables_dump(1, '_') ;

The columns are separated by an underscore instead of the default colon:

TABLE(0, 0):
a_b_c
1_2_3
4_5_6

Following Perl.com’s Advance Subroutines article, I tried to pass a hash containing parameter names and values to clarify the meaning of the parameters:

$parser->tables_dump({show_content => 1, col_sep => '_'}) ;

Perl doesn’t understand this. It ignores the value of col_sep and outputs with the default value:

TABLE(0, 0):
a:b:c
1:2:3
4:5:6

I get the same output if I don’t try to change the separator:

$parser->tables_dump({show_content => 1}) ;

Even if I specify nonsense parameter names, I get the same output:

$parser->tables_dump({tweedledum => 1, tweedledee => '_'}) ;

Can I call this function using the named parameter style, or should I just settle for positional?

Answer 1

Following Perl.com’s Advance Subroutines article, I tried to pass a
hash containing parameter names and values to clarify the meaning of
the parameters:

That article covers a way to write subroutines such that they will accept a hashref of named parameters. If you’re calling a sub that isn’t written to accept that, then it won’t know how to handle it correctly.

$parser->tables_dump({show_content => 1, col_sep => '_'}) ;

Perl doesn’t understand this. It ignores the value of col_sep and
outputs with the default value:

Not to be overly pedantic, but Perl understands that just fine. However, tables_dump is only written to accept a list of scalar parameters. When you call it that way, it receives a single scalar parameter. This parameter just happens to be a reference to a hash, but tables_dump doesn’t know or care about that, so it uses the reference as the value of $show_content. This is probably equivalent to passing 1 for show_content, since both 1 and any possible reference will evaluate as “true” in boolean context and I assume that $show_content is only ever used as a boolean.

Since there is no second parameter, nothing gets assigned to $col_sep, so it uses the default separator, as you observed.