perlsub states the following:

When using a signature, the arguments are still available in the special array variable @_ , in addition to the lexical variables of the signature.

We can confirm this:

sub baz ($this, $that) {
    warn '@_ contains ', scalar(@_), " elements.\n";
}

baz('hello', 'world') # @_ contains 2 elements.

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But what happens when one of the parameters is optional?

sub foo ($this, $that = 'world') {
    warn '@_ contains ', scalar(@_), " elements.\n";
}

foo('hello'); # @_ contains 1 elements.

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Here's an example with tests:

#!/usr/bin/env perl

use strict;
use warnings;
use feature qw(signatures);
no warnings qw(experimental::signatures);
use Test::More tests => 8;

sub baz($this, $that) {
    ok defined($this), "\$this is defined and contains $this";
    ok defined($that), "\$that is defined and contains $that";
    is scalar(@_), 2, '@_ has two elements.';
    is_deeply \@_, [$this, $that]
        or diag 'Parameters: ', explain {
            this => $this,
            that => $that,
            at_underscore => \@_
        };
}

sub foo ($this, $that='bar') {
    ok defined($this), "\$this is defined and contains $this";
    is $that, 'bar', '\$that contains "bar"';
    is scalar(@_), 2, '@_ has two elements.';
    is_deeply \@_, [$this, $that]
        or diag 'Parameters: ', explain {
            this => $this,
            that => $that,
            at_underscore => \@_
        };
}

note "\n\nTesting baz()";
baz('hello', 'world');

note "\n\nTesting foo()";
foo("a");

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The output is...

1..8
# 
# 
# Testing baz()
ok 1 - $this is defined and contains hello
ok 2 - $that is defined and contains world
ok 3 - @_ has two elements.
ok 4
# 
# 
# Testing foo()
ok 5 - $this is defined and contains a
ok 6 - \$that contains "bar"
not ok 7 - @_ has two elements.
#   Failed test '@_ has two elements.'
#   at mytest2.pl line 24.
#          got: '1'
#     expected: '2'
not ok 8
#   Failed test at mytest2.pl line 25.
#     Structures begin differing at:
#          $got->[1] = Does not exist
#     $expected->[1] = 'bar'
# Parameters: {
#   'at_underscore' => [
#     'a'
#   ],
#   'that' => 'bar',
#   'this' => 'a'
# }
# Looks like you failed 2 tests of 8.

shell returned 2

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This has probably been discussed a lot somewhere.

The question: Is the reason that scalar(@_) does not contain aliases to optional values so that we can determine how many parameters were actually passed, and if so, should it be documented?


Dave

In reply to Contents of @_ using subroutine signatures with default values by davido

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