The following works:
#!/usr/bin/perl
use strict;
use warnings;
use Test::More tests => 5;
use Parse::RecDescent ();
my $p = Parse::RecDescent->new(<<'__END_OF_GRAMMAR__');
{
use strict;
use warnings;
}
parse : rec /\Z/ { $item[1] }
rec : POS_INT rec_list[ $item[1] ] { [ $item[0] => $item[2] ] }
rec_list : { $arg[0] == 0 ? [] : undef }
| ELEM rec_list[ $arg[0]-1 ] { [ $item[1], @{$item[2]} ] }
POS_INT : /\d+/
ELEM : /\S+/
__END_OF_GRAMMAR__
ok($p->parse('0'));
ok($p->parse('1 foo'));
ok($p->parse('2 foo bar'));
ok(!$p->parse('1'));
ok(!$p->parse('1 foo bar'));
Changes:
-
rec_list recursively builds a list from one element and a list. The terminating condition of the recursion is the count of items, which is passed as an argument to the rule.
-
I added a check for end of "file" (/\Z/). This catches "1 foo bar". It's always good to check if you have leftover text to parse (unless you want to allow leftover text).
-
I prefer <<'__END_OF_GRAMMAR__' over q{...} because it handles backslashes more intuitively.
-
Your use strict and use warnings are in a different scope than the grammar. For them to apply to the grammar, you need to include them in the grammar.
-
I renamed int to pos_int to make it clear that signs are not acceptable. A check on the numbers magnitude inside of rec wouldn't hurt.
-
I uppercased tokens. It's just a style I use.
This is the same grammar as above, but with (yet-to-be-customized) error reporting:
my $p = Parse::RecDescent->new(<<'__END_OF_GRAMMAR__');
{
use strict;
use warnings;
}
parse : rec eof { $item[1] }
eof : /\Z/
| <error>
rec : POS_INT rec_list[ $item[1] ] { [ $item[0] => $item[2] ] }
rec_list : rec_list_[ $arg[0] ]
| <error>
rec_list_ : { $arg[0] == 0 ? [] : undef }
| ELEM rec_list_[ $arg[0]-1 ] { [ $item[1], @{$item[2]} ] }
POS_INT : /\d+/
ELEM : /\S+/
__END_OF_GRAMMAR__
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