mattford63 has asked for the wisdom of the Perl Monks concerning the following question:

Dear Monks,

Here's some parser code I've written in RecDescent.

The data format has dynamic columns e.g., the cpu column specifies there are 4 cpus and that the next 4 columns contain cpu values. Using another prayer here Parse::RecDescent and Dynamically Matched Subrule Repetition I've got some code, via recusion, that does what I want.

However,if I add an autoaction or an autotree directive (uncomment these in the code to replicate). The code breaks - the recursion fails. There is something i'm not understanding regarding the way code blocks return (I think). It says in the manual that autoactions (and autotree) will not modify rules that have code blocks so I don't get why the recursion stops. Any thoughts wise ones?

(an aside: is { [ $item[1] ] } equivalent to { $return=[ $item[1] ]} )?

#!/usr/bin/perl

use strict;
use warnings;

use Parse::RecDescent;
use Data::Dumper;

#$::RD_AUTOACTION = q { print $item[0]."=".$item[1]."\n" };

my $parser=Parse::RecDescent->new(<<'__END_OF_GRAMMAR__');
#<autotree>
line: time load cpus avg_time avg_load eof {\%item}
time: int
load: int
avg_time: int
avg_load: int

#Recursively build a list of cpus based on the number of cpus reported
cpus: cpu_num cpu_list[ $item{cpu_num} ] { $return=[ $item[0], $item[2
+] ] }
cpu_list: { $arg[0] == 0 ? $return=[] : undef }
        | cpu_elem cpu_list[ $arg[0]-1 ] { $return=[ $item[1], @{$item
+[2]} ] }
cpu_elem: int
cpu_num: int

int: /\d+/
eof: /\Z/

__END_OF_GRAMMAR__
my @input=( "10 12 4 3 2 9 2 7 567",
            "8 9 3 1 2 3 8 8",
            "6 2 1 1 4 8",
            "6 2 0 1 4",
           );

foreach my $input(@input){
            print Dumper($parser->line($input));
        };

[download]

Replies are listed 'Best First'.
Re: Parse::RecDescent and auto{actions,trees} and stuff
by pc88mxer (Vicar) on Jul 16, 2008 at 14:33 UTC
    Your autoaction is returning 1 instead of, perhaps, $item[1]. The 1 is the return value of the print statement in your autoaction code. That's why you are seeing in your output: 
    int=10
time=1
int=12
load=1
...

    [download]
    instead of: 
    int=10
time=10
int=12
load=12
...

    [download]

    Also, in your case you can safely remove the assignments to $return from your code. Assigning to $return is useful if, for instance, the return value is determined by an action block which is not the last: 

    production: stuff { $return = ... } and other { ... } things { ... }

    [download]
    Normally the return value would be whatever is returned by the action following things, but in this case it is set by the code in the action following stuff.
[reply]
[d/l]
[select]
      Hi,

      Well that explains things nicely and fixs for me my autoaction issue :-)

      But any idea why autotree won't work ?

      :-(
[reply]
        <autotree> only specifies actions for those rules which don't already have actions.
[reply]
[d/l]

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