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#!/usr/bin/perl -ws
# THE COMMONEST REASON FOR WANTING LEFT RECURSION
use strict;
use Parse::RecDescent; $::RD_HINT = 1;
my $parse = Parse::RecDescent->new(<<'EndGrammar');
main: expr /\Z/ { $item[1] }
| <error>
expr: left_assoc[qw{term add_op term}]
| term
add_op: '+' { sub { $_[0] + $_[1] } }
| '-' { sub { $_[0] - $_[1] } }
term: left_assoc[qw{factor mult_op factor}]
| factor
mult_op: '*' { sub { $_[0] * $_[1] } }
| '/' { sub { $_[0] / $_[1] } }
factor: number
| '(' expr ')' { $item[2] }
number: /[-+]?\d+(\.\d+)?/
# THE BLACK MAGIC THAT MAKES IT WORK...
left_assoc: left_assoc_left[@arg[0,1]](s) <matchrule:$arg[2]>
{ my @terms = $item[1]
? ((map { @$_ } @{$item[1]}), $item[2])
: $item[1];
splice @terms, 0, 3, $terms[1]->(@terms[0,2])
while @terms>1;
$terms[0];
}
left_assoc_left: <matchrule:$arg[0]> <matchrule:$arg[1]>
{ [ @item[1..2] ] }
EndGrammar
while (<DATA>) {
print $parse->main($_), "\n";
}
__DATA__
+1-1+1-1+1-1+1-1+1
7*7-6*8
121/(121/11)/121*11
1/(10-1/(1/(10-1)))