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#!/usr/bin/perl -ws
# THE COMMONEST REASON FOR WANTING LEFT RECURSION
use strict;
use Parse::RecDescent; $::RD_HINT = 1;
sub Parse::RecDescent::evalop {
$_[0][0] = $_[0][$_+1](@{$_[0]}[0,$_+2])
for map 2*($_-1), 1..@{$_[0]}/2;
return $_[0][0];
}
my $parse = Parse::RecDescent->new(<<'EndGrammar');
main: expr /\Z/ { $item[1] }
| <error>
expr: <leftop:term add_op term>
{ evalop($item[1]) }
add_op: '+' { sub { $_[0] += $_[1] } }
| '-' { sub { $_[0] -= $_[1] } }
term: <leftop:factor mult_op factor>
{ evalop($item[1]) }
mult_op: '*' { sub { $_[0] *= $_[1] } }
| '/' { sub { $_[0] /= $_[1] } }
factor: number
| '(' expr ')' { $item[2] }
number: /[-+]?\d+(\.\d+)?/
EndGrammar
while (<DATA>) {
print "$_ = ", $parse->main($_), "\n";
}
while (print "> " and defined($_=<>)) {
print "= ", $parse->main($_), "\n";
}
__DATA__
2+3
2*3
+1-1+1-1+1-1+1-1+1
7*7-6*8
121/(121/11)/121*11
1/(10-1/(1/(10-1)))