In Monads in Perl, sleepingsquirrel gives an example of how to parse a prefix-notation equation using monads. After reading the article, I really couldn't see the point of monads (they seem just like functions with an implicit state). However, the prefix equations used in the article seemed like a perfect place to apply the sexeger technique.

As an added bonus, this version features per-operator callbacks. 

use strict;
use warnings 'all';

# to call:
my %handlers = (
    'Div' => sub { $_[0] /  $_[1] },
    'Mul' => sub { $_[0] *  $_[1] },
    'Mod' => sub { $_[0] %  $_[1] },
    'Exp' => sub { $_[0] ** $_[1] }
);
print eval_prefix_equation("Mul Mod Exp Div Mul Div Div Con 18 Con 2 C
+on 3 Con 4 Con 6 Con 2 Con 3 Con 4", \%handlers);


# the method:
sub eval_prefix_equation {
    my ($s,$h) = @_;
    my $r = reverse $s;
    
    # qr/Con (\d+(?:\.\d+)?/ backwards
    my $con = qr/(\d+(?:\.\d+)?) noC/;

    my $op = '(' . join('|', map { scalar reverse $_ } keys %$h) .')';
    
    # just a sexeger; the while-loop condition resets our pos for us.
    $r =~ s/ $con \s+ $con \s+ $op
           / reverse('Con ' . $h->{reverse $3}(
                 scalar(reverse $2),
                 scalar(reverse $1)
             ));
           /ex while $r =~ /$op/;
    return scalar reverse $r;
}

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In reply to Parsing/Evaluating a prefix-notation equation using sexeger by jryan

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