Parsing/Evaluating a prefix-notation equation using sexeger

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 $r;
}
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Re: Parsing/Evaluating a prefix-notation equation using sexeger by Roy Johnson (Monsignor) on Sep 01, 2004 at 17:01 UTC
Why do you consider sexeger preferable to ordinary regexes for this? `sub my_epe { my ($s,$h) = @_; my $op = '(' . join('\|', keys %$h) . ')'; my $con = qr/Con (\d+(?:\.\d+)?)/; 1 while $s=~s/$op \s+ $con \s+ $con/'Con '.$h->{$1}($2,$3)/ex; $s; }` [download] Caution: Contents may have been coded under pressure.	[reply] [d/l]
Re^2: Parsing/Evaluating a prefix-notation equation using sexeger by jryan (Vicar) on Sep 01, 2004 at 22:38 UTC
That's a really good point. :) I guess I saw that sexeger might be applicable, got excited, and just flew with it. A huge ++ for bringing me back down to earth.	[reply]
Re: Parsing/Evaluating a prefix-notation equation using sexeger by sleepingsquirrel (Chaplain) on Sep 17, 2004 at 22:04 UTC
You inspired me to implement monadic parser combinators in perl. I will probably document the thing better, but here you go. Grab Monadic_parser and the corresponding needed module Do_notation.pm. For extra credit figure out a way to use perl's native regex engine instead of the roll-your-own kind used (i.e. `/\d*/` instead of `many(digits())`). Maybe monads in perl aren't so useless after all. Read more... (5 kB) -- All code is 100% tested and functional unless otherwise noted.	[reply] [d/l] [select]