#!/usr/bin/perl -w
use strict;
use Parse::RecDescent;
$::RD_HINT   = 1;
$::RD_WARN   = 1;
$::RD_ERRORS = 1;

our %VARIABLE;
sub expression
{
    my ($left, $op, $right) = @_;
    $left = $VARIABLE{$left} if $left =~ /[^-+0-9]/;
    return eval "$left $op $right";
}

my @expressions = (
    'a=2',
    'a=1+3',
    'print 5*7',
);

my $parser1 = Parse::RecDescent->new(grammar1());

foreach my $expression (@expressions) {
    print "$expression\n";
    $parser1->startrule($expression);
}

sub grammar1 {
    return <<'__EOGRAMMAR__';

    #
    # Terminals (macros that can't expand further)
    #

    OP       : m([-+*/%])      # Mathematical operators
    INTEGER  : /[-+]?\d+/      # Signed integers
    VARIABLE : /\w[a-z0-9_]*/i # Variable

    expression : INTEGER OP expression
                 { return main::expression(@item) }
               | VARIABLE OP expression
                 { return main::expression(@item) 
               | INTEGER
               | VARIABLE
                 { return $main::VARIABLE{$item{VARIABLE}} }

    print_instruction  : /print/i expression
                         { print $item{expression}."\n" }
    assign_instruction : VARIABLE "=" expression
                         { $main::VARIABLE{$item{VARIABLE}} = $item{expression} }

    instruction : print_instruction
                | assign_instruction

    startrule : instruction(s /;/)

__EOGRAMMAR__
}