Advanced Perl Programming

Jeeves Implementation

In the following pages, we implement all the components of the Jeeves framework. You may find it helpful to run jeeves for a sample problem and have a copy of the output handy.

AST Module

The AST module is a very simple library, so we will look at only a few of the more interesting procedures below.

An AST node is a container of properties, so a hash table suits the job perfectly. Each node is given a name for ease of debugging:

package Ast;
use strict;
sub new {
    my ($pkg, $name) = @_;
    bless {'ast_node_name' => $name}, $pkg;
}

new, add_prop , and add_prop_list are used by all specification parsers to create AST objects:

sub add_prop {
    my ($node, $prop_name, $prop_value) = @_;
    $node->{$prop_name} = $prop_value;
}
sub add_prop_list {
    my ($node, $prop_name, $node_ref) = @_;
    if (! exists $node->{$prop_name}) {
        $node->{$prop_name} = [];
    }
    push (@{$node->{$prop_name}}, $node_ref);
}

add_prop simply adds a name-value pair to the AST object. add_prop_list creates a list-valued property. The property value is an anonymous array that contains references to other AST nodes. You can have your own list-valued properties, but you should never use them as an argument to @foreach because it assumes that the elements of that list are AST nodes.

my @saved_values_stack;
sub visit { no strict 'refs'; my $node = shift; package main; my ($var, $val, $old_val, %saved_values); while (($var,$val) = each %{$node}) { if (defined ($old_val = $$var)) { $saved_values{$var} = $old_val; } $$var ...