If you have made it this far...

If you have managed to make it this far and still understand what's going on, you are doing very well indeed. At this point we now have a full compiler that works with basic scripts, although it does not yet respect any form of operator precedence. If you are having trouble putting the script together in the right order (sorry, but I really didn't want to print out the entire script more than once, and it's printed out later at the end of this chapter), here's a guide:

    define("FOO_NUMBER", 0);
    define("FOO_MULTIPLY", 7);
    define("IS_OPERATOR", 0);
    define("IS_OPERAND", 1);

    class token { }
    function getval($token) { }
    function execute(&$stack) {}
    function main() {}
    function gettoken() {}

    $script = fopen("script.foo", "r");
    $characters = array_merge(range('a', 'z'), range('A', 'z'));
    $characters[] = "_";
    $variables = array();

    function cleanup() {
        GLOBAL $script;



Note that I snuck in a little shutdown function just to make sure and close the file handle before the script terminates. If you have your script completed, you should now be able to execute it and have it parse script.foo.

Here's what I put in my script.foo file:

a = 45;
b = a;
print a;
d = a + b + a;
c = "Foo bar baz";
print "Baz bar foo";
print d;
print c;

If you've followed all the instructions carefully, you should get the following output:

Baz bar foo
Foo bar baz

It seems we have been quite successful! However, the compiler is not done just quite yet - there is one tweak left to make before it's perfect.


Next chapter: Operator precedence >>

Previous chapter: Finally, execution

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Copyright ©2015 Paul Hudson. Follow me: @twostraws.