Post 6: Expression Evaluator - Finale

We have, in the previous posts, seen how to build an expression evaluator and integrate it into our calling environment.

The concepts we have discussed provide a solid basis for an evaluation engine that can be extended further.

For example, we could support custom functions with a defun function:

(defun <function name> <list of formal parameters> <function body>)

It's easy to see how we could implement the defun function as a known function, which would store the body and formal parameter list in a hash table indexed by the function name. We could then modify the Apply function to bind values to the formal parameters and evaluate the body.

We could support variable binding with the setq function:

(setq <var_name> <expr>)

We could do this by storing the value of the expression in a global symbol table indexed by the var_name, and modifying the Apply function to do look for the value of a variable first in the scoped symbol table and then the global one.

We could support conditional operations with the cond function, which returns the result associated with the first test expression that evaluates to 'true':

(cond ((test_1) (result_1))  ... ((test_n) (result_n)))

We could even expand our processing to perform evaluation of the function name:

((cond ((> w 20) "square") (t "cube")) p)

so that we get the value of (square p) if w > 20 and (cube p) otherwise!

With a few more modifications, we can support recursive functions such as the classic expression for a factorial

(defun decr (x) (- x 1))
(defun fact (x) (cond ((> x 0) (* x (fact (decr x)))) (t 1)))

Executing the above strings in order would allow us to compute factorials of any integer from that point on!

double fact10 = 10.ToString().Evaluate(null);

Cool?

Contact me if you would like to get your hands on this library!

One last thing - in honour of my newest niece who was born just as the first full implementation of this project was completed, I'm calling this library Eden. We'll be likely releasing BrightSword.Eden as a free download from the company home page soon.