Scheme-Evaluator

This is a modified version of evaluator for Scheme based on the book STRUCTURE AND INTERPRETATION OF COMPUTER PROGRAMS (SICP)

Scheme

Scheme is a minimalist, multi-paradigm dialect of Lisp. It's simple and elegant.

Install

There is a version provided by GNU
Or you can install DrRacket

The Evaluator

This is a modifeid version of the Scheme interpreter written in Scheme which support lazy evaluation.

How to Run

To run the evaluator:

scheme -load mceval.scm

The Lazy Evaluation

In Scheme, if we define the following try

(define (try a b)
  (cond ((= a 0) 1)
  (else b)))

If we run (try 0 (/ 1 0)), the system will throw me an error because it will evaluate (/ 1 0), even though it is not used in the try function.

However, in my modified version, user can choose to delay some of the argument which can improve computation efficiency and provide more flexibility in coding. For example, if we define try in this way in the new evaluator:

(define (try a (delayed b))
  (cond ((= a 0) 1)
  (else b)))

and now if we run (try 0 (/ 1 0)) again, the system will simply return 1 instead of throwing an error.

A Different Version of Stream

In Scheme, there is a built in stream which is similar to cons but with a delayed second argument. Now with the new dealyed tag, we can build a new-cons which inherits the built in cons but add a delayed tag for the second argument.

Usage:

(define (integers-from n)
  (new-cons n (integers-from (+ n 1))))

which will give us a function that can return a infinite list of integers starting from n.

This is a great example of how delayed argument can provide better computation efficiency.

Other functionality

Besides the delayed tag, there are other improvement from the original meta-circular evaluator introduced in SICP.

let which is the same as the built in let
let* which is the same as the built in let*
make-unbound!which will unbound the variable in the current scope with its associated value

WesleyyC / Scheme-Evaluator