Compatibility

This interpreter will run on mzscheme v372 and mit-scheme 9.1.1.

Design

I used 2 ideas from Sussman and Abelson's Meta-Circular Evaluator (Structure and Interpretation of Computer Programs): expressing the environment as stack/list of frames and using syntactic transformations. The first idea is necessary to implement the rules of nested defines (i.e., you should not be able to redefine a name in your existing scope). The second idea helps to reduce the amount of code necessary to support special forms. Like Sussman and Abelson's interpreter, my interpreter translates cond expressions to equivalent if expressions. I extend this idea to both letrec, let*, and even and and or.

I've also made a conscious effort to limit the number of primitives registered in the global environment; instead, many of these procedures can be found in a standalone file called stdlib.scm. As such, it may be necessary to load this file before attempting to use any of the normal builtins. Here's a list of the procedures stdlib.scm provides:

• <
• >=
• append
• assoc
• fold
• foldr
• length
• list
• list?
• map
• null?

IMPORTANT: Because interpreter.scm needs stdlib.scm to bootstrap itself, it loads that file automatically. However, if you'd like to use any of the procedures defined in stdlib.scm in a standalone fashion (e.g., after starting the repl), then you'll need to load it yourself.

Demo instructions

Here's a sample session of how to exercise the interpreter.

;; Load interpreter into Scheme
(load "interpreter.scm")
;; Start up the read-eval-print-loop
(repl)

;; Now we test the macro facility
;; Load stdlib.scm for "list" procedure
(load "stdlib.scm")

;; Define weird macro
(define-macro (strange-mult a b) (list '* a (list '- b 1)))

;; Set up some variables that will be necessary for evaulating our
;; macro
(define y 3) (let ((x 5)) (strange-mult (+ 3 2) (+ x y)))


;; Now let's test self-evaluation
(load "interpreter.scm")
;; ... Wait a couple of minutes

;; Change the prompt for this invocation of repl, so that it's
;; easier to distinguish which interpreter we are really in
(define *prompt* "*meta* ]=> ")

;; Now start the read-eval-printloop
(repl)

;; Load stdlib.scm again, because we're going to test something standalone. Load takes a while (be patient)

(load "stdlib.scm")

;; After load finishes, we define a factorial function

(define (fac x)
  (if (= x 0)
      1
      (* x (fac (- x 1)))))

;; Now let's test it. This will take about a minute or two.

(fac 4)

;; Exit second interpreter

(exit)

;; Exit first interpreter

(exit)