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;; A Scheme interpreter with debugging features.
(load "rn-match.r5rs")
(define (lookup x env)
(if (eq? x (caar env))
(cdar env)
(lookup x (cdr env))))
(define (update-env! x v env)
(if (eq? x (caar env))
(set-cdr! (car env) v)
(update-env! x v (cdr env))))
(define (value-of e env)
(match e
(,x (guard (symbol? x)) (lookup x env))
(,n (guard (number? n)) n)
((lambda (,x) ,e)
(lambda (v) (value-of e (cons (cons x v) env))))
((set! ,x ,[e])
(update-env! x e env))
((begin ,e* ... ,e)
(let loop ((e* e*))
(if (pair? e*)
(value-of (car e*) env)
(loop (cdr e*)))))
(value-of e env)))
((debug) (debugger env))
((,op ,[e1] ,[e2]) (guard (memq op '(+ * -)))
((eval op) e1 e2))
((,[e1] ,[e2])
(e1 e2))))
(define (debugger env)
(printf "debug> ")
(let ((cmd (read))
(continue #t))
(match cmd
((continue) (set! continue #f))
((eval ,e)
(printf "~s => ~s\n" e (value-of e env)))
((show-env) (show-env env))
(,else (printf "unknown command\n")))
(if continue
(debugger env)
(printf "running...\n"))))
(define (show-env env)
(if (pair? env)
(let ((depth (show-env (cdr env))))
(printf "~d. ~a: ~s\n" depth (caar env) (cdar env))
(+ 1 depth))
(((lambda (x)
(lambda (y)
(+ x y))))