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; This ain't a Scheme interpreter, despite the filename.
; Uses: startup scmhelp.lsp
(define read-eval-print-loop
(lambda ()
(let ((exp '*))
(while (begin
(write '>)
(not (eq? the-eof-object (set! exp (read)))))
(print (eval-exp (macroexpand exp) init-env))))))
(define eval-exp
(lambda (exp env)
((evaluator exp) exp env)))
(define evaluator
(lambda (exp)
(if (atom? exp)
(if (symbol? exp) lookup-variable self-evaluating)
(or (get (car exp) 'evaluator)
(lambda (exp env)
(apply-proc (eval-exp (car exp) env)
(map (lambda (rand) (eval-exp rand env)) (cdr exp))))))))
(define self-evaluating
(lambda (exp env) exp))
(put 'quote 'evaluator
(lambda (exp env) (cadr exp)))
(put 'lambda 'evaluator make-closure)
(put 'if 'evaluator
(lambda (exp env)
(if (eval-exp (test-exp exp) env)
(eval-exp (then-exp exp) env)
(eval-exp (else-exp exp) env))))
(put 'define 'evaluator
(lambda (exp env)
(define-variable-value (cadr exp) (eval-exp (caddr exp) env) env)
(cadr exp)))
(define apply-proc
(lambda (proc args)
(if (primitive? proc)
(apply proc args)
(eval-exp (closure-body proc)
(extend-env (closure-formals proc) args (closure-env proc))))))
; Here we go
(define init-env (extend-env '() '() '()))
(read-eval-print-loop)
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