看看这个page,它有完整的说明。只要做到这一点:
#lang racket
(require (planet dyoo/simply-scheme:2:2))
另外要注意的是,’
字符是不正确,对引用使用'
,这可能是因为你复制粘贴代码错误排版。
当然,在完成上述操作之后,您必须定义第18章中介绍的过程,它们没有在您刚刚导入的包中定义!这将肯定工作:
(define (parse expr)
(parse-helper expr '() '()))
(define (parse-helper expr operators operands)
(cond ((null? expr)
(if (null? operators)
(car operands)
(handle-op '() operators operands)))
((number? (car expr))
(parse-helper (cdr expr)
operators
(cons (make-node (car expr) '()) operands)))
((list? (car expr))
(parse-helper (cdr expr)
operators
(cons (parse (car expr)) operands)))
(else (if (or (null? operators)
(> (precedence (car expr))
(precedence (car operators))))
(parse-helper (cdr expr)
(cons (car expr) operators)
operands)
(handle-op expr operators operands)))))
(define (handle-op expr operators operands)
(parse-helper expr
(cdr operators)
(cons (make-node (car operators)
(list (cadr operands) (car operands)))
(cddr operands))))
(define (precedence oper)
(if (member? oper '(+ -)) 1 2))
(define (compute tree)
(if (number? (datum tree))
(datum tree)
((function-named-by (datum tree))
(compute (car (children tree)))
(compute (cadr (children tree))))))
(define (function-named-by oper)
(cond ((equal? oper '+) +)
((equal? oper '-) -)
((equal? oper '*) *)
((equal? oper '/) /)
(else (error "no such operator as" oper))))
(parse '(4 + 3 * 7 - 5/(3 + 4) + 6))
=> '(+ (- (+ (4) (* (3) (7))) (/ (5) (+ (3) (4)))) (6))
(compute (parse '(4 + 3 * 7 - 5/(3 + 4) + 6)))
=> 30 2/7
只是为了重申:你还没有定义'parse',所以错误信息是正确的:它没有被“绑定”,因为没有内置的'parse'定义。所以你恰当地使用这种语言:语言不是问题的根源。 – dyoo