Scheme 方案跟踪变量检查

我需要函数来检查变量是否有阴影？函数应该返回#t或#f，这取决于是否有变量阴影。 我使用DrRacket实现代码（）。到目前为止，我有这个

#lang plai

(define-type WAE
  [num (n number?)]
  [add (lhs WAE?)(rhs WAE?)]
  [sub (lhs WAE?)(rhs WAE?)]
  [with (name symbol?)(named-expr WAE?)(body WAE?)]
  [id (name symbol?)])

(define (parse sexpr)
  (cond
    [(number? sexpr) (num sexpr)]
    [(symbol? sexpr) (id sexpr)]
    [(list? sexpr)
      (case (first sexpr)
        [(add) (add (parse (second sexpr)) (parse (third sexpr)))]
        [(sub) (sub (parse (second sexpr)) (parse (third sexpr)))]
        [(with) (with (first (second sexpr)) 
                      (parse (second (second sexpr)))
                      (parse(third sexpr)) )]]
     [else (error "unexpected token")]))

(define (symbol<? a b)
  (string<? (symbol->string a) (symbol->string b)))

我不知道

#lang plai

，但我相信代码的第二部分可能有语法错误

(define (shadowed? wae)
  (let 
   >>> Possible error >>> ([l(type-case WAE wae
      [num (n) n]
      [add (lft rght) (+ (shadowed-variable? lft) (shadowed-variable? rght))]
      [sub (lft rght) (- (shadowed-variable? lft) (shadowed-variable? rght))]
      [with (x i b) (shadowed-variable? (sub b x (shadowed-variable? i)))]
      [id (s) (error 'shadowed-variable? "free variable")])]) 
    >>>> Error >>> (if (remove-duplicates (flatten l) symbol=?) #t #f)))

试试这个，让我们知道

(define (shadowed? wae)
  (let 
    ([l (type-case WAE wae
      [num (n) n]
      [add (lft rght) (+ (shadowed-variable? lft) (shadowed-variable? rght))]
      [sub (lft rght) (- (shadowed-variable? lft) (shadowed-variable? rght))]
      [with (x i b) (shadowed-variable? (sub b x (shadowed-variable? i)))]
      [id (s) (error 'shadowed-variable? "free variable")])]) 
    (if (remove-duplicates (flatten l)) symbol=?) #t #f))

---

你能不能比“它不起作用”更具体一点？发生了什么不应该发生，或不应该发生但应该发生的事情？是的，可能是错误，但它仍然不能正常工作。例如，（带阴影的？（id'x））应该返回#t，（带阴影的？（num 1））应该返回#f。我到了这里error@vladang为什么

（隐藏的？（id'x））

应该是

#t

？您的

id

案例中是否有

error'阴影变量？“自由变量”）

。另外，请尝试REPL中的

（删除重复项（展平1）symbol=？）

，看看会发生什么。

(define (shadowed? wae)
  (let 
    ([l (type-case WAE wae
      [num (n) n]
      [add (lft rght) (+ (shadowed-variable? lft) (shadowed-variable? rght))]
      [sub (lft rght) (- (shadowed-variable? lft) (shadowed-variable? rght))]
      [with (x i b) (shadowed-variable? (sub b x (shadowed-variable? i)))]
      [id (s) (error 'shadowed-variable? "free variable")])]) 
    (if (remove-duplicates (flatten l)) symbol=?) #t #f))