Loops 在Scheme中如何在循环结束时返回默认值？

我正在尝试在Scheme中实现回溯搜索。到目前为止，我有以下几点：

(define (backtrack n graph assignment)  
    (cond (assignment-complete n assignment) (assignment) )

    (define u (select-u graph assignment))

    (define c 1)
    (define result 0)

    (let forLoop ()
        (when (valid-choice graph assignment c)
             (hash-set! assignment u c)

             (set! result (backtrack n graph assignment))

             (cond ((not (eq? result #f)) result))

             (hash-remove! assignment u)            
        )

        (set! c (+ c 1))
        (when (>= n c) (forLoop))
    )

   #f ; I believe this is where I'm having problems
)

我的功能分配完成并通过单元测试。参数赋值是一个hash表make with（make hash），因此应该可以

---

我相信我遇到的问题与在循环结束时返回false有关，如果没有递归返回非false值（应该是有效的赋值）。是否有一个与显式返回语句等效的方案？

您的问题的答案是肯定的：

这将设置一个出口，以便您可以从函数体内部返回结果值。通常的方案方法是将您的返回表单作为最后一个表单，因此返回其值：

    (let forLoop ()
        (when (valid-choice graph assignment c)
             (hash-set! assignment u c)
             (set! result (backtrack n graph assignment))
             (cond
                 ((not (eq? result #f))
                   result))       ; the value of `cond` form is ignored
             (hash-remove! assignment u))
                                  ; the value of `when` form is ignored
        (set! c (+ c 1))
        (if (>= n c)     ; `if` must be the last form 
           (forLoop)     ; so that `forLoop` is tail-recursive
           ;; else:
           return-value) ; <<------ the last form's value 
    )                    ; is returned from `let` form

   ;; (let forLoop ...) must be the last form in your function
   ;;                   so its value is returned from the function
)

此代码不呼叫

（分配完成n个分配）

。相反，它检查变量

赋值完成

是否有非空值，如果没有，则检查

赋值

变量，但在任何情况下，其返回值都会被忽略。可能缺少了更多的括号和/或

else

子句

    (let forLoop ()
        (when (valid-choice graph assignment c)
             (hash-set! assignment u c)
             (set! result (backtrack n graph assignment))
             (cond
                 ((not (eq? result #f))
                   result))       ; the value of `cond` form is ignored
             (hash-remove! assignment u))
                                  ; the value of `when` form is ignored
        (set! c (+ c 1))
        (if (>= n c)     ; `if` must be the last form 
           (forLoop)     ; so that `forLoop` is tail-recursive
           ;; else:
           return-value) ; <<------ the last form's value 
    )                    ; is returned from `let` form

   ;; (let forLoop ...) must be the last form in your function
   ;;                   so its value is returned from the function
)

(cond (assignment-complete n assignment) (assignment) )