Racket 匹配可选模式并绑定属性的拼接语法类

下面定义了一个拼接语法类。syntax类匹配由两个语句（第一个模式）、一个语句（第三个和第二个模式）组成的序列，甚至可能根本不匹配这些语句（最后一个模式）

正如您所看到的，有相当多的“重复”代码，因为每个模式要么返回模式中捕获的某些内容的属性，要么返回一个空的内容。我的问题是，目前该语句从来都不是真正可选的，因为最后一个模式必须匹配某些内容。在本例中，一组空括号

（）

问题是：我怎样才能使这个陈述真正具有选择性？作为一个附带问题，代码是否可以通过更好地使用头部模式来压缩

(define-splicing-syntax-class signal-subscriptions-and-declarations
  #:description "subscriptions to signals and signal declarations"

  ; Match the case where both a subscription and declaration statement is present
  (pattern (~seq subscribed:signal-subscriptions  declared:signal-declarations)
           #:with (subscription-collection  ...) #'(subscribed.signal-collection ...)
           #:with (subscription-signal-id   ...) #'(subscribed.signal-identifier ...)
           #:with (declaration-signal-id    ...) #'(declared.signal-identifier ...))

  ; Match the case where no declaration statement is present
  (pattern subscribed:signal-subscriptions
           #:with (subscription-collection ...) #'(subscribed.signal-collection ...)
           #:with (subscription-signal-id  ...) #'(subscribed.signal-identifier ...)
           #:with (declaration-signal-id   ...) #'())

  ; Match the case where no subscription statement is present
  (pattern declared:signal-declarations
           #:with (subscription-collection ...) #'()
           #:with (subscription-signal-id  ...) #'()
           #:with (declaration-signal-id   ...) #'(declared.signal-identifier ...))

  (pattern ()
           #:with (subscription-collection ...) #'()
           #:with (subscription-signal-id  ...) #'()
           #:with (declaration-signal-id   ...) #'()))

---

听起来你有两个不同的东西，都是可选的。因此，有两个单独的语法类是有意义的，如下所示：

(define-splicing-syntax-class opt-signal-subscriptions
  ;; Match the case where a subscription is present
  [pattern (~seq subscribed:signal-subscriptions)
           #:with (subscription-collection  ...) #'(subscribed.signal-collection ...)
           #:with (subscription-signal-id   ...) #'(subscribed.signal-identifier ...)]
  ;; Match the case where no subscription is present
  [pattern (~seq)
           #:with (subscription-collection  ...) #'()
           #:with (subscription-signal-id   ...) #'()])

(define-splicing-syntax-class opt-signal-declarations
  ;; Match the case where a declaration statement is present
  [pattern (~seq declared:signal-declarations)
           #:with (declaration-signal-id    ...) #'(declared.signal-identifier ...)]
  ;; Match the case where no declaration statement is present
  [pattern (~seq)
           #:with (declaration-signal-id    ...) #'()])

(define-splicing-syntax-class signal-subscriptions-and-declarations
  #:description "subscriptions to signals and signal declarations"
  #:auto-nested-attributes
  [pattern (~seq :opt-signal-subscriptions :opt-signal-declarations)])

两者都使用空的

（~seq）

大小写（匹配0个术语）使其成为可选的，而不是匹配1个术语的

（）

。然后，可以这样定义与原始语法类类似的语法类：

(define-splicing-syntax-class opt-signal-subscriptions
  ;; Match the case where a subscription is present
  [pattern (~seq subscribed:signal-subscriptions)
           #:with (subscription-collection  ...) #'(subscribed.signal-collection ...)
           #:with (subscription-signal-id   ...) #'(subscribed.signal-identifier ...)]
  ;; Match the case where no subscription is present
  [pattern (~seq)
           #:with (subscription-collection  ...) #'()
           #:with (subscription-signal-id   ...) #'()])

(define-splicing-syntax-class opt-signal-declarations
  ;; Match the case where a declaration statement is present
  [pattern (~seq declared:signal-declarations)
           #:with (declaration-signal-id    ...) #'(declared.signal-identifier ...)]
  ;; Match the case where no declaration statement is present
  [pattern (~seq)
           #:with (declaration-signal-id    ...) #'()])

(define-splicing-syntax-class signal-subscriptions-and-declarations
  #:description "subscriptions to signals and signal declarations"
  #:auto-nested-attributes
  [pattern (~seq :opt-signal-subscriptions :opt-signal-declarations)])

---

这与原来的不同，因为它可以匹配0、1或2个术语，而您的术语至少需要1个术语，当两个选项都不存在时，必须是

（）

作为注释，在

信号订阅和声明

中，您需要命名订阅和声明模式变量，并在

#：with

子句中包含要向类用户提供的所有内容（来自订阅和声明变量）哦，抱歉，我的意思是包含自动嵌套属性，这是另一种方法。我不知道。酷。谢谢