如何检查Racket语法中是否存在关键字？

我想检查语法对象中是否存在关键字。 当关键字后面跟一个表达式时很容易：

(syntax-parse #'(hello #:world "sunny")
  [(_ (~optional (~seq #:world <adjective>:str)))
   #'(string-append "Hello world! Today's weather is "
                    (~? <adjective> "unknown"))])

---

如何将

#：world

的存在绑定到语法属性？

您可以使用

~和

将语法绑定到属性：

(syntax-parse #'(hello #:world)
  [(_ (~optional (~and world? #:world)))
   #:with maybe-world (if (attribute world?) #'" world" #'"")
   #'(string-append "Hello" maybe-world "!")])

---

从文档：

另见。

(syntax-parse #'(hello #:world)
  [(_ (~optional (~and world? #:world)))
   #:with maybe-world (if (attribute world?) #'" world" #'"")
   #'(string-append "Hello" maybe-world "!")])

One use for ~and-patterns is preserving a whole term (including its lexical
context, source location, etc) while also examining its structure.
Syntax classes are useful for the same purpose, but ~and can be lighter weight.