racket - 如何检查 Racket 语法中是否存在关键字？

Question

我想检查语法对象中是否存在关键字。
当关键字后跟一个表达式时，这很容易:

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

但是，如果我只想检查关键字是否存在，则不再需要检查语法绑定(bind):

(syntax-parse #'(hello #:world)
  [(_ (~optional #:world))
   #:with maybe-world (if (attribute #:world) #'" world" #'"") ; doesn't work
   #'(string-append "Hello" maybe-world "!")])

如何绑定(bind) #:world 的存在到语法属性？

Answer 1

您可以使用 ~and将语法绑定(bind)到属性:

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

来自文档 https://docs.racket-lang.org/syntax/stxparse-patterns.html?q=~and#(elem.(pattern-link.(~7eand._s))) :

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.