Question

我要检查语法对象中是否存在关键字。关键字后跟一个表达式很容易：

(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，就没有语法绑定可以检查：

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

如何将#:world的存在绑定到语法属性？

Answer 1

您可以使用~and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 "!")])

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.

如何检查Racket语法中是否存在关键字？

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