考虑两个宏的场景： outer-macro定义了一些实体的一般结构， inner-macro在外部宏的范围inner-macro扩展。 我的意图被捕获在下面的代码中，其中预期的输出是一个打印语句。 此示例将引发内部宏的模式的以下错误： (_ value ...) 。
syntax: no pattern variables before ellipsis in template in: ...我打算以与外部宏观的body ...模式相同的方式value ... 事实上，“价值”的清单正是我所需要的（不一定非常灵活的省略号模式）。 可悲的是，这种方式不起作用。 我如何在内部宏中捕获可变数量的参数？
#lang racket (require racket/stxparam (for-syntax syntax/parse)) (define-syntax-parameter inner-macro (lambda (stx) (raise-syntax-error 'inner-macro "generic error message" stx))) (define-syntax (outter-macro stx) (syntax-parse stx [(_ body:expr ...+) #'(syntax-parameterize ([inner-macro (lambda (stx) (syntax-case stx () [(_ value ...) (printf "values are: ~a~n" (list value ...))]))]) body ...)])) (outter-macro (inner-macro 'a 'b 'c)) ; expected result ; > "values are: (a b c)"Consider a scenario of two macros: the outer-macro defines a general structure of some entity, and the inner-macro expands in the scope of the outer macro. My intent is captured in the following code, where the expected output is a print statement. This example throws the following error for the pattern of the inner macro: (_ value ...).
syntax: no pattern variables before ellipsis in template in: ...I intend to use value ... in the same way as the body ... pattern of the outer macro. In fact, a list of the 'values' is exactly what I need (not necessarily a very flexible 'ellipsis pattern'). Sadly it does not work this way. How can I capture a variable amount of arguments in the inner macro?
#lang racket (require racket/stxparam (for-syntax syntax/parse)) (define-syntax-parameter inner-macro (lambda (stx) (raise-syntax-error 'inner-macro "generic error message" stx))) (define-syntax (outter-macro stx) (syntax-parse stx [(_ body:expr ...+) #'(syntax-parameterize ([inner-macro (lambda (stx) (syntax-case stx () [(_ value ...) (printf "values are: ~a~n" (list value ...))]))]) body ...)])) (outter-macro (inner-macro 'a 'b 'c)) ; expected result ; > "values are: (a b c)"最满意答案
要在语法模板中“逃避”省略号，可以使用语法(... <form>) ，其中<form>是语法模板，其中...序列按字面意思处理。 因此，你可以包装一段语法来包含直接的省略号：
> #'(... (syntax-rules () [(x ...) (list x ...)])) #<syntax:4:9 (syntax-rules () ((x ...) (li...>你可以用它来包围你的内部宏定义来转义内部的椭圆：
(define-syntax (outer-macro stx) (syntax-parse stx [(_ body:expr ...+) #'(syntax-parameterize ([inner-macro (lambda (stx) (... (syntax-case stx () [(_ value ...) (printf "values are: ~a~n" (list value ...))])))]) body ...)]))但是，这实际上并不正确，因为你的syntax-case体是错误的，它不会返回一个语法对象。 你只是在(printf ...)之前缺少一个#' （或者你可以使用syntax-rules ），所以正确的实现应该如下：
(define-syntax (outer-macro stx) (syntax-parse stx [(_ body:expr ...+) #'(syntax-parameterize ([inner-macro (lambda (stx) (... (syntax-case stx () [(_ value ...) #'(printf "values are: ~a~n" (list value ...))])))]) body ...)]))这应该按预期工作。
To “escape” ellipses in syntax templates, you can use the syntax (... <form>), where <form> is a syntax template where ... sequences are treated literally. Therefore, you can wrap a piece of syntax to include literal ellipses:
> #'(... (syntax-rules () [(x ...) (list x ...)])) #<syntax:4:9 (syntax-rules () ((x ...) (li...>You can use this to surround your inner macro definition to escape the inner ellipses:
(define-syntax (outer-macro stx) (syntax-parse stx [(_ body:expr ...+) #'(syntax-parameterize ([inner-macro (lambda (stx) (... (syntax-case stx () [(_ value ...) (printf "values are: ~a~n" (list value ...))])))]) body ...)]))However, this is actually not quite right, because your syntax-case body is wrong—it does not return a syntax object. You are just missing a #' before the (printf ...) (or you could use syntax-rules), so the correct implementation should be the following:
(define-syntax (outer-macro stx) (syntax-parse stx [(_ body:expr ...+) #'(syntax-parameterize ([inner-macro (lambda (stx) (... (syntax-case stx () [(_ value ...) #'(printf "values are: ~a~n" (list value ...))])))]) body ...)]))This should work as intended.
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