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我正在尝试define-let在球拍中编写一个宏，它“保存” a 的标题(let ((var value) ...) ...)，即只是(var value) ...部分，并允许稍后重新使用它。

下面的代码按预期工作：

#lang racket

;; define-let allows saving the header part of a let, and re-use it later
(define-syntax (define-let stx1)
  (syntax-case stx1 ()
    [(_ name [var value] ...)
     #`(define-syntax (name stx2)
         (syntax-case stx2 ()
           [(_ . body)
            #`(let ([#,(datum->syntax stx2 'var) value] ...)
                . body)]))]))

;; Save the header (let ([x "works]) ...) in the macro foo
(define-let foo [x "works"])
;; Use the header, should have the same semantics as:
;; (let ([x "BAD"])
;;   (let ([x "works])
;;     (displayln x))
(let ([x "BAD"])
  (foo (displayln x))) ;; Displays "works".

问题是宏破坏了卫生：如下例所示，y在 a 中声明的define-let由宏产生的变量 应该是一个新的、不受约束的符号，因为卫生，但它设法从宏中泄漏, 并且在(displayln y).

;; In the following macro, hygiene should make y unavailable
(define-syntax (hygiene-test stx)
  (syntax-case stx ()
    [(_ name val)
     #'(define-let name [y val])]))

;; Therefore, the y in the above macro shouldn't bind the y in (displayln y).
(hygiene-test bar "wrong")
(let ((y "okay"))
  (bar (displayln y))) ;; But it displays "wrong".

我如何编写define-let宏，使其表现得像第一个示例中一样，但在标识符由宏生成时也保持卫生，"okay"在第二个示例中给出？

4

1 回答 1

3

遵循 Chris 的提示“语法参数”，这是一个解决方案：

#lang racket
(require racket/stxparam
         (for-syntax syntax/strip-context))

(define-syntax (define-let stx1)
  (syntax-case stx1 ()
    [(_ name [var expr] ...)
     (with-syntax ([(value ...) (generate-temporaries #'(expr ...))])
       #`(begin
           (define-syntax-parameter var (syntax-rules ()))
           ...
           (define value expr)
           ...
           (define-syntax (name stx2)
             (syntax-case stx2 ()
               [(_ . body)
                (with-syntax ([body (replace-context #'stx1 #'body)])
                  #'(syntax-parameterize ([var (syntax-id-rules () [_ value])] ...)
                     . body))]))))]))

(define-let foo [x "works"])

(let ([x "BAD"])
  (foo (displayln x)))       ; => works

(let ([x "BAD"])
  (foo 
   (let ([x "still works"])
     (displayln x))))        ; => still works

更新

该解决方案通过了评论中的附加测试。新的解决方案将主体的上下文转移到要绑定的变量。

#lang racket
(require (for-syntax syntax/strip-context))

(define-syntax (define-let stx1)
  (syntax-case stx1 ()
    [(_ name [var expr] ...)
     #`(begin
         (define-syntax (name stx2)
           (syntax-case stx2 ()
             [(_ . body)
              (with-syntax ([(var ...) (map (λ (v) (replace-context #'body v))
                                            (syntax->list #'(var ...)))])
                #'(let ([var expr] ...) 
                    . body))])))]))

(define-let foo [x "works"])

(let ([x "BAD"])
  (foo (displayln x)))       ; => works

(let ([x "BAD"])
  (foo 
   (let ([x "still works"])
     (displayln x))))        ; => still works


(let ([z "cool"]) 
  (foo (displayln z)))       ; => cool
于 2015-06-08T20:30:17.340 回答