如何使用Racket宏定义函数? [英] How do I define functions using Racket macros?

问题描述

我正在尝试编写一个宏，该宏定义具有相关功能的特殊数据结构类.

I am trying to write a macro that defines a special class of data structure with associated functions.

我知道这是可能的；它是在核心语言本身中多次执行的.

I know this is possible; it is done multiple times in the core language itself.

作为一个具体示例，我将如何在Scheme本身中定义define-struct宏.它需要创建make-struct，struct-<<field>>等函数.

As a specific example, how would I define the define-struct macro in Scheme itself. It needs to create make-struct, struct-<<field>>, etc functions.

我尝试使用define进行此操作，但是，这仅在宏的词法范围内定义了函数.

I tried doing this using define, however, this only defines the function in the macro's lexical scope.

我该如何在宏中实际定义一个函数?

How can I actually define a function in a macro?

推荐答案

答案的关键是datum->syntax.基本思想是要获取一些随机数据并将其转换为语法-在这种情况下，请将符号转换为标识符.标识符基本上是一个带有一些词汇信息的符号，这些词汇信息(非常粗略地)表明了其绑定方式.使用datum->syntax，您可以精确地做到这一点:它期望从其复制绑定的位置使用现有语法，并获得语法包装中包含的值的数据(此处为符号).

The key for an answer is datum->syntax. The basic idea is that you want to take some random data and turn it into a syntax -- in this case, turn a symbol into an identifier. An identifier is basically a symbol with some lexical information that (very roughly) indicates how it is bound. Using datum->syntax you can do exactly that: it expects an existing piece of syntax which is where it copies the binding from, and a datum (a symbol here) which is the value that is contained in the syntax wrapper.

下面是一个使用此工具演示类似define-struct的工具的示例:

Here's an example that demonstrates a define-struct-like tool using this:

#lang scheme
;; implements a defstruct-like macro that uses association lists
(define-syntax (defstruct-lite stx)
  (syntax-case stx ()
    [(defstruct-lite name field ...)
     (let ([make-id
            (lambda (template . ids)
              (let ([str (apply format template (map syntax->datum ids))])
                (datum->syntax stx (string->symbol str))))])
       (with-syntax ([make-name (make-id "make-~a" #'name)]
                     [name?     (make-id "~a?" #'name)]
                     [(arg ...) (generate-temporaries #'(field ...))]
                     [(name-field ...)
                      (map (lambda (f) (make-id "~a-~a" #'name f))
                           (syntax->list #'(field ...)))])
         #'(begin
             (define (make-name arg ...) (list 'name (cons 'field arg) ...))
             (define (name? x) (and (pair? x) (eq? 'name (car x))))
             (define (name-field x)
               (and (name? x) (cdr (assq 'field (cdr x)))))
             ...)))]))

这是一个使用它的示例:

And here's an example of using it:

(defstruct-lite point x y)
(point-y (make-point 1 2))

这篇关于如何使用Racket宏定义函数?的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！