用大括号 {} 替换 Racket 中的“开始" [英] Curly brackets {} to replace 'begin' in Racket
问题描述
有没有宏可以用大括号{}来表示语句块,从而代替'begin'关键字.因此,而不是:
Is it possible to have a macro to use curly brackets {} to indicate a block of statments, so as to replace the 'begin' keyword. Hence, instead of:
(if (condition)
(begin
(statement1)
(statement2)
(statement3)
(statement4))
(else-statement))
我们可以使用:
(if (condition) {
(statement1)
(statement2)
(statement3)
(statement4) }
(else-statement))
如何实现?感谢您的回答.
How can this be accomplished? Thanks for your answers.
推荐答案
这是完全可能的,并且有几种方法可以做到.(开始之前的快速说明,我将使用 block
而不是 begin
因为它在使用内部定义时表现更好.)
This is completely possible, and there are several ways to do it. (Quick note before I start, I'm going to use block
instead of begin
because it behaves better with internal definitions.)
一种稍微有点老套的方法是重新定义函数应用程序的含义,以便对花括号进行特殊处理.您可以通过定义一个 #%app
宏来实现:
One slightly hack-y way is to redefine what function application means so that curly-braces are treated specially. You can do this by defining an #%app
macro:
#lang racket
(require racket/block syntax/parse/define (prefix-in - racket))
;; This #%app macro redefines what function application means so that
;; { def-or-expr ... } expands into (block def-or-expr ...)
;; Otherwise it uses normal function application
(define-syntax-parser #%app
[{_ def-or-expr:expr ...}
#:when (equal? #\{ (syntax-property this-syntax 'paren-shape))
;; group them in a block
#'(block def-or-expr ...)]
[(_ f:expr arg ...)
#:when (not (equal? #\{ (syntax-property this-syntax 'paren-shape)))
;; expand to the old #%app form, from (prefix-in - racket)
#'(-#%app f arg ...)])
;; using it:
(define (f x)
(if (< 5 x) {
(define y (- x 5))
(f y)
}
x))
(f 1) ; 1
(f 5) ; 5
(f 6) ; 1
(f 10) ; 5
(f 11) ; 1
方法二:扩展阅读器
另一种方法是定义一种新的 #lang
语言并使用 {
字符的不同条目扩展可读表.让我去做那个......
Method 2: Extending the Reader
Another way would be to define a new #lang
language and extend the readtable with a different entry for the {
character. Let me go and do that ...
要定义#lang
语言,您需要将阅读器实现放在your-language/lang/reader.rkt
中.这里是 curly-block/lang/reader.rkt
,其中 curly-block
目录作为单集合包安装(raco pkg install path/to/curly-block
).
To define a #lang
language, you need to put the reader implementation in your-language/lang/reader.rkt
. Here that's curly-block/lang/reader.rkt
, where the curly-block
directory is installed as a single-collection package (raco pkg install path/to/curly-block
).
curly-block/lang/reader.rkt
;; s-exp syntax/module-reader is a language for defining new languages.
#lang s-exp syntax/module-reader
racket
#:wrapper1 (lambda (th)
(parameterize ([current-readtable (make-curly-block-readtable (current-readtable))])
(th)))
;; This extends the orig-readtable with an entry for `{` that translates
;; { def-or-expr ... } into (block def-or-expr ...)
(define (make-curly-block-readtable orig-readtable)
(make-readtable orig-readtable
#\{ 'terminating-macro curly-block-proc))
;; This is the function that the new readtable will use when in encounters a `{`
(define (curly-block-proc char in src ln col pos)
;; This reads the list of things ending with the character that closes `char`
;; The #f means it uses the racket reader for the first step, so that `{`
;; uses the normal behavior, grouping expressions into a reader-level list
(define lst (read-syntax/recursive src in char #f))
(cons 'block lst))
使用:
#lang curly-block
(require racket/block)
(define (f x)
(if (< 5 x) {
(define y (- x 5))
(f y)
}
x))
(f 1) ; 1
(f 5) ; 5
(f 6) ; 1
(f 10) ; 5
(f 11) ; 1
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