在clojure，如何应用宏到列表？ [英] In clojure, how to apply a macro to a list?

问题描述

在 clojure ， apply 不能应用于宏。例如（apply和[true false]）引发异常。我正在考虑以下解决方法：

 （defmacro apply-macro [func args]`（〜func〜@ args））
  

乍一看，它似乎工作得很好：

 （apply-macro和[true 5]）; 5 
（apply-macro和[true 5 0]）; 0 
（let [a 0]（apply-macro和[true a]））; 0 
  

但是，当我向它传递一个指向一个向量的变量时， p>

 （let [a [true]]（apply-macro和a））; java.lang.IllegalArgumentException：
;不知道如何创建ISeq从：clojure.lang.Symbol 
  

什么失望！

任何想法如何解决 apply-macro ？

a 在编译时只是一个符号。因此，编译时宏无法看到它包含什么并做必要的扩展。因此，您需要使用 eval 在运行时展开宏。

$ b $一种方法是将宏包装在调用eval的函数中，这可以用这个方便的functionize宏来完成：

 （defmacro functionize [macro] 
`（fn [& args＃]（eval（cons'〜macro args＃））））
 
（let [a [true]]（apply（functionize和）a））
 => true 
  

如果你愿意，你也可以在functionize方面定义apply-macro：

 （defmacro apply-macro [macro args] 
`（apply（functionize〜macro）〜args））
 
（let [a [true false]]（apply-macro和a））
 => false 
  

说完这些，我仍然认为最好的做法是避免宏完全并不是真正需要的：它们增加了额外的复杂性，最适合于你真正需要编译时代码生成的情况。在这种情况下，您不会： Alex Taggart的回答给出了一个很好的示例，说明如何在没有任何宏的情况下实现类似的目标，这在大多数情况下可能更合适。

In clojure, apply cannot be applied to a macro. For instance (apply and [true false]) raises an exception. I was thinking about following workaround:

(defmacro apply-macro[func args] `(~func ~@args))

At first glance, it seemed to work pretty well:

(apply-macro and [true 5]); 5
(apply-macro and [true 5 0]); 0
(let [a 0] (apply-macro and [true a])); 0

But, when I passed to it a variable that points to a vector, it collapsed.

(let [a [true]] (apply-macro and a));  java.lang.IllegalArgumentException:
   ;Don't know how to create ISeq from: clojure.lang.Symbol

What a disappointment!!!!

Any idea how to fix apply-macro?

解决方案

The problem is that a is just a symbol at compile time. So there's no way for a compile-time macro to see what it contains and do the necessary expansion. As a result, you need to expand the macro at run-time using eval.

One way to do this is just to wrap the macro in a function that calls eval, which can be done with this handy "functionize" macro:

(defmacro functionize [macro]
  `(fn [& args#] (eval (cons '~macro args#))))

(let [a [true]] (apply (functionize and) a))
=> true

If you like, you could also define apply-macro in terms of functionize:

(defmacro apply-macro [macro args]
   `(apply (functionize ~macro) ~args))

(let [a [true false]] (apply-macro and a))
=> false

Having said all this, I still think the best thing to do is to avoid macros entirely when they are not really needed: they add extra complexity and are best reserved for cases when you really need compile time code generation. In this case you don't: Alex Taggart's answer gives a good example of how to achieve a similar objective without any macros, which is probably more appropriate in most situations.

这篇关于在clojure，如何应用宏到列表？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！