在Clojure中，使用递归实现宏时如何进行代码模板化 [英] In clojure, how to do code templating when implementing a macro using recursion

问题描述

我正在尝试实现一个宏，以将一个中缀列表递归转换为一个前缀列表.我遇到如下问题:

I am trying to implement a macro to recursively converting an infix list into a prefix one. I encounter a problem as follows:

;;this works
(defmacro recursive-infix [form]
  (list (second form) (first form)
        (if (not (seq? (nth form 2)))
          (nth form 2)
          (recursive-infix (nth form 2)))))

;;this doesn't work
(defmacro my-recursive-infix [form]
  `(~(second form) ~(first form)
        (if (not (seq? ~(nth form 2)))
          ~(nth form 2)
          (my-recursive-infix ~(nth form 2)))))

(macroexpand '(recursive-infix (10 + 10))) 
;;get (+ 10 10)

(macroexpand '(my-recursive-infix (10 + 10))) 
;;get (+ 10 (if (clojure.core/not (clojure.core/seq? 10)) 10 (user/my-recursive-infix 10)))

(recursive-infix (10 + 10))
;;get 20

(my-recursive-infix (10 + 10))
;;Don't know how to create ISeq from: java.lang.Integer [Thrown class java.lang.IllegalArgumentException]

问题出在哪里?如何使用代码模板正确定义宏?

Where is the problem? How to correctly define a macro with code templating?

P.S.我将代码更改为此并且可以正常工作，为什么?有什么区别?:

P.S. I changed the code into this and it works, why? what is the difference?:

(defmacro my-recursive-infix [form]
  (if (not (seq? (nth form 2)))
    `(~(second form) ~(first form) ~(nth form 2))
    `(~(second form) ~(first form) (my-recursive-infix (nth form 2)))))

推荐答案

在原始版本中，(if (not ...))检查是在编译时进行的.您已将其包含在扩展代码中.因此，这是一个最小的更改，可以使它按您想要的方式工作-与原始内容实际上是相同的，只是翻转"了引用的内容和没有的内容.

In the original version, the (if (not ...)) check was happening at compile-time; you've instead included it in the expanded code. So here's a minimal change that would get it to act like you want - it's effectively the same as the original, but "flipping" what's quoted and what's not.

 (defmacro my-recursive-infix [form]
  (let [third (nth form 2)]
    `(~(second form) ~(first form)
      ~(if (not (seq? third))
         third
         `(my-recursive-infix ~third)))))

但是，使用结构化提前而不是就地提取表单部分会更好一些:

However, it's a bit nicer to use destructuring to pull out the pieces of the form ahead of time, rather than in-place:

(defmacro my-recursive-infix [form]
  (let [[x op y] form]
    `(~op ~x ~(if (not (seq? y))
                y
                `(my-recursive-infix ~y)))))

更好的是，实际上是将非递归大小写移到外部，以便(a)适用于文字数字，并且(b)代码看起来更像是扩展为:

And better still, really, is to move the non-recursive case outside, so that (a) it works for literal numbers, and (b) the code looks more like what it expands to:

(defmacro my-recursive-infix [form]
  (if-not (seq? form)
    form
    (let [[x op y] form]
      `(~op ~x (my-recursive-infix ~y)))))

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