在Common Lisp FORMAT中，递归格式如何工作 [英] In Common Lisp FORMAT how does recursive formatting work

问题描述

我需要生成一个带有可变字符串长度的带空格的字符串.不太聪明的解决方案涉及到format:

I need to generate a space-padded string with a variable string length. The not-so-clever solution that works involved nesting of format:

(format nil (format nil "~~~d,,a" 10) "asdf")

现在，我想通过使用format的~?进行递归处理使其更加智能.我希望这样的事情可以满足我的要求:

Now, I wanted to make it a bit more clever by using format's ~? for recursive processing. I would expect that something like this should do what I want:

(format nil "~@?" "~~~d,,a" 10 "asdf")

，但是我得到的只是格式字符串，即~10,,a，而不是填充的asdf字符串.也许我在这里误解了递归"一词，但是我希望CL形成了内部格式字符串后，应该继续实际使用它.我想念什么吗?

but what I get is just the formatting string, i.e. ~10,,a, not the padded asdf string. Perhaps I misunderstood the word 'recursive' here, but I would expect that having formed the inner format string, CL should proceed to actually use it. Am I missing something?

推荐答案

格式指令的可变参数

FORMAT允许您将v用作控制字符串中指令的参数，以从参数列表中弹出参数.

FORMAT allows you to use v as an argument to a directive in the control string to pop arguments from the argument list.

CL-USER> (format nil "~va" 10 "asdf")
"asdf      "

可能使用了多个v.

CL-USER> (format nil "~v,,,va" 10 #\- "asdf")
"asdf------"

使用~?

Recursive processing with ~?

递归处理指令旨在将不同的调用"嵌入到控制字符串中的FORMAT.例如，可能会使用提示输入是"或否"答案的功能.

The recursive processing directive is meant for embedding a different "call" to FORMAT inside a control string. For example, a function to prompt for a yes or no answer might be implemented with it.

(defun y-or-n-prompt (control-string &rest args)
  (format t "~&~? [y/n]: " control-string args)
  ;;...
  )

现在，呼叫者可以像使用FORMAT一样使用此格式设置提示，而不必担心该提示对用户的外观细节(在开头或[y/n]:提示处添加新行)最后).

The caller can now format a prompt with this as they would with FORMAT without having to worry about the details of what the prompt should look like to the user (adding a new line at the beginning or the [y/n]: prompt at the end).

CL-USER> (y-or-n-prompt "foo ~d bar" 12)
foo 12 bar [y/n]: 
NIL

~?的结果将不被FORMAT处理，因此不能用于构建控制字符串.通常，在运行时构建控制字符串不是一个好主意，因为它易于出错(例如，您必须转义字符串中所有不需要的波浪号)，并阻止实现在编译时处理控制字符串.

The result of ~? will not be processed by FORMAT, so it cannot be used to build a control string. In general, building control strings at run time is a bad idea, because it's error prone (for example, you must escape any unwanted tildes in the string) and prevents the implementation from processing the control string at compile time.

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