使用#a.k.a.阅读宏 [英] Use of # a.k.a. read-macro

问题描述

阅读道格·霍伊特(Doug Hoyte)的著作《让Lambda越过》，我发现了以下对#.符号的描述，也就是阅读宏:

Reading book "Let Over Lambda" by Doug Hoyte, I found the following description of #. sign, a.k.a. read-macro:

#随COMMON LISP内置的基本读取宏是#.读取时间eval宏.通过此读取宏，您可以将对象嵌入无法序列化但可以用少量lisp代码创建的读取形式.

A basic read macro that comes built in with COMMON LISP is the #. read-time eval macro. This read macro lets you embed objects into the forms you read that can't be serialised, but can be created with a bit of lisp code.

从第4章开始，可以在这里找到本书的大部分内容: http://letoverlambda.com/index.cl/toc

It's from Chapter 4, most part of the book can be found here: http://letoverlambda.com/index.cl/toc

这是本书中的示例，该示例显示了每次相同的表达式可能如何被不同地读取:

This is example from the book that shows how the same expression may be read differently every time:

* '(football-game
     (game-started-at
       #.(get-internal-real-time))
     (coin-flip
       #.(if (zerop (random 2)) 'heads 'tails)))

(FOOTBALL-GAME
  (GAME-STARTED-AT 187)
  (COIN-FLIP HEADS))

* '(football-game
     (game-started-at
       #.(get-internal-real-time))
     (coin-flip
   #.(if (zerop (random 2)) 'heads 'tails)))

(FOOTBALL-GAME
  (GAME-STARTED-AT 309)
  (COIN-FLIP TAILS))

接下来，作者演示了一些铁杆花样，并使用#宏创建了变体.

Next, author demonstrates some hardcore tricks, creating variations with # macro.

因此，事实证明#'也是某种读取宏，它通常在表示函数名称的符号之前使用.但是有必要，他在那里的工作到底是什么?

So, it turns out that #' is also some kind of reading macro, and it's usually used before symbols that represent names of functions. But is it necessary, and what's exactly his job there?

我可以使用#'或不使用#'来放置高阶函数的符号:

I can put symbol for higher-order functions with #' or without it:

CL-USER> (defun test nil t)
TEST
CL-USER> (funcall #'test)
T
CL-USER> (funcall 'test)
T

取得同样的成功.

推荐答案

您可以通过两种方式调用函数的全局定义:

You can call global definitions of functions both ways:

CL-USER> (defun test nil t)
TEST
CL-USER> (funcall #'test)
T
CL-USER> (funcall 'test)
T

但是看到这个:

CL-USER 10 > (defun foo () 42)
FOO

CL-USER 11 > (flet ((foo () 82))
               (print (foo))
               (print (funcall 'foo))
               (print (funcall #'foo)))

82   ; the local definition
42   ; the symbol references the global definition
82   ; (function foo) / #'foo  references the local definition

(funcall 'foo)从符号中查找功能.

(funcall #'foo)从词法环境中调用该函数.如果没有，则使用全局定义.

(funcall #'foo) calls the function from the lexical environment. If there is none the global definition is used.

#'foo是(function foo)的简写形式.

CL-USER 19 > '#'foo
(FUNCTION FOO)

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