lisp里面，eval-when 到底怎么用？

我是害虫

谢谢，看了一些英文资料，还是搞不太清楚

miniqq

eval-when一般用来处理源文件,这个事关 side-effect的学问.
常用有三种方式:load-toplevel    :compile-toplevel   :execute

load 文件时里面所有的form都会被顺序执行
compile时相当于 load fasl 文件(机器码) (emacs里有 eval-when-compile)
execute / eval 直接一条条执行
如下:
(eval-when (compile) (some forms...))
(eval-when (load) (some forms...))
(eval-when (eval) (some forms...)

我是害虫

Here are some additional examples.

(let ((x 1))
  (eval-when (:execute :load-toplevel :compile-toplevel)
    (setf (symbol-function 'foo1) #'(lambda () x))))

The eval-when in the preceding expression is not at top level, so only the :execute keyword is considered. At compile time, this has no effect. At load time (if the let is at top level), or at execution time (if the let is embedded in some other form which does not execute until later), this sets (symbol-function 'foo1) to a function that returns 1.

(eval-when (:execute :load-toplevel :compile-toplevel)
  (let ((x 2))
    (eval-when (:execute :load-toplevel :compile-toplevel)
      (setf (symbol-function 'foo2) #'(lambda () x)))))

If the preceding expression occurs at the top level of a file to be compiled, it has both a compile time and a load-time effect of setting (symbol-function 'foo2) to a function that returns 2.

(eval-when (:execute :load-toplevel :compile-toplevel)
  (setf (symbol-function 'foo3) #'(lambda () 3)))

If the preceding expression occurs at the top level of a file to be compiled, it has both a compile time and a load-time effect of setting the function cell of foo3 to a function that returns 3.

(eval-when (:compile-toplevel)
  (eval-when (:compile-toplevel)  
    (print 'foo4)))

The preceding expression always does nothing; it simply returns nil.

(eval-when (:compile-toplevel)  
  (eval-when (:execute)
    (print 'foo5)))

If the preceding form occurs at the top level of a file to be compiled, foo5 is printed at compile time. If this form occurs in a non-top-level position, nothing is printed at compile time. Regardless of context, nothing is ever printed at load time or execution time.

(eval-when (:execute :load-toplevel)
  (eval-when (:compile-toplevel)
    (print 'foo6)))

If the preceding form occurs at the top level of a file to be compiled, foo6 is printed at compile time. If this form occurs in a non-top-level position, nothing is printed at compile time. Regardless of context, nothing is ever printed at load time or execution time.

这是几个例子，特别是eval-when的嵌套，看得有点晕，请您解释一下
谢谢

miniqq

注释不都很明显嘛,楼主别钻牛角尖哦.
看看eval-when的用法,
(eval-when (way)
  *body*)
way包括: :execute :load-toplevel :compile-toplevel
如果eval-when这个form在toplevel就执行load或者compile.类似:
(if (load-toplevel) load) 对*body*进行load
(if (compile-toplevel) compile) 对*body*进行 compile
(else (execute) eval) 直接执行*body*
这三种情况.

我觉得你好像是不理解 eval, load, compile都做些什么!
不是你不理解eval-when.

忘记了说eval-when的(way)列表可以包括:compile-toplevel, :load-toplevel, :execute, compile, load, or eval.  几个符号

不知道对你有没有帮助.

xdshting

1，这是cltl中的一个例子
(eval-when (eval compile load)
  (defmacro foo (x) `(car ,x))
  (print (foo '(a b c))))     
你能解释一下，这个form的运行过程吗，比如（compile-file。。。）时做什么，（load ***.fas）时做什么

2，下面是我的结果，clisp2.48，特别是对于（load  "mm.fas"）输出了两个A，是因为print的关系，但是为什么complie的时候只输出一个A，这和compile有关系？很不解，（load  "mm.lisp"）也是输出两个A

3，
(defmacro foo (x) `(car ,x))

(eval-when (:execute :compile-toplevel :load-toplevel)
  (print (foo '(a b c))))   
此定义与上面的定义功能上有什么区别？
谢谢

miniqq

http://www.cs.cmu.edu/Groups/AI/ ... iss147-writeup.html
好没耐心打字哦.
eval-when总会返回最后一个form的执行结果.

a.lisp
  (defmacro foo (x) `(car ,x))
  (print (foo '(a b c)))

你对比下(compile-file 'a.lisp) 与 你的(compile-file 'mm.lisp)

top-level: 当代码在顶层时才...
假如:
(defun test ()
(eval-when ....)
这样,compile后你是调用不到foo的.因为 test系顶层.

win_hate

两个 a，其中一个是 print 表达式的值，一般这个值在交互时才会显示。我的 clisp 是 2.44.1 的，load, compile-file 时都显示一个 a. 不过这个只涉及 clisp 的设计，跟语言关系不大。

miniqq

楼上,要用 :verbose :print 才看到两个 a
否则只看到返回值.

win_hate

这个是代码，我把它叫 1.lisp

1. 
   
2. (eval-when (eval compile load)
   
3.   (defmacro foo (x) `(car ,x))
   
4.   (print (foo '(a b c))))
   

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我这里无论用什么参数，总是一个 a

1. 
   
2. [1]> (compile-file "1.lisp")
   
3. ;; Compiling file /home/zxl/1.lisp ...
   
4. A
   
5. ;; Wrote file /home/zxl/1.fas
   
6. 0 errors, 0 warnings
   
7. #P"/home/zxl/1.fas" ;
   
8. NIL ;
   
9. NIL
   

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1. 
   
2. [2]> (compile-file "1.lisp" :verbose t :print t)
   
3. ;; Compiling file /home/zxl/1.lisp ...
   
4. ; (EVAL-WHEN (EVAL COMPILE LOAD) (DEFMACRO FOO # ...) ...)
   
5. A
   
6. ;; Wrote file /home/zxl/1.fas
   
7. 0 errors, 0 warnings
   
8. #P"/home/zxl/1.fas" ;
   
9. NIL ;
   
10. NIL
    

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1. 
   
2. [3]> (compile-file "1.lisp" :verbose nil :print nil)
   
3. 
   
4. A
   
5. #P"/home/zxl/1.fas" ;
   
6. NIL ;
   
7. NIL
   

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miniqq

两个 a，其中一个是 print 表达式的值，一般这个值在交互时才会显示。我的 clisp 是 2.44.1 的，load, comp ...
win_hate 发表于 2010-07-17 12:00

    我是指 load时 会打印两个 a, compile-file当然是一个.