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lisp,赋值函数的差异

common-lisp lisp

johnc · 技术社区 · 14 年前

我对lisp还不太熟悉,所以很抱歉问这么简单的问题,

虽然我理解DEFVAR和DEFPARAMETER之间的区别(DEFVAR只设置未定义的变量),LET只用于局部作用域,但是SETF相对于前面提到的其他赋值函数的用途是什么?

2 回复 | 直到 9 年前

Rainer Joswig mmmmmm 14 年前

SETF和SETQ设置变量(全局或局部、特殊或词法),但不定义它们。

编辑:

Paul说在CLISP SETF中定义了变量。它不是这样的。我们来看看:

我们有以下计划:

(defun foo (a)
  (setf baz (* a a))
  a)

(defun bar (a)
  (setf baz (* a a))
  a)

[1]> (compile-file "/tmp/test.lisp")
;; Compiling file /tmp/test.lisp ...
WARNING in FOO in lines 2..4 :
BAZ is neither declared nor bound,
it will be treated as if it were declared SPECIAL.
WARNING in BAR in lines 6..8 :
BAZ is neither declared nor bound,
it will be treated as if it were declared SPECIAL.
;; Wrote file /tmp/test.fas
0 errors, 2 warnings

在这两个函数中,CLISP都警告我们变量BAZ既没有声明也没有绑定。CLISP没有拒绝代码,而是将变量BAZ视为特殊变量。

[1]> (load "/tmp/test")
;; Loading file /tmp/test.fas ...
;; Loaded file /tmp/test.fas
T
[2]> (foo 2)
2
[3]> (bar 3)
3
[4]> baz
9
[5]> (compile-file "/tmp/test.lisp")
;; Compiling file /tmp/test.lisp ...
WARNING in FOO in lines 2..4 :
BAZ is neither declared nor bound,
it will be treated as if it were declared SPECIAL.
WARNING in BAR in lines 6..8 :
BAZ is neither declared nor bound,
it will be treated as if it were declared SPECIAL.

不,即使在执行SETF语句之后,CLISP也认为变量BAZ没有被声明。

; in: DEFUN BAR
;     (SETF BAZ (* A A))
; ==>
;   (SETQ BAZ (* A A))
;
; caught WARNING:
;   undefined variable: BAZ

; in: DEFUN FOO
;     (SETF BAZ (* A A))
; ==>
;   (SETQ BAZ (* A A))
;
; caught WARNING:
;   undefined variable: BAZ

LispWorks编译器有这样一句话:

;;;*** Warning in FOO: BAZ assumed special in SETQ
; FOO
;;;*** Warning in BAR: BAZ assumed special in SETQ
; BAR

Doug Currie 14 年前

SETF 影响现有的绑定或“位置”。您可以使用它来更改用其创建的任何对象的值 LET 或 DEF...

在Common Lisp中, setf 是“影响宏的广义位置”。参见 CLHS 5.1.1 - Overview of Places and Generalized Reference