blocks|key|1556715|text|setq就像带引号的set+--+(set+'foo+'(bar+baz))就像(setq+foo+'(bar+baz))。另一方面，setf确实很微妙--它就像一个“间接的”。我建议http://www.n-a-n-o.com/lisp/cmucl-tutorials/LISP-tutorial-16.html作为一种更好的方式来开始理解它，而不是这里的任何答案。简而言之，setf将第一个参数作为“引用”，这样(aref+myarray+3)就可以(作为setf的第一个arg+)在数组中设置项。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|1556716|entityMap|0|LINK|mutability|MUTABLE|url|http://www.n-a-n-o.com/lisp/cmucl-tutorials/LISP-tutorial-16.html^0|0|4|A|3|H|L|14|L|1V|4|59|4|5S|G|6E|4|2K|1T|0|0^^$0|@$1|2|3|4|5|6|7|N|8|@$9|O|A|P|B|C]|$9|Q|A|R|B|C]|$9|S|A|T|B|C]|$9|U|A|V|B|C]|$9|W|A|X|B|C]|$9|Y|A|Z|B|C]|$9|10|A|11|B|C]|$9|12|A|13|B|C]]|D|@$9|14|A|15|1|16]]|E|$]]|$1|F|3|-4|5|6|7|17|8|@]|D|@]|E|$]]]|G|$H|$5|I|J|K|E|$L|M]]]]

<code>setq</code> is just like <code>set</code> with a quoted first arg -- <code>(set 'foo '(bar baz))</code> is just like <code>(setq foo '(bar baz))</code>. <code>setf</code>, on the other hand, is subtle indeed -- it's like an "indirection". I suggest <a href="http://www.n-a-n-o.com/lisp/cmucl-tutorials/LISP-tutorial-16.html" rel="noreferrer">http://www.n-a-n-o.com/lisp/cmucl-tutorials/LISP-tutorial-16.html</a> as a better way to get started understanding it than any answer here can give... in short, though, <code>setf</code> takes the first argument as a "reference", so that e.g. <code>(aref myarray 3)</code> will work (as the first arg to <code>setf</code>) to set an item inside an array.

blocks|key|1556762|text|最初，在Lisp中，没有词法变量--只有动态变量。没有SETQ或SETF，只有SET函数。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1556763|现在是这样写的：|1556764|(setf+(symbol-value+'*foo*)+42)|code-block|syntax|javascript|1556765|是这样写的：|1556766|(set+(quote+*foo*)+42)|1556767|最终缩写为SETQ+(SET+Quoted)：|1556768|(setq+*foo*+42)|1556769|然后出现了词法变量，并且也开始使用SETQ对它们进行赋值--因此它不再是SET的简单包装器。|1556770|后来，有人发明了SETF+(设置字段)，作为一种通用的赋值给数据结构的方法，以反映其他语言的l值：|1556771|x.car+:=+42;|1556772|将被写成|1556773|(setf+(car+x)+42)|1556774|为了对称性和通用性，SETF还提供了SETQ的功能。在这一点上，可以正确地说SETQ是一个低级原语，而SETF是一个高级操作。|1556775|然后发生了符号宏。为了使符号宏能够透明地工作，人们意识到，如果被赋值的“变量”真的是一个符号宏，那么SETQ必须像SETF一样工作：|1556776|(defvar+*hidden*+(cons+42+42))
(define-symbol-macro+foo+(car+*hidden*))

foo+=>+42

(setq+foo+13)

foo+=>+13

*hidden*+=>+(13+.+42)|1556777|因此，我们来到了今天:+SET和SETQ是旧方言的萎缩残余物，可能会从Common+Lisp的最终继任者那里启动。|1556778|entityMap^0|0|0|0|0|0|0|0|0|0|0|0|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|1A|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|1B|8|@]|9|@]|A|$]]|$1|D|3|E|5|F|7|1C|8|@]|9|@]|A|$G|H]]|$1|I|3|J|5|6|7|1D|8|@]|9|@]|A|$]]|$1|K|3|L|5|F|7|1E|8|@]|9|@]|A|$G|H]]|$1|M|3|N|5|6|7|1F|8|@]|9|@]|A|$]]|$1|O|3|P|5|F|7|1G|8|@]|9|@]|A|$G|H]]|$1|Q|3|R|5|6|7|1H|8|@]|9|@]|A|$]]|$1|S|3|T|5|6|7|1I|8|@]|9|@]|A|$]]|$1|U|3|V|5|F|7|1J|8|@]|9|@]|A|$G|H]]|$1|W|3|X|5|6|7|1K|8|@]|9|@]|A|$]]|$1|Y|3|Z|5|F|7|1L|8|@]|9|@]|A|$G|H]]|$1|10|3|11|5|6|7|1M|8|@]|9|@]|A|$]]|$1|12|3|13|5|6|7|1N|8|@]|9|@]|A|$]]|$1|14|3|15|5|F|7|1O|8|@]|9|@]|A|$G|H]]|$1|16|3|17|5|6|7|1P|8|@]|9|@]|A|$]]|$1|18|3|-4|5|6|7|1Q|8|@]|9|@]|A|$]]]|19|$]]

Originally, in Lisp, there were no lexical variables -- only dynamic ones. And
there was no SETQ or SETF, just the SET function.

What is now written as:

<pre><code>(setf (symbol-value '*foo*) 42)
</code></pre>

was written as:

<pre><code>(set (quote *foo*) 42)
</code></pre>

which was eventually abbreviavated to SETQ (SET Quoted):

<pre><code>(setq *foo* 42)
</code></pre>

Then lexical variables happened, and SETQ came to be used for assignment to them too -- so it was no longer a simple wrapper around SET.

Later, someone invented SETF (SET Field) as a generic way of assigning values to data structures, to mirror the l-values of other languages:

<pre><code>x.car := 42;
</code></pre>

would be written as

<pre><code>(setf (car x) 42)
</code></pre>

For symmetry and generality, SETF also provided the functionality of SETQ. At this point it would have been correct to say that SETQ was a Low-level primitive, and SETF a high-level operation.

Then symbol macros happened. So that symbol macros could work transparently, it was realized that SETQ would have to act like SETF if the "variable" being assigned to was really a symbol macro:

<pre><code>(defvar *hidden* (cons 42 42))
(define-symbol-macro foo (car *hidden*))

foo =&gt; 42

(setq foo 13)

foo =&gt; 13

*hidden* =&gt; (13 . 42)
</code></pre>

So we arrive in the present day: SET and SETQ are atrophied remains of older dialects, and will probably be booted from eventual successors of Common Lisp.

blocks|key|1341232|text|(set+ls+'(1+2+3+4))+=>+Error+-+ls+has+no+value

(set+'ls+'(1+2+3+4))+=>+OK

(setq+ls+'(1+2+3+4))+=>+OK+-+make+ls+to+(quote+ls)+and+then+have+the+usual+set

(setf+ls+'(1+2+3+4))+=>+OK+-+same+as+setq+so+far+BUT

(setf+(car+ls)+10)+=>+Makes+ls+'(10+2+3+4)+-+not+duplicated+by+setq/set|type|code-block|depth|inlineStyleRanges|entityRanges|data|syntax|javascript|1341233|unstyled|entityMap^0|0^^$0|@$1|2|3|4|5|6|7|G|8|@]|9|@]|A|$B|C]]|$1|D|3|-4|5|E|7|H|8|@]|9|@]|A|$]]]|F|$]]

<pre><code>(set ls '(1 2 3 4)) =&gt; Error - ls has no value

(set 'ls '(1 2 3 4)) =&gt; OK

(setq ls '(1 2 3 4)) =&gt; OK - make ls to (quote ls) and then have the usual set

(setf ls '(1 2 3 4)) =&gt; OK - same as setq so far BUT

(setf (car ls) 10) =&gt; Makes ls '(10 2 3 4) - not duplicated by setq/set
</code></pre>

blocks|key|1341294|text|我想补充前面的回答，setf是一个宏，它调用特定的函数，这取决于作为第一个参数传递的内容。使用不同类型的参数比较setf的宏展开结果：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1341295|(macroexpand+'(setf+a+1))

(macroexpand+'(setf+(car+(list+3+2+1))+1))

(macroexpand+'(setf+(aref+#(3+2+1)+0)+1))|code-block|syntax|javascript|1341296|对于某些类型的参数，将调用"setf函数“：|1341297|(defstruct+strct+field)
(macroexpand+'(setf+(strct-field+(make-strct))+1))|1341298|entityMap^0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|M|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|N|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|O|8|@]|9|@]|A|$]]|$1|I|3|J|5|D|7|P|8|@]|9|@]|A|$E|F]]|$1|K|3|-4|5|6|7|Q|8|@]|9|@]|A|$]]]|L|$]]

I would like to add to previous answers that setf is macro that call specific function depending on what was passed as its first argument.
Compare results of macro expansion of setf with different types of arguments:

<pre><code>(macroexpand '(setf a 1))

(macroexpand '(setf (car (list 3 2 1)) 1))

(macroexpand '(setf (aref #(3 2 1) 0) 1))
</code></pre>

For some types of arguments "setf function" will be called:

<pre><code>(defstruct strct field)
(macroexpand '(setf (strct-field (make-strct)) 1))
</code></pre>

blocks|key|1556918|text|您可以使用setf代替set或setq，但反之亦然，因为如果变量有单独的元素，setf还可以设置变量的单独元素的值。请看下面的示例：|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|1556919|所有四个示例都将列表(1，2，3)赋给名为foo的变量。|1556920|(set+(quote+foo)+(list+1+2+3))++++;foo+=>+(1+2+3)
(1+2+3)

(set+'foo+'(1+2+3))+++;foo+=>+(1+2+3)+same+function,+simpler+expression
(1+2+3)

(setq+foo+'(1+2+3))+++;foo+=>+(1+2+3)+similar+function,+different+syntax
(1+2+3)

(setf+foo+'(1+2+3))+++;foo+=>+(1+2+3)+more+capable+function
(1+2+3)|code-block|syntax|javascript|1556921|setf增加了将foo中的列表成员设置为新值的功能。|1556922|foo+++++++++++++++++++;foo+=>+(1+2+3)+as+defined+above
(1+2+3)

(car+foo)+++++++++++++;the+first+item+in+foo+is+1
1

(setf+(car+foo)+4)++++;set+or+setq+will+fail+since+(car+foo)+is+not+a+symbol
4

foo+++++++++++++++++++;the+fist+item+in+foo+was+set+to+4+by+setf
(4+2+3)|1556923|但是，您可以在foo中定义表示单个项目的符号宏|1556924|(define-symbol-macro+foo-car+(car+foo))++++;+assumes+FOO+=>+(1+2+3)
FOO-CAR

foo-car+++++++++++++++;foo-car+is+now+a+symbol+for+the+1st+item+in+foo
1

(setq+foo-car+4)++++++;set+or+setq+can+set+the+symbol+foo-car+
4

foo+++++++++++++++++++;Lisp+macros+are+so+cool
(4+2+3)|1556925|如果您尚未定义变量，并且不想在稍后的代码中为其赋值，则可以使用defvar。|1556926|(defvar+foo2)
(define-symbol-macro+foo-car+(car+foo2))|1556927|entityMap^0|5|4|B|3|F|4|13|4|0|0|0|0|4|8|3|0|0|7|3|0|0|V|6|0|0^^$0|@$1|2|3|4|5|6|7|10|8|@$9|11|A|12|B|C]|$9|13|A|14|B|C]|$9|15|A|16|B|C]|$9|17|A|18|B|C]]|D|@]|E|$]]|$1|F|3|G|5|6|7|19|8|@]|D|@]|E|$]]|$1|H|3|I|5|J|7|1A|8|@]|D|@]|E|$K|L]]|$1|M|3|N|5|6|7|1B|8|@$9|1C|A|1D|B|C]|$9|1E|A|1F|B|C]]|D|@]|E|$]]|$1|O|3|P|5|J|7|1G|8|@]|D|@]|E|$K|L]]|$1|Q|3|R|5|6|7|1H|8|@$9|1I|A|1J|B|C]]|D|@]|E|$]]|$1|S|3|T|5|J|7|1K|8|@]|D|@]|E|$K|L]]|$1|U|3|V|5|6|7|1L|8|@$9|1M|A|1N|B|C]]|D|@]|E|$]]|$1|W|3|X|5|J|7|1O|8|@]|D|@]|E|$K|L]]|$1|Y|3|-4|5|6|7|1P|8|@]|D|@]|E|$]]]|Z|$]]

You can use <code>setf</code> in place of <code>set</code> or <code>setq</code> but not vice versa since <code>setf</code> can also set the value of individual elements of a variable if the variable has individual elements. See the exaples below:

All four examples will assign the list (1, 2, 3) to the variable named foo. 

<pre><code>(set (quote foo) (list 1 2 3)) ;foo =&gt; (1 2 3)
(1 2 3)

(set 'foo '(1 2 3)) ;foo =&gt; (1 2 3) same function, simpler expression
(1 2 3)

(setq foo '(1 2 3)) ;foo =&gt; (1 2 3) similar function, different syntax
(1 2 3)

(setf foo '(1 2 3)) ;foo =&gt; (1 2 3) more capable function
(1 2 3)
</code></pre>

<code>setf</code> has the added capability of setting a member of the list in <code>foo</code> to a new value.

<pre><code>foo ;foo =&gt; (1 2 3) as defined above
(1 2 3)

(car foo) ;the first item in foo is 1
1

(setf (car foo) 4) ;set or setq will fail since (car foo) is not a symbol
4

foo ;the fist item in foo was set to 4 by setf
(4 2 3)
</code></pre>

However, you can define a symbol macro that reprents a single item within <code>foo</code>

<pre><code>(define-symbol-macro foo-car (car foo)) ; assumes FOO =&gt; (1 2 3)
FOO-CAR

foo-car ;foo-car is now a symbol for the 1st item in foo
1

(setq foo-car 4) ;set or setq can set the symbol foo-car 
4

foo ;Lisp macros are so cool
(4 2 3)
</code></pre>

You can use <code>defvar</code> if you have not already defined the variable and do not want to give it a value until later in your code.

<pre><code>(defvar foo2)
(define-symbol-macro foo-car (car foo2))
</code></pre>

What is the difference between "set", "setq", and "setf" in Common Lisp?

Difference between `set`, `setq`, and `setf` in Common Lisp?

数据结构

Common Lisp中的"set“、"setq”和"setf“有什么区别？

问Common Lisp中`set`、`setq`和`setf`的区别？
EN

社区

活动

资源

关于

腾讯云开发者

热门产品

热门推荐

更多推荐

问Common Lisp中`set`、`setq`和`setf`的区别？EN

社区

活动

资源

关于

腾讯云开发者

热门产品

热门推荐

更多推荐

问Common Lisp中`set`、`setq`和`setf`的区别？
EN