00000004.htm

一份很好的linux入门资料

previously&nbsp;typed&nbsp;forms.&nbsp;LISP&nbsp;always&nbsp;saves&nbsp;its&nbsp;most&nbsp;recent&nbsp;three&nbsp;<BR>
results;&nbsp;it&nbsp;stores&nbsp;them&nbsp;as&nbsp;the&nbsp;values&nbsp;of&nbsp;the&nbsp;symbols&nbsp;*,&nbsp;**,&nbsp;and&nbsp;***.&nbsp;<BR>
For&nbsp;example:&nbsp;<BR>
&nbsp;<BR>
＞<I>&nbsp;3&nbsp;</I><BR>
3&nbsp;<BR>
＞<I>&nbsp;4&nbsp;</I><BR>
4&nbsp;<BR>
＞<I>&nbsp;5&nbsp;</I><BR>
5&nbsp;<BR>
＞<I>&nbsp;***&nbsp;</I><BR>
3&nbsp;<BR>
＞<I>&nbsp;***&nbsp;</I><BR>
4&nbsp;<BR>
＞<I>&nbsp;***&nbsp;</I><BR>
5&nbsp;<BR>
＞<I>&nbsp;**&nbsp;</I><BR>
4&nbsp;<BR>
＞<I>&nbsp;*&nbsp;</I><BR>
4&nbsp;<BR>
&nbsp;<BR>
&nbsp;<BR>
&nbsp;<BR>
Special&nbsp;forms&nbsp;<BR>
&nbsp;<BR>
There&nbsp;are&nbsp;a&nbsp;number&nbsp;of&nbsp;special&nbsp;forms&nbsp;which&nbsp;look&nbsp;like&nbsp;function&nbsp;calls&nbsp;but&nbsp;<BR>
aren't.&nbsp;These&nbsp;include&nbsp;control&nbsp;constructs&nbsp;such&nbsp;as&nbsp;if&nbsp;statements&nbsp;and&nbsp;do&nbsp;<BR>
loops;&nbsp;assignments&nbsp;like&nbsp;setq,&nbsp;setf,&nbsp;push,&nbsp;and&nbsp;pop;&nbsp;definitions&nbsp;such&nbsp;as&nbsp;<BR>
defun&nbsp;and&nbsp;defstruct;&nbsp;and&nbsp;binding&nbsp;constructs&nbsp;such&nbsp;as&nbsp;let.&nbsp;(Not&nbsp;all&nbsp;of&nbsp;<BR>
these&nbsp;special&nbsp;forms&nbsp;have&nbsp;been&nbsp;mentioned&nbsp;yet.&nbsp;See&nbsp;below.)&nbsp;<BR>
&nbsp;<BR>
One&nbsp;useful&nbsp;special&nbsp;form&nbsp;is&nbsp;the&nbsp;quote&nbsp;form:&nbsp;quote&nbsp;prevents&nbsp;its&nbsp;argument&nbsp;<BR>
from&nbsp;being&nbsp;evaluated.&nbsp;For&nbsp;example:&nbsp;<BR>
&nbsp;<BR>
＞<I>&nbsp;(setq&nbsp;a&nbsp;3)&nbsp;</I><BR>
3&nbsp;<BR>
＞<I>&nbsp;a&nbsp;</I><BR>
3&nbsp;<BR>
＞<I>&nbsp;(quote&nbsp;a)&nbsp;</I><BR>
A&nbsp;<BR>
＞<I>&nbsp;'a&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;;'a&nbsp;is&nbsp;an&nbsp;abbreviation&nbsp;for&nbsp;(quote&nbsp;a)&nbsp;</I><BR>
A&nbsp;<BR>
&nbsp;<BR>
Another&nbsp;similar&nbsp;special&nbsp;form&nbsp;is&nbsp;the&nbsp;function&nbsp;form:&nbsp;function&nbsp;causes&nbsp;its&nbsp;<BR>
argument&nbsp;to&nbsp;be&nbsp;interpreted&nbsp;as&nbsp;a&nbsp;function&nbsp;rather&nbsp;than&nbsp;being&nbsp;evaluated.&nbsp;<BR>
For&nbsp;example:&nbsp;<BR>
&nbsp;<BR>
＞<I>&nbsp;(setq&nbsp;+&nbsp;3)&nbsp;</I><BR>
3&nbsp;<BR>
＞<I>&nbsp;+&nbsp;</I><BR>
3&nbsp;<BR>
＞<I>&nbsp;'+&nbsp;</I><BR>
+&nbsp;<BR>
＞<I>&nbsp;(function&nbsp;+)&nbsp;</I><BR>
#&lt;Function&nbsp;+&nbsp;@&nbsp;#x-fbef9de&gt;&nbsp;<BR>
＞<I>&nbsp;#'+&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;;#'+&nbsp;is&nbsp;an&nbsp;abbreviation&nbsp;for&nbsp;(function&nbsp;+)&nbsp;</I><BR>
#&lt;Function&nbsp;+&nbsp;@&nbsp;#x-fbef9de&gt;&nbsp;<BR>
&nbsp;<BR>
The&nbsp;function&nbsp;special&nbsp;form&nbsp;is&nbsp;useful&nbsp;when&nbsp;you&nbsp;want&nbsp;to&nbsp;pass&nbsp;a&nbsp;function&nbsp;as&nbsp;<BR>
an&nbsp;argument&nbsp;to&nbsp;another&nbsp;function.&nbsp;See&nbsp;below&nbsp;for&nbsp;some&nbsp;examples&nbsp;of&nbsp;<BR>
functions&nbsp;which&nbsp;take&nbsp;functions&nbsp;as&nbsp;arguments.&nbsp;<BR>
&nbsp;<BR>
&nbsp;<BR>
&nbsp;<BR>
Binding&nbsp;<BR>
&nbsp;<BR>
Binding&nbsp;is&nbsp;lexically&nbsp;scoped&nbsp;assignment.&nbsp;It&nbsp;happens&nbsp;to&nbsp;the&nbsp;variables&nbsp;in&nbsp;<BR>
a&nbsp;function's&nbsp;parameter&nbsp;list&nbsp;whenever&nbsp;the&nbsp;function&nbsp;is&nbsp;called:&nbsp;the&nbsp;formal&nbsp;<BR>
parameters&nbsp;are&nbsp;bound&nbsp;to&nbsp;the&nbsp;actual&nbsp;parameters&nbsp;for&nbsp;the&nbsp;duration&nbsp;of&nbsp;the&nbsp;<BR>
function&nbsp;call.&nbsp;You&nbsp;can&nbsp;bind&nbsp;variables&nbsp;anywhere&nbsp;in&nbsp;a&nbsp;program&nbsp;with&nbsp;the&nbsp;<BR>
let&nbsp;special&nbsp;form,&nbsp;which&nbsp;looks&nbsp;like&nbsp;this:&nbsp;<BR>
&nbsp;<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;(let&nbsp;((var1&nbsp;val1)&nbsp;<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;(var2&nbsp;val2)&nbsp;<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;...&nbsp;<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;)&nbsp;<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;body&nbsp;<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;)&nbsp;<BR>
&nbsp;<BR>
Let&nbsp;binds&nbsp;var1&nbsp;to&nbsp;val1,&nbsp;var2&nbsp;to&nbsp;val2,&nbsp;and&nbsp;so&nbsp;forth;&nbsp;then&nbsp;it&nbsp;executes&nbsp;<BR>
the&nbsp;statements&nbsp;in&nbsp;its&nbsp;body.&nbsp;The&nbsp;body&nbsp;of&nbsp;a&nbsp;let&nbsp;follows&nbsp;exactly&nbsp;the&nbsp;same&nbsp;<BR>
rules&nbsp;that&nbsp;a&nbsp;function&nbsp;body&nbsp;does.&nbsp;Some&nbsp;examples:&nbsp;<BR>
&nbsp;<BR>
＞<I>&nbsp;(let&nbsp;((a&nbsp;3))&nbsp;(+&nbsp;a&nbsp;1))&nbsp;</I><BR>
4&nbsp;<BR>
＞<I>&nbsp;(let&nbsp;((a&nbsp;2)&nbsp;</I><BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;(b&nbsp;3)&nbsp;<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;(c&nbsp;0))&nbsp;<BR>
&nbsp;&nbsp;&nbsp;&nbsp;(setq&nbsp;c&nbsp;(+&nbsp;a&nbsp;b))&nbsp;<BR>
&nbsp;&nbsp;&nbsp;&nbsp;c&nbsp;<BR>
&nbsp;&nbsp;)&nbsp;<BR>
5&nbsp;<BR>
＞<I>&nbsp;(setq&nbsp;c&nbsp;4)&nbsp;</I><BR>
4&nbsp;<BR>
＞<I>&nbsp;(let&nbsp;((c&nbsp;5))&nbsp;c)&nbsp;</I><BR>
5&nbsp;<BR>
＞<I>&nbsp;c&nbsp;</I><BR>
4&nbsp;<BR>
&nbsp;<BR>
Instead&nbsp;of&nbsp;(let&nbsp;((a&nbsp;nil)&nbsp;(b&nbsp;nil))&nbsp;...),&nbsp;you&nbsp;can&nbsp;write&nbsp;(let&nbsp;(a&nbsp;b)&nbsp;...).&nbsp;<BR>
&nbsp;<BR>
The&nbsp;val1,&nbsp;val2,&nbsp;etc.&nbsp;inside&nbsp;a&nbsp;let&nbsp;cannot&nbsp;reference&nbsp;the&nbsp;variables&nbsp;var1,&nbsp;<BR>
var2,&nbsp;etc.&nbsp;that&nbsp;the&nbsp;let&nbsp;is&nbsp;binding.&nbsp;For&nbsp;example,&nbsp;<BR>
&nbsp;<BR>
＞<I>&nbsp;(let&nbsp;((x&nbsp;1)&nbsp;</I><BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;(y&nbsp;(+&nbsp;x&nbsp;1)))&nbsp;<BR>
&nbsp;&nbsp;&nbsp;&nbsp;y&nbsp;<BR>
&nbsp;&nbsp;)&nbsp;<BR>
Error:&nbsp;Attempt&nbsp;to&nbsp;take&nbsp;the&nbsp;value&nbsp;of&nbsp;the&nbsp;unbound&nbsp;symbol&nbsp;X&nbsp;<BR>
&nbsp;<BR>
If&nbsp;the&nbsp;symbol&nbsp;x&nbsp;already&nbsp;has&nbsp;a&nbsp;global&nbsp;value,&nbsp;stranger&nbsp;happenings&nbsp;will&nbsp;<BR>
result:&nbsp;<BR>
&nbsp;<BR>
＞<I>&nbsp;(setq&nbsp;x&nbsp;7)&nbsp;</I><BR>
7&nbsp;<BR>
＞<I>&nbsp;(let&nbsp;((x&nbsp;1)&nbsp;</I><BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;(y&nbsp;(+&nbsp;x&nbsp;1)))&nbsp;<BR>
&nbsp;&nbsp;&nbsp;&nbsp;y&nbsp;<BR>
&nbsp;&nbsp;)&nbsp;<BR>
8&nbsp;<BR>
&nbsp;<BR>
The&nbsp;let*&nbsp;special&nbsp;form&nbsp;is&nbsp;just&nbsp;like&nbsp;let&nbsp;except&nbsp;that&nbsp;it&nbsp;allows&nbsp;values&nbsp;to&nbsp;<BR>
reference&nbsp;variables&nbsp;defined&nbsp;earlier&nbsp;in&nbsp;the&nbsp;let*.&nbsp;For&nbsp;example,&nbsp;<BR>
&nbsp;<BR>
＞<I>&nbsp;(setq&nbsp;x&nbsp;7)&nbsp;</I><BR>
7&nbsp;<BR>
＞<I>&nbsp;(let*&nbsp;((x&nbsp;1)&nbsp;</I><BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;(y&nbsp;(+&nbsp;x&nbsp;1)))&nbsp;<BR>
&nbsp;&nbsp;&nbsp;&nbsp;y&nbsp;<BR>
&nbsp;&nbsp;)&nbsp;<BR>
2&nbsp;<BR>
&nbsp;<BR>
The&nbsp;form&nbsp;<BR>
&nbsp;<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;(let*&nbsp;((x&nbsp;a)&nbsp;<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;(y&nbsp;b))&nbsp;<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;...&nbsp;<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;)&nbsp;<BR>
&nbsp;<BR>
is&nbsp;equivalent&nbsp;to&nbsp;<BR>
&nbsp;<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;(let&nbsp;((x&nbsp;a))&nbsp;<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;(let&nbsp;((y&nbsp;b))&nbsp;<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;...&nbsp;<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;)&nbsp;)&nbsp;<BR>
&nbsp;<BR>
&nbsp;<BR>
&nbsp;<BR>
Dynamic&nbsp;Scoping&nbsp;<BR>
&nbsp;<BR>
The&nbsp;let&nbsp;and&nbsp;let*&nbsp;forms&nbsp;provide&nbsp;lexical&nbsp;scoping,&nbsp;which&nbsp;is&nbsp;what&nbsp;you&nbsp;<BR>
expect&nbsp;if&nbsp;you're&nbsp;used&nbsp;to&nbsp;programming&nbsp;in&nbsp;C&nbsp;or&nbsp;Pascal.&nbsp;Dynamic&nbsp;scoping&nbsp;is&nbsp;<BR>
what&nbsp;you&nbsp;get&nbsp;in&nbsp;BASIC:&nbsp;if&nbsp;you&nbsp;assign&nbsp;a&nbsp;value&nbsp;to&nbsp;a&nbsp;dynamically&nbsp;scoped&nbsp;<BR>
variable,&nbsp;every&nbsp;mention&nbsp;of&nbsp;that&nbsp;variable&nbsp;returns&nbsp;that&nbsp;value&nbsp;until&nbsp;you&nbsp;<BR>
assign&nbsp;another&nbsp;value&nbsp;to&nbsp;the&nbsp;same&nbsp;variable.&nbsp;<BR>
&nbsp;<BR>
In&nbsp;LISP,&nbsp;dynamically&nbsp;scoped&nbsp;variables&nbsp;are&nbsp;called&nbsp;special&nbsp;variables.&nbsp;You&nbsp;<BR>
can&nbsp;declare&nbsp;a&nbsp;special&nbsp;variable&nbsp;with&nbsp;the&nbsp;defvar&nbsp;special&nbsp;form.&nbsp;Here&nbsp;are&nbsp;<BR>
some&nbsp;examples&nbsp;of&nbsp;lexically&nbsp;and&nbsp;dynamically&nbsp;scoped&nbsp;variables.&nbsp;<BR>
&nbsp;<BR>
In&nbsp;this&nbsp;example,&nbsp;the&nbsp;function&nbsp;check-regular&nbsp;references&nbsp;a&nbsp;regular&nbsp;(ie,&nbsp;<BR>
lexically&nbsp;scoped)&nbsp;variable.&nbsp;Since&nbsp;check-regular&nbsp;is&nbsp;lexically&nbsp;outside&nbsp;of&nbsp;<BR>
the&nbsp;let&nbsp;which&nbsp;binds&nbsp;regular,&nbsp;check-regular&nbsp;returns&nbsp;the&nbsp;variable's&nbsp;<BR>
global&nbsp;value.&nbsp;<BR>
&nbsp;<BR>
＞<I>&nbsp;(setq&nbsp;regular&nbsp;5)&nbsp;</I><BR>
5&nbsp;<BR>
＞<I>&nbsp;(defun&nbsp;check-regular&nbsp;()&nbsp;regular)&nbsp;</I><BR>
CHECK-REGULAR&nbsp;<BR>
＞<I>&nbsp;(check-regular)&nbsp;</I><BR>
5&nbsp;<BR>
＞<I>&nbsp;(let&nbsp;((regular&nbsp;6))&nbsp;(check-regular))&nbsp;</I><BR>
5&nbsp;<BR>
&nbsp;<BR>
In&nbsp;this&nbsp;example,&nbsp;the&nbsp;function&nbsp;check-special&nbsp;references&nbsp;a&nbsp;special&nbsp;(ie,&nbsp;<BR>
dynamically&nbsp;scoped)&nbsp;variable.&nbsp;Since&nbsp;the&nbsp;call&nbsp;to&nbsp;check-special&nbsp;is&nbsp;<BR>
temporally&nbsp;inside&nbsp;of&nbsp;the&nbsp;let&nbsp;which&nbsp;binds&nbsp;special,&nbsp;check-special&nbsp;returns&nbsp;<BR>
the&nbsp;variable's&nbsp;local&nbsp;value.&nbsp;<BR>
&nbsp;<BR>
＞<I>&nbsp;(defvar&nbsp;*special*&nbsp;5)&nbsp;</I><BR>
*SPECIAL*&nbsp;<BR>
＞<I>&nbsp;(defun&nbsp;check-special&nbsp;()&nbsp;*special*)&nbsp;</I><BR>
CHECK-SPECIAL&nbsp;<BR>
＞<I>&nbsp;(check-special)&nbsp;</I><BR>
5&nbsp;<BR>
＞<I>&nbsp;(let&nbsp;((*special*&nbsp;6))&nbsp;(check-special))&nbsp;</I><BR>
6&nbsp;<BR>
&nbsp;<BR>
By&nbsp;convention,&nbsp;the&nbsp;name&nbsp;of&nbsp;a&nbsp;special&nbsp;variable&nbsp;begins&nbsp;and&nbsp;ends&nbsp;with&nbsp;a&nbsp;*.&nbsp;<BR>
Special&nbsp;variables&nbsp;are&nbsp;chiefly&nbsp;used&nbsp;as&nbsp;global&nbsp;variables,&nbsp;since&nbsp;<BR>
programmers&nbsp;usually&nbsp;expect&nbsp;lexical&nbsp;scoping&nbsp;for&nbsp;local&nbsp;variables&nbsp;and&nbsp;<BR>
dynamic&nbsp;scoping&nbsp;for&nbsp;global&nbsp;variables.&nbsp;<BR>
&nbsp;<BR>
For&nbsp;more&nbsp;information&nbsp;on&nbsp;the&nbsp;difference&nbsp;between&nbsp;lexical&nbsp;and&nbsp;dynamic&nbsp;<BR>
scoping,&nbsp;see&nbsp;_Common&nbsp;LISP:&nbsp;the&nbsp;Language_.&nbsp;<BR>
&nbsp;<BR>
&nbsp;<BR>
&nbsp;<BR>
Arrays&nbsp;<BR>
&nbsp;<BR>
The&nbsp;function&nbsp;make-array&nbsp;makes&nbsp;an&nbsp;array.&nbsp;The&nbsp;aref&nbsp;function&nbsp;accesses&nbsp;its&nbsp;<BR>
elements.&nbsp;All&nbsp;elements&nbsp;of&nbsp;an&nbsp;array&nbsp;are&nbsp;initially&nbsp;set&nbsp;to&nbsp;nil.&nbsp;For&nbsp;<BR>
example:&nbsp;<BR>
&nbsp;<BR>
＞<I>&nbsp;(make-array&nbsp;'(3&nbsp;3))&nbsp;</I><BR>
#2a((NIL&nbsp;NIL&nbsp;NIL)&nbsp;(NIL&nbsp;NIL&nbsp;NIL)&nbsp;(NIL&nbsp;NIL&nbsp;NIL))&nbsp;<BR>