Emacs Lisp：リスト項目を重複して挿入するのを避けるには？

Question

文字列が既にEmacs Lispのリストに入っているかどうかを確認するにはどうすればよいですか？特定のパス文字列が既にexec-pathにあるかどうかを確認し、そうでなければそのリストに追加する必要があります。ありがとう！

Answer 1

機能追加・ツー・リスト（4 1 2 3）

章Fから

に等しいaもたらす

(setq a '(1 2 3)) 
(add-to-list 'a 4) 
(add-to-list 'a 3) 

を添加する前に自動的にチェックする追加ツーリスト：

 
add-to-list is a compiled Lisp function in `subr.el'. 
(add-to-list list-var element &optional append compare-fn) 

Add element to the value of list-var if it isn't there yet. 
The test for presence of element is done with `equal', 
or with compare-fn if that's non-nil. 
If element is added, it is added at the beginning of the list, 
unless the optional argument append is non-nil, in which case 
element is added at the end. 

The return value is the new value of list-var. 

If you want to use `add-to-list' on a variable that is not defined 
until a certain package is loaded, you should put the call to `add-to-list' 
into a hook function that will be run only after loading the package. 
`eval-after-load' provides one way to do this. In some cases 
other hooks, such as major mode hooks, can do the job.