How do I check if a string is already in a list in Emacs Lisp? I need to check if a certain path string is already in exec-path, and then add it to that list if it's not.
The function add-to-list will check automatically before adding
(setq a '(1 2 3))
(add-to-list 'a 4)
(add-to-list 'a 3)
will result in a equal to (4 1 2 3)
From Emacs 26 C-h f add-to-list:
(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. This is handy to add some elements to configuration variables, but please do not abuse it in Elisp code, where you are usually better off using ‘push’ or ‘cl-pushnew’. 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.