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Here is an example from kdic.el (KanjiDictionary). The variable
‘kdic-ask-for’ is one of meaning kanji, or reading. Calling kdic-after will return the next symbol in that list, or the first one the we have reached the end of the list.
(defun kdic-after (option options) "Return the next value after OPTION in the list OPTIONS." (let ((result (cadr (memq option options)))) (or result (car options))))
(setq kdic-ask-for (kdic-after kdic-ask-for '(meaning kanji reading)))
‘memq’ finds a symbol in the list and returns the tail of the list starting with the symbol found. The cdr of that tail will return another tail, starting with the “next” symbol in the list. cadr will return the head of that list, therefore card will only return the next symbol in the list.
Here’s a drawing and some code to illustrate it:
+---------+---+ +-------+---+ +---------+-----+ | meaning | *-+--->| kanji | *-+--->| reading | nil | +---------+---+ +-------+---+ +---------+-----+
Looking for kanji we get a list:
(memq 'kanji '(meaning kanji reading)) => (kanji reading)
Let us extract the tail starting with the next symbol is a list. In this case it has only one element, but it is a list nevertheless -- notice the parens!
(cdr (memq 'kanji '(meaning kanji reading))) => (reading)
Let us keep the head of that list. Remember, (cadr x) is a shorthand for (car (cdr x)).
(cadr (memq 'kanji '(meaning kanji reading))) => reading
What happens when we look at the last symbol in the list, however?
(cadr (memq 'reading '(meaning kanji reading))) => nil
That’s why there is the line (or result (car options)) in the little lisp function above. If result is nil, then we want to return the beginning of the list.