What is identity doing? [closed]

Question

I've seen this sort of thing a few times:

(mapconcat 'identity '("" "home" "alex " "elisp" "erc") "/")
   => "/home/alex /elisp/erc"

which means mapconcatwants a function in the second position, hence just shove it the "identity" function if you don't want anything done to your input list first. But this

(defun shk-yas/helm-prompt (prompt choices &optional display-fn)
  "Use helm to select a snippet. Put this into `yas/prompt-functions.'"
  (interactive)
  (setq display-fn (or display-fn 'identity))

has me confused. Likewise this

(org-element-map (org-element-parse-buffer) 'headline #'identity)

I assume it has something to do with the code calling a function, but we don't want to give it one -- other than something reflexive like an identity. Again, the first example is clear, but not so much the next two.

Answer 1

The first example becomes a little clearer if you look further down the function. Just a bit past what you quoted we have

(mapcar (lambda (x) (funcall display-fn x)) choices)

So a list of "choices" is being made and display-fn is being applied to each choice, presumably to modify how they get displayed. A sensible default is that each choice is displayed as itself, which means display-fn should be identity a function that just returns its argument. You can see that display-fn is an optional argument, but optional arguments default to nil. (or display-fn 'identity) is just a clever way of saying "if display-fn is nil return identity otherwise return display-fn". It works because or returns its first non-nil argument. Writing (if display-fn display-fn 'identity) would do the same thing.

The second example is similar mapconcat. The third argument is a function that gets applied to every headline in the buffer. The manual says:

FUN is the function called on the matching element or object. It has to accept one argument: the element or object itself.

Just like mapconcat we have to give a function to transform the data, even if we don't want to. The identity function is perfect for this as it doesn't do anything to the data.