Higher-order functions in Elisp

Question

I created a function that returns a function in Elisp:

(defun singleton-set (elem)
  (defun f (n) (= n elem))
  f)

I try to run this in IELM, a

Answer 1

From the NEWS for Emacs 24:

Lisp changes in Emacs 24.1

• Code can now use lexical scoping by default instead of dynamic scoping. The lexical-binding variable enables lexical scoping for local variables. It is typically set via a file-local variable in the first line of the file, in which case it applies to all the code in that file.

So, in Emacs 24:

(setq lexical-binding t)
(defun singleton-set (elem) (lambda (n) (= n elem)))
(mapcar (singleton-set 1) '(0 1 2 3))
    ===> (nil t nil nil)