function that takes a function as argument and returns a new function

Question

I'd like to write an elisp-function that has a function as argument and returns a new function.

Let's say the new function double-the-function should write the returned value of the argument-function twice. (The argument function operates on a string and returns a string. This is ensured.)

So:

(reverse "abc") returns: "cba"

If I define the function f as follows:

(setq f (double-the-function reverse))

I expect that:

(f "abc") returns "cbacba"

which is equal to (concat (reverse cba) (reverse "cba")).

How to define double-the-function?

Answer 1

;; -*- lexical-binding : t -*-
(defun double-the-function (fn)
  (lambda (string)
    (let ((result (funcall fn string)))
      (concat result result))))

(funcall (double-the-function 'reverse) "abc")