Is this a closure?

Question

Some guy asserts that the following piece of code is an illustration of closure in Lisp. I'm not familiar with Lisp, but believe he is wrong. I don't see any free variables, it seems to me as an example of ordinary high level functions. Could you please judge...

 (defun func (callback)
   callback()
)

(defun f1() 1)
(defun f1() 2)

func(f1)
func(f2)

Answer 1

No.

There is no function being defined inside func that would enclose local variables inside func. Here is a contrived example based on yours Here is a good example:

Input:

(define f 
  (lambda (first-word last-word) 
    (lambda (middle-word)
      (string-append first-word middle-word last-word))))

(define f1 (f "The" "cat."))
(define f2 (f "My" "adventure."))

(f1 " black ")
(f1 " sneaky ")

(f2 " dangerous ")
(f2 " dreadful ")

Output:

Welcome to DrScheme, version 4.1.3 [3m].
Language: Pretty Big; memory limit: 128 megabytes.
"The black cat."
"The sneaky cat."
"My dangerous adventure."
"My dreadful adventure."
> 

f defines and returns a closure in which the first and last words are enclosed, and which are then reused by calling the newly-created functions f1 and f2.

---

This post has several hundred views, therefore if non-schemers are reading this, here is the same silly example in python:

def f(first_word, last_word):
    """ Function f() returns another function! """
    def inner(middle_word):
        """ Function inner() is the one that really gets called
        later in our examples that produce output text. Function f()
        "loads" variables into function inner().  Function inner()
        is called a closure because it encloses over variables
        defined outside of the scope in which inner() was defined. """ 
        return ' '.join([first_word, middle_word, last_word])
    return inner

f1 = f('The', 'cat.')
f2 = f('My', 'adventure.')

f1('black')
Output: 'The black cat.'

f1('sneaky')
Output: 'The sneaky cat.'

f2('dangerous')
Output: 'My dangerous adventure.'

f2('dreadful')
Output: 'My dreadful adventure.'