CODEWITHSUNDEEP
lispclosures
fe2s
fe2s

Reputation: 435

Is this a closure?

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)

Upvotes: 5

Views: 1307

Answers (2)

Caleb Hattingh
Caleb Hattingh

Reputation: 9225

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.'

Upvotes: 18

nourdine
nourdine

Reputation: 7597

Here's my contribute as a JavaScript programmer:

A closure is a function that has access to variables defined in its lexical scope (a scope that might not be present anymore when the closure is actually invoked). Here:

function funktionFactory(context) {
   // this is the lexical scope of the following anonymous function
   return function() {
      // do things with context
   }
}

Once funktionFactory has returned the lexical scope is gone forever BUT (and it's a big 'but') if the returned function is still referenced (and therefore not garbage collected), then such function (a closure) can still play with the original variable context. Here:

var closure = funktionFactory({
   name: "foo"
});

no one but closure can access the name property of the context object (unreachable for any other entity in the software when funktionFactory has returned).

So to answer your question: is func a closure? nope. And callback? neither!

Upvotes: 0

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