Programming – Is Every Function a Closure?

cclosuresjavascriptpythonterminology

Wikipedia says, that closure – is a function, which has an access to variables, declared outside of the function. There is even an example:

function startAt(x)
   function incrementBy(y)
       return x + y
   return incrementBy

variable closure1 = startAt(1)
variable closure2 = startAt(5)

But according to most programming languages (including python, javascript, swift, etc.) the next example is correct (written in python):

# Script starts
test = "check"

def func(test2):

    def func2():
        return test2 == test

    return func2()

print func("check") // returns TRUE
# Script ends

Basically, func is not a closure, but it obviously uses variable test, declared outside of the function. Does that mean func IS a closure?

Even in C++ you can run this:

std::string test = "check";

bool func(std::string test2) {
    if (test2 == test)
        return true;
    else
        return false;
}

int main() {
    if (func("check"))
        std::cout << "TRUE" << std::endl; // prints TRUE
}

Eventually, this makes every function being a closure. Where am I wrong?

Best Answer

No, not every function is a closure.

Wikipedia says:

... closure ... is a function or reference to a function together with a referencing environment — a table storing a reference to each of the non-local variables (also called free variables or upvalues) of that function.

I'd add "non-local and non-global", but the idea is correct.

Neither your C++ nor Python examples use closures. In both cases it's just scoping rules allow functions to see their outer scope and global scope.

"Closure" happens in the 1st example - incrementBy is constructed in and then returned from it's outer function, capturing argument x. When you assign variable closure1 = startAt(1), you end up having a closure (function) inside closure1 var which captured argument, which value happened to be 1, so when you call closure1(2) the result is 3 (1 + 2).

Think of it as memorizing some information about closure's declaration scope: incrementBy retain a memory about insides of startAt, specifically a value of it's argument x.

In lambda calculus, as I know, those "non-local" variables are called "free", and functions with free variables are called "open terms". Closure is a process of "closing" open terms by "fixing" values of those free variables in aforementioned "environment table". Hence the name.

It's worth noting that in Python and JS closure happens implicitly, while in PHP you have to explicitly tell which variables you want to close over (capture): http://php.net/manual/en/functions.anonymous.php - note use keyword in declarations:

// equivalent to the 1st example
function startAt($x) { //        vvvvvvvv          vv
    $incrementBy = function ($y) use ($x) { return $x + $y };
    return $incrementBy;
}

Related Solutions

Language Features – What Is a Closure?

_{(Disclaimer: this is a basic explanation; as far as the definition goes, I'm simplifying a little bit)}

The most simple way to think of a closure is a function that can be stored as a variable (referred to as a "first-class function"), that has a special ability to access other variables local to the scope it was created in.

Example (JavaScript):

var setKeyPress = function(callback) {
    document.onkeypress = callback;
};

var initialize = function() {
    var black = false;

    document.onclick = function() {
        black = !black;
        document.body.style.backgroundColor = black ? "#000000" : "transparent";
    }

    var displayValOfBlack = function() {
        alert(black);
    }

    setKeyPress(displayValOfBlack);
};

initialize();

The functions¹ assigned to document.onclick and displayValOfBlack are closures. You can see that they both reference the boolean variable black, but that variable is assigned outside the function. Because black is local to the scope where the function was defined, the pointer to this variable is preserved.

If you put this in an HTML page:

Click to change to black
Hit [enter] to see "true"
Click again, changes back to white
Hit [enter] to see "false"

This demonstrates that both have access to the same black, and can be used to store state without any wrapper object.

The call to setKeyPress is to demonstrate how a function can be passed just like any variable. The scope preserved in the closure is still the one where the function was defined.

Closures are commonly used as event handlers, especially in JavaScript and ActionScript. Good use of closures will help you implicitly bind variables to event handlers without having to create an object wrapper. However, careless use will lead to memory leaks (such as when an unused but preserved event handler is the only thing to hold on to large objects in memory, especially DOM objects, preventing garbage collection).

^{1: Actually, all functions in JavaScript are closures.}

JavaScript Closures – Importance and Benefits

Because javascript doesn't have feature like namespaces, and you can mess up pretty easily with all sort of global objects.

So it is important to be able to isolate some code in its own execution environment. Closure are perfect for that.

This usage of closure doesn't make sense in a language like C# where you have namespaces, classes and so on to isolates code and not putting everything in the global scope.

A very common practice for javascript code is writting it like this :

(function(){
    // Some code
})();

As you can see, this is an anonymous function declaration, followed immediately by its execution. Thus, everything defined within the function is impossible to access from outside, and you will not mess up the global scope. The execution context of this function will remain alive as long as some code uses it, like nested functions defined within this context, that you can pass as callback or whatever.

Javascript is a very different language than C#. It's not object oriented, it is prototype oriented. This leads to very different practices at the end.

Anyway, closures are good, so use them, even in C#!

EDIT: After some discuss on stackoverflow's chat, I think this anwer has to be precised.

The function in the sample code isn't a closure. However, this fucntion can define local variable and nested functions. All nested functions that use these local variables are closures.

This is usefull to share some data across a set of functions without messing up the global scope. This is the most common use of closure in javascript.

Closure are way more powerfull than just sharing some data like this, but let's be realistic, most programmers don't know a thing about functionnal programming. In C# you would have used class or a namespace for these kind of use, but JS does not provide this functionnality.

You can do way more with closure than just protect the global scope, but this is what you'll see in JS source code.

Best Answer

Related Solutions

Language Features – What Is a Closure?

JavaScript Closures – Importance and Benefits

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