Closures

Closures are a bit hard to understand concept for someone from an Object Oriented programming background, mainly because popular OO programming languages, ~~(Java/C++) does not have this feature~~ (As bjzaba pointed out in reddit, C++ 11 has Closures) haven’t embraced or promoted this feature until recently. It is more of a functional programming concept, although many Object Oriented languages has started to support Closures through first class functions or higher order functions.

I first heard about Closures while developing something in Javascript. If you have used the popular javascript library jQuery, you have already used closures, knowingly or unknowingly.

Here is my attempt to explain Closures, through examples in few programming languages. I will try to be as simple as possible.

From Wikipedia:

In programming languages, a closure (also lexical closure or function
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 up values) of that
function. A closure—unlike a plain function pointer—allows a
function to access those non-local variables even when invoked outside
its immediate lexical scope.

What that means:
1. Closure is a function (or a reference to a function)
2. You get a pointer to closure
3. So you can pass it around like an object
4. It knows about non-local variables
5. It can access those non-local variables, even when invoked outside of its scope
6. So we say, closures ‘closes’ on its environment
7. A function may create a closure and return it.

Few programming languages, that support Closures

Lisp
Javascript
Scala
Clojure
Ruby
Python
Haskell
PHP
Go

In closures procedure/method/function contexts become first-class. That means, with closures you can create functions that return functions, although that is only an outcome. An important point to understand here is, the closure methods refer to the context in which it was created, not to the one it was called.

To better understand closures one has to understand a variable’s scope, the best read about that would be understanding javascript variable scope.

Closures store references to the outer function’s variables; they do not store the actual value. So if we change the value of reference in closure it changes value outside of its scope.

You may implement closures using Anonymous functions, but all anonymous functions need not be a closure, although many of them are.

Why would anyone use Closures?

Reusability
Do more with less code
Make functional code stateful

Closures help us to write more expressive and concise code(once you get a hang of it!). We know objects have a state, using Closure we can give state to functions as well.

Now, let us take a look at examples of how to use closures in a few programming languages.

Examples

All of the following examples do the same thing: Create a closure to increment a number by another number.

1. Closure example in Javascript

This would be the easiest to understand code.

var incrementBy = function(x) {
    return function(y) {
        return x+y;
    };
};

// this will remember the value of 'x' forever
var incrementBy2 = incrementBy(2);
var incrementBy3 = incrementBy(3);

console.log(incrementBy2(4));
console.log(incrementBy3(8));
// Here you are creating a closure and calling it immediately
console.log(incrementBy(5)(8));

In the Javascript example above, we define a function incrementBy(x), which returns a function, that accepts parameter ‘y’ and returns sum of x and y. Here the value of ‘x’ will go into the closure of the returned function and will be accessible whenever the function is invoked.

Note that when calling incrementBy2(4) our closure remembers the value 2 (i.e ‘x’) that was passed earlier when doing var incrementBy2 = incrementBy(2);. And when invoking incrementBy2(4) we are actually passing the value of y as 4. Hence the statement return x+y will transform to return 2+4. Cool right!!?

2. Closure example in Scala

object Closure {

    // define the closure using one line of code, power of Scala
    def incrementBy(x:Int) = (y:Int) => x + y        

    def main(args: Array[String]) = {

        var incrementBy3 = incrementBy(3)
        val incrementBy5 = incrementBy(5)

        println(incrementBy3(5))
        println(incrementBy5(10))
        println(incrementBy(20)(20))    
    }    
}

The example in Scala is similar to the example in Javascript except for Scala’s awesome one liner syntax.

3. Closure example in Lisp (Clisp)

(defun increment(y)
    (defun incrementBy(x)
        (+ y x)
    )
)

(increment 4)
(incrementBy 5) ; 9
(incrementBy 10) ; 14

There are several ways of doing this is lisp. This is only one way of doing it. Here you are not getting a pointer to the closure function and will probably make it useless. See this link to see how to return functions is Clisp.

4. Closure example in Clojure

(def increment (fn [y] 
    (def incrementBy (fn [x]
      (+ x y)
      ))
))

(increment 4)
(incrementBy 5) ; 9
(incrementBy 10) ; 14

As you can see, the syntax of clojure and lisp are extremely similar.

5. Closure example in Python

def incrementBy(x):
    def increment(y):
        return x+y
    return increment

incrementBy4 = incrementBy(4)
incrementBy6 = incrementBy(6)

print incrementBy4(5)  # 9
print incrementBy6(10) # 16

The above example in Python is pretty similar to the one in Javascript and is easy to understand. Here we define the closure function increment(y) and then return it. Just remember to take care of the tabs!

6. Closure example in Ruby

class Closure

  # Ruby example using Proc.new
  def incrementBy(x)
    return Proc.new {|y| x+y }
  end

  # Ruby example using lambda
  def incrementByLambda(x)
    lambda {|y| x+y}
  end

end

closure = Closure.new
increment5 = closure.incrementBy(5)
incrementByLambda7 = closure.incrementByLambda(7)

puts increment5.call(6)
puts incrementByLambda7.call(6)

Ruby can do this is in two ways, using Proc and using lambda. The lambda functions in Ruby are similar to lambda’s in lisp.

So that was examples about closures. Hope that gives you at least some idea about Closures. If you know how do the same in any other languages, please feel free to share. Also checkout the references, they are good reads.

References:

Thanks to Craig for sharing Go snippet

Reddit thread here: http://redd.it/223b7p

Sunday, March 30, 2014