March 30, 2008

Functional ActionScript – Part II

Welcome to the second part of my series on Functional ActionScript. Part I was a brief introduction to some concepts of functional programming in ActionScript. In this second part, I will present you some examples to ActionScript's built-in functional APIs on Array. However, first I would like to introduce you to a neat little trick that will save us some typing and make our code more clear.


If you take a look at the documentation of the following methods that we will discuss later (every, some, filter, forEach, and map) you will notice that they all take a callback that, apart from the return type maybe, has a signature that looks like this:

function callback(item:*, index:int, array:Array):*

Being pragmatic, I rarely have use for the last two arguments, index and array. Therefore, I wrote myself a little wrapper function that looks like this:

function wrap(f:Function):Function
      function(x:*, index:int, array:Array):*
          return f(x)

Basically, it takes a simple function like:

function even(x:int):Boolean
    return x % 2 == 0

…and returns a function which conforms to the callback signature shown above. Another great example for the power of higher-order functions.

The Party

After having been introduced to friend number one, namely map, in Part I, I suggest we get to know some new friends but first a small convention:


I will use the following convention to denote trace output:

Friend Number Two: every

If you want to check if all the elements of an Array satisfy a certain condition, just write a test function and drop it into Array.every.

Example: Everybody Even?

For example, let's see if all integer in list are even:
var list:Array = [1, 2, 3, 4, 5, 6, 7, 8, 9]
First, we take the even function from above which takes an int, tests if it's even and returns the corresponding Boolean:
function even(x:int):Boolean
    return x % 2 == 0
Then, wrap even with wrapdoh! — drop it into Array.every and see what happens:

//? false

Friend Number Three: some

Array.some works along the lines of every but returns true as soon as one of the elements passes the supplied test.

Example: Anybody Odd?

In the following example, we check if any (meaning: one or more) of the elements in list is odd:
var list:Array = [1, 2, 3, 4, 5, 6, 7, 8, 9]
Our test function:
function odd(x:int):Boolean
    return !even(x)
The test:

//? true

Friend Number Four: filter

Array.filter is really handy. Pass it a test function and it returns you an Array with all the elements that passed the test.

Example: Who's Even, Who's Odd?

Get all even elements in list:
var list:Array = [1, 2, 3, 4, 5, 6, 7, 8, 9]


//? 2,4,6,8
…and all odd elements:

//? 1,3,5,7,9

Friend Number Five: forEach

Array.forEach is pretty much the same as with a subtle but important difference: forEach executes a function on each element in an Array but unlike map has not the purpose to modify the elements. Therefore forEach returns void and map returns an Array. This may or may not sound confusing. However, the following examples will make the difference clear…

Example: Hello

Let's say hello to all elements in list:
var list:Array = [1, 2, 3, 4, 5, 6, 7, 8, 9]

function sayHello(element:*,
    trace("Hello, Number", element)


//? Hello, Number 1
//? Hello, Number 2
//? Hello, Number 3
//? Hello, Number 4
//? Hello, Number 5
//? Hello, Number 6
//? Hello, Number 7
//? Hello, Number 8
//? Hello, Number 9
In this example I purposely didn't use my carefully crafted wrap function from above to show you how ugly the callback function can end up (line 3–6).

Old Friend: map

We've already met map in the first part on Functional ActionScript but I allow myself to introduce her here once again. takes a function, applies it to all elements in an Array and returns an Array with all modified elements.

Example: We're Square

Square all elements in list:
var list:Array = [1, 2, 3, 4, 5, 6, 7, 8, 9]

function square(x:Number):Number
    return x * x

//? 1,4,9,16,25,36,49,64,81
…or take the square root of all elements:
function squareRoot(x:int):Number
    return Math.sqrt(x)

//? 1,1.4142135623730951,1.7320508075688772,2,
//? 2.23606797749979,2.449489742783178,
//? 2.6457513110645907,2.8284271247461903,3

Friends Forever

When I'm talking about friends, I actually mean friends. Not only will the functions above be nice to you but they also get along very well with each other. Let's see how…

Example: Rendez-Vous

Let's look at this real-world scenario: If any of the elements in list is odd, you want to pick out the even elements, square them and then say hello to them. No sooner said than done:
var list:Array = [1, 2, 3, 4, 5, 6, 7, 8, 9]


//? Hello, Number 4
//? Hello, Number 16
//? Hello, Number 36
//? Hello, Number 64
Isn't the expressivess of this code just beautiful? Finding a more useless example is left as an exercise to the reader.

Doggy Bag (a.k.a Source Code)

Like what you saw? Have look at it, download it, and play with it!


View Source | Download Source (ZIP, 3KB)

Thank you for your attention and stay tuned for Part III of Functional ActionScript