Journal of the overlooked tiny bits in young programming language. Will update weekly or two, as a way to build the habit of writing something on a regular basis. Authored by Riza and Augie as Editor In Chief.

Functional Programming Basic Features

A couple week ago I read this article about why you should take a look at functional programming. Well, when you read and follow this blog you already did. One part that interesting about that article is the basic features of FP, which is:

  1. First-class functions
  2. Higher-order functions
  3. Closure
  4. Immutable state

This is good practice for us to learn the very basic of functional programming through Elixir. So let's start!

First-Class Functions

The basic definition of first-class functions is simply that you can store functions into a variable. You can also passes the functions around and invoked from other functions. This also known as anonymous functions in Elixir to be specific.

Let’s see the example.

add = fn num1, num2 ->
  num1 + num2

substract = fn num1, num2 ->
  num1 - num2

perform_calculation = fn num1, num2, func ->
  func.(num1, num2)

IO.inspect add.(1, 2)

IO.inspect substract.(2, 4)

IO.inspect perform_calculation.(5, 5, add)
IO.inspect perform_calculation.(5, 5, substract)
IO.inspect perform_calculation.(5, 5, fn a, b -> a * b end)

Higher-Order Functions

Elixir not only lets you put functions into variables, but also allow you to pass functions as another function’s arguments. In mathematics, a higher-order function in general is a function that takes one or more functions as an input and or returns a function as an output as well. This one feature are where Elixir’s power really starts to shine. You can do higher-order functions in other languages, but Elixir treats higher-order functions as a native and feel natural. Let’s call it first-class higher-order functions :)

For example, take a look at these code below:

iex> square = fn x -> x * x end
#Function<6.17052888 in :erl_eval.expr/5>
iex> Enum.map(1..10, square)
[1, 4, 9, 16, 25, 36, 49, 64, 81, 100]

In the first line we define an anonymous function that take a number and square it and assigned to the variable called square. Then we use Enum.map that takes 2 arguments. The first is just sequence of numbers and the second argument is a function that will be apply to each element of sequence.


Closure is a concept that has these attributes:

As you can see, even though you changed outside_var value, the result still 5. That’s because we changed the value after we define the print function.

Immutable State

Elixir are immutable. By being immutable, Elixir also helps eliminate common cases where concurrent code has race conditions because two different entities are trying to change a data structure at the same time. Let's see how Elixir being immutable.

iex> tuple = {:ok, "hello"}
{:ok, "hello"}
iex> put_elem(tuple, 1, "world")
{:ok, "world"}
iex> tuple
{:ok, "hello"}

But yes, you still can re-assign variable. In the background, Elixir still immutable but in the front Elixir looks like support mutable state. With this, Elixir being the best of both world: immutable and mutable. And with this decision, Elixir can become 'bridge' for people who interested in FP so they're not overwhelm with all the functionality stuff. At least they still sign and re-assign variables like other languages.

iex(1)> num = 22
iex(2)> ^num = 23
** (MatchError) no match of right hand side value: 23

iex(3)> num = 23

In the first line, we assign variable called num to number 22. Then we try to match variable num which is 22 with 23. We do pattern matching here with hat (^) char before variable, not re-assign value. And the last line of code is we rebound variable num and re-assign it to 23 as a value. In this case, num is just a container for data and you can re-bind it to new data. When you re-bind it the old data will be promptly chucked by the runtime leaving your memory free to store new data.

As I mentioned above, at it's core Elixir still immutable and you have no worries about side-effect of variable re-assignment. Let's see the code.

defmodule Assignment do
    def change_me(string) do
        string = 2

We try to change any variable that will passing the function into 2. Let's see when we run the these code.

$ iex assignment.ex 
Erlang/OTP 17 [erts-6.2] [source] [64-bit] [smp:8:8] [async-threads:10] [hipe] [kernel-poll:false]

assignment.ex:2: warning: variable string is unused
assignment.ex:3: warning: variable string is unused
Interactive Elixir (1.0.2) - press Ctrl+C to exit (type h() ENTER for help)
iex(1)> greeting = "Hello"
iex(2)> Assignment.change_me(greeting)
iex(3)> greeting

First of all, Elixir compiler will complaint about string variable is unused. Then we assign variable called greeting to string "Hello". Then we try to change the greeting variable with Assignment.change_me/1 function we've created before. It then return 2. But if we check greeting variable, it's still prints out "Hello" as the original value.