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Using Pattern Matching to write a more idiomatic functional code

Juan Flores Juan Flores
Reading time: 2 min

As a functional programming newbie and someone that has done OOP for a few years, I often find myself writing code in Elixir that looks more like C# or Java.

Today I wanted to show how we can use the power of pattern matching to rethink how we define functions when we are writing code in Functional Programming.

What is Pattern Matching?

First things first, let’s see what pattern matching is. In programming we typically assign values to a variable with an = sign:

var x = "Elixir" + "is great";

Now, remember what = sign was in your Math class:

x + 1 = 3

That means the two terms on either side of the = sign are equivalent.

That’s what it is in Elixir:

[1, x, 5] = [1, 10, 5]
#x = 10

The only difference here is that Elixir will also solve the equation for us.

Pattern Matching in Functions

Elixir will try to match a function call to a specific implementation:

defmodule SimpleMaths do

  def sum([head|tail]), do: head + sum(tail)

  def sum([]), do: 0

end

SimpleMaths.sum([1, 2, 3])
#6

Here we provide two definitions for sum function. When we pass in an empty list, Elixir will pattern match into the second definition and return a 0. When we pass in a list, it will pattern match the parameter and get the head and the tail. Here we are simply accumulating the values on the list in order to perform the sum.

Using Functional Programming idioms

I remember when I started learning English, I learnt the grammar and vocabulary to make simple structures. Then I tried to build more complex structures with the former simple structures. In order to do that I applied ideas and concepts that I knew from my mother tongue into my new language. Life was so good. Then suddenly something arose: my English was grammatically correct, yet native speakers struggled to understand. I wasn’t really speaking English.

Let’s see how this is relevant to programming. Let’s define a module in Elixir to make payments:

defmodule Payments do

  def pay_with_paypal(login: login, password: password) do
    #pay with Paypal
	%{login => login, password => password} |> validate |> do_transaction
  end

  def pay_with_card(number: number, expires: expires, code: code) do
    #pay with card
	%{number => number, expires => expires, code => code} |> connectwithBank |> validate |> do_transaction
  end

end

We have two ways of making a payment, either with paypal or with credit card. That code above is me speaking Elixir in a C# way. In C# I need to rely on a change in the function name to identify different implementations. Despite being grammatically correct, the two functions are going to do pretty much the same.

Let’s try to be a bit more idiomatic:

defmodule Payments do

  def pay(:paypal, login: login, password: password) do
    #pay with Paypal
	%{login => login, password => password} |> validate |> do_transaction
  end

  def pay(:card, number: number, expires: expires, code: code) do
    #pay with card
	%{number => number, expires => expires, code => code} |> connectwithBank |> validate |> do_transaction
  end

end

Now when calling the pay function, Elixir will use the one with the appropriate matching pattern:

Payments.pay(:paypal, login: "juanito", password: "123")

Notice that we have used an atom to help with the matching, it also helps with clarity of intent and expressiveness of the code.