Once you start developing any program besides the very simple one you will absolutely start structuring your code in subroutines. In the Object Oriented Programming, they called methods. But as soon as Elixir is a functional programming language, subroutines here are functions. Let’s take a look what functions can give us.

## Named functions

Most of the time we are using named functions. In Elixir, those functions are live inside modules.

Let’s take a look at a simple example.

defmodule Greetings do
def say_hello(name) do
IO.puts "Hello, #{name}!"
end
end


So here we have a module Greetings. To define modules we are using defmodule keyword. The module consists of say_hello/1 function who accepts a single argument name and then prints a hello message on the screen. As you can see functions defined using def keyword.

iex> Greetings.say_hello("John")
Hello, John!


We can also call functions without parentheses because they are optional in Elixir:

iex> Greetings.say_hello "John"
Hello, John!


### Arity

You are probably noticed that I have referred to a function as a say_hello/1. Ok, we can see that the name of the function is say_hello. But what does /1 mean?

In Elixir we can define several functions with the same name. In order to understand that those functions are actually different the arity is used. Term “Arity” is not Elixir specific. As Wikipedia says:

In logic, mathematics, and computer science, the arity of a function or operation is the number of arguments or operands that the function takes.

So we have a function with a single argument, so its arity is 1.

We can define more functions in this module. Let’s do this:

def say_hello(first_name, last_name) do
IO.puts "Hello, #{first_name} #{last_name}!"
end

def say_hello(title, first_name, last_name) do
IO.puts "Hello, #{title} #{first_name} #{last_name}!"
end

iex> Greetings.say_hello("John", "Doe")
Hello, John Doe!

iex> Greetings.say_hello("Mr.", "John", "Doe")
Hello, Mr. John Doe!


So now we have say_hello/1, say_hello/2 and say_hello/3.

### Default params

Now imagine we are greeting gentlemen most of the time, but we still want to be polite and refer to them using “Mr.” title.

So we can try to make title to have a default value of “Mr.”. To achieve it our attribute in function definition should be followed by \\ <value>, which is \\ "Mr." in our case. Let’s try to change our function in the following way:

def say_hello(title \\ "Mr.", first_name, last_name) do
IO.puts "Hello, #{title} #{first_name} #{last_name}!"
end


But once we try to compile our code we will get:

warning: this clause cannot match because a previous clause at line 6 always matches


That happens because now Elixir does not know which function to call, by excluding title we have two functions with arity 2.

We may think: “Ok, let’s swap the order of the functions and put our say_hello/3 in front of say_hello/2 so Elixir can pick up default parameter earlier. But then we get:

** (CompileError) hello.ex:10: def say_hello/2 conflicts with defaults from def say_hello/3


So we need to take default parameters into account and check if that does not overlaps with our already existed functions with the same name.

We have caught ourselves into the trap by trying to set a default value for title.

Let’s take a look a working example with default parameters to understand how does it work.

defmodule DefaultParams do
def func(a, b \\ "b", c) do
IO.inspect [a, b, c]
end
end

iex> DefaultParams.func(1, 2)
[1, "b", 2]

iex> DefaultParams.func(1, 2, 3)
[1, 2, 3]


In that example, we have 3 attributes and one of them has a default value. That means if we call the function with only two attributes Elixir would set values for a and c param.

### Private functions

By default all the functions defined inside the module are public. There is a way to define private functions as well. Private functions will not be visible from the outside of the module. To do that we are using defp keyword.

### Capture functions

Sometimes we need to store the function into a variable and pass it to another function. By store function into a variable, I am not talking about the value that function returns, I am talking about the function itself. That means we are not executing function body right away, but pass it into another function and then call it inside.

That process is known as function capture.

To do achieve this we can use following structure:

variable = &ModuleName.function_name/arity


Once we have the function captured we can call it: 

variable.(arg1, arg2)


Pay attention there is a dot . between variable and parentheses.

Let’s take a look at the following example to see it in action:

defmodule Math do
a + b
end
end

defmodule CaptureExample do
def do_something(c, d, func) do
func.(c, d)
end
end


Now we can use our code in the following way:

# We can pature the function into variable

# We can call that function with different arguments
iex> sum.(1, 2)
3

# We can pass variable as argument to another function
iex> CaptureExample.do_something(1, 2, sum)
3

# Or we can capture and pass it as argument at the same time
3


### Short form

Sometimes a body of our functions can be so small, so we may want to write a complete function definition as a one-liner. That is also possible in elixir.

Let’s rewrite our previous example and make it shorter.

defmodule Math do
def add(a, b), do: a + b
end

defmodule CaptureExample do
def do_something(c, d, func), do: func.(c, d)
end


What we have got here is a function name with arguments followed by comma , then beginning of the bloc followed by colon : and then function body itself.

I would recommend avoiding using this approach for nontrivial functions. These functions would harder to read for other people and even for you after some time.

Let’s summarise what did we learn here in an example.

### FizzBuzz example

Yes that is solution to FizzBuzz problem without using any conditionals

defmodule FizzBuzz do
def fizz_buzz(n) do
calculate_fizz_buzz(rem(n, 3), rem(n, 5), n)
end

defp calculate_fizz_buzz(0, 0, _), do: "FizzBuzz"
defp calculate_fizz_buzz(0, _, _), do: "Fizz"
defp calculate_fizz_buzz(_, 0, _), do: "Buzz"
defp calculate_fizz_buzz(_, _, n), do: n
end

→ iex fizz_buzz.ex

iex> Enum.map(1..20, &FizzBuzz.fizz_buzz/1)
[1, 2, "Fizz", 4, "Buzz", "Fizz", 7, 8, "Fizz", "Buzz",
11, "Fizz", 13, 14,"FizzBuzz", 16, 17, "Fizz", 19, "Buzz"]


In this example, we have four private calculate_fizz_buzz/3 functions with the same arity. As soon as we are using Pattern Matching in the function arguments, Elixir can understand what function do we want to call.

Pay attention that the order of how do we define these functions are important, because if we put our (_, _, n) function in front of others it would overlap any other function.

Then we call that function with rem(n, 3), rem(n, 5), n (rem/2 returns the remainder after dividing one number by another).

In the Interactive Elixir, we use Enum.map/2 to go from 1 to 20 and use function capture to pass our fizz_buzz/1 function and see the result.

## Anonymous functions

Sometimes we may need shorter or one-time functions. We don’t even want to bother by organizing them in the modules. We may need this kind of function just to use once or only to pass as an argument.

So to respond to these needs Elixir provides us Anonymous functions.

To define these functions we are using fn keyword using the following format:

fn
arguments -> body
arguments -> body
end


A function which adds two numbers together will look like

iex> sum = fn a, b -> a + b end
#Function<12.99386804/2 in :erl_eval.expr/5>

iex> sum.(1, 2)
3


We can see that we are calling it using the same way as we did with captured functions. Well, actually capture is converting named function to anonymous and that is the way how do we call anonymous functions in Elixir. It worth to mention that parentheses, in this case, are mandatory.

### Multiple bodies

Using Pattern Matching we can have several bodies for functions. For example:

iex> sum = fn
...> 0, b -> b
...> a, b -> a + b
...> end
#Function<12.99386804/2 in :erl_eval.expr/5>

iex> sum.(0, 1)
1
iex> sum.(1, 1)
2


But we cannot have a different number of arguments within the same function. So the following code is invalid:

iex> sum = fn
...> a, b -> a + b
...> a, b, c -> a + b + c
...> end
** (CompileError) iex: cannot mix clauses with different arities in anonymous functions


The & is not only used to capture the function. It can be used to write a shorter version of anonymous functions.

What? Even shorter? Well, Yes.

Now our sum function looks like that:

# Before
iex> sum = fn a, b -> a + b end
#Function<12.99386804/2 in :erl_eval.expr/5>

# After
iex> sum = &(&1 + &2)
&:erlang.+/2

iex> sum.(1, 2)
3


Using &() we are defining function body where placeholders &1 and &2 are the first and second argument of the function.

Take a look at our updated example from function capture section:

iex> CaptureExample.do_something(1, 2, &(&1 + &2))
3


### FizzBuzz example

And as an example, there is a FizzBuzz problem solved using anonymous functions.

iex> fizz_or_buzz = fn
...>   (0, 0, _) -> "FizzBuzz"
...>   (0, _, _) -> "Fizz"
...>   (_, 0, _) -> "Buzz"
...>   (_, _, n) -> n
...> end
#Function<6.99386804/1 in :erl_eval.expr/5>

iex> fizz_buzz = fn n -> fizz_or_buzz.(rem(n, 3), rem(n, 5), n) end
#Function<6.99386804/1 in :erl_eval.expr/5>

iex> Enum.map(1..20, fizz_buzz)
[1, 2, "Fizz", 4, "Buzz", "Fizz", 7, 8, "Fizz", "Buzz",
11, "Fizz", 13, 14, "FizzBuzz", 16, 17, "Fizz", 19, "Buzz"]
`

## Summary

As we can see the function is a cornerstone in the Elixir. That is not surprising because Elixir is a functional language. We have discovered a lot of new things here and laid the solid foundation for next steps.

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