Recursion

Since data is immutable, we don’t have traditional loops, like the `for` loop, that you might be familiar with from imperative languages. Instead, we have recursion where a function is called recursively until a condition is reached that stops that action from continuing.

Let’s look at how we would iterate over a list. We want to start by grabbing the first item:

``````defmodule Recursion do
def each(x) do
[ head | tail ] = x
end
end
``````

Then, we want to call the function with the current item:

``````defmodule Recursion do
def each(x) do
[ head | tail ] = x
end
end
``````

Finally, we call `each` with the tail of the list, which is everything we haven’t operated on yet:

``````defmodule Recursion do
def each([]), do: nil
def each(x) do
[ head | tail ] = x
each.(tail, function)
end
end
``````

We also define the case where the list is empty, so we don’t call `each` infinitely.

For instance, let’s say we have a function where we want to implement the Fibonacci sequence. Fibonacci starts with “1,1” and then adds the previous two numbers to get the next one in the sequence. The simplest way for handling fibonacci functions is to use recursion:

``````defmodule Math do
def fibonacci(x) when x <= 1, do: x
def fibonacci(x), do: fibonacci(x - 1) + fibonacci(x - 2)
end
``````

Although this might look complex from the outset, it’s a fairly succinct method for defining such a complex algorithm.

Elixir implements recursive functions efficiently so we can rely on recursive statements as a safe and reliable solution. They are an important part of the language so knowing how it works is helpful but you’ll rarely use recursion to manipulate lists. The `Enum` module already provides many of the conveniences for working with lists. We’ll use `Enum` a bit in the Phoenix class.