Elixir & Phoenix: Scalable Backend Development · Lesson

Recursion and Higher-Order Functions

Understand recursion as a fundamental functional concept and explore higher-order functions for abstracting behavior.

Recursion and Higher-Order Functions is a free Elixir & Phoenix: Scalable Backend Development lesson on CoddyKit — lesson 3 of 4. You can read the complete lesson below for free — then practise it hands-on in the browser with a built-in code editor and a 24/7 AI tutor. It is part of the Elixir & Phoenix: Scalable Backend Development learning path, one of 4 lessons in the course, and your progress syncs across the web and the CoddyKit app.

Meet Recursion in Elixir

Welcome to recursion! In functional programming, recursion is a powerful technique where a function calls itself to solve a problem.

Instead of using loops (like for or while in other languages), Elixir often relies on recursion to iterate over data or repeat actions. It's a core concept you'll use a lot!

The Two Pillars of Recursion

Every recursive function needs two main parts to work correctly:

Base Case: This is the stopping condition. It defines when the function should stop calling itself and return a direct result. Without it, your function would run forever!
Recursive Step: This is where the function calls itself again, but with a smaller or simpler version of the original problem. Each call moves closer to the base case.

Recursion in Action: Factorial

Let's see recursion with a classic example: calculating the factorial of a number. The factorial of n (written as n!) is the product of all positive integers less than or equal to n. For example, 5! = 5 * 4 * 3 * 2 * 1 = 120.

Notice how factorial(n) calls factorial(n - 1) until it hits the base case of 0.

defmodule Math do
  def factorial(0), do: 1
  def factorial(n) when n > 0, do: n * factorial(n - 1)
end

IO.puts "Factorial of 5: #{Math.factorial(5)}"

Efficient Recursion: Tail Calls

While recursion is great, naive recursion can sometimes lead to performance issues or 'stack overflows' for very deep calls.

Elixir (and the Erlang VM) offers Tail Call Optimization (TCO). If the recursive call is the very last operation in a function, the VM can optimize it, preventing new stack frames from being created. This makes tail-recursive functions as efficient as loops!

Optimizing with Tail Recursion

To achieve TCO, we often use an accumulator. This is an extra argument passed to the function that collects the result as the recursion progresses.

Compare this version to the previous one. The recursive call factorial(n - 1, n * acc) is the last thing happening in the function, making it tail-recursive.

defmodule Math do
  # Public interface, calls the private tail-recursive function
  def factorial(n), do: factorial(n, 1)

  # Private tail-recursive function with accumulator
  defp factorial(0, acc), do: acc
  defp factorial(n, acc) when n > 0, do: factorial(n - 1, n * acc)
end

IO.puts "Tail factorial of 5: #{Math.factorial(5)}"

Functions as First-Class Citizens

Now, let's explore Higher-Order Functions (HOFs). In Elixir, functions are 'first-class citizens'. This means you can:

Pass functions as arguments to other functions.
Return functions as results from other functions.
Assign functions to variables.

HOFs enable powerful abstractions, making your code more concise, flexible, and reusable.

Transforming Lists with Enum.map

Enum.map/2 is one of the most common HOFs. It takes an enumerable (like a list) and a function. It applies that function to each element and returns a new list with the transformed elements.

It never modifies the original list, embracing Elixir's immutability.

numbers = [1, 2, 3, 4]
doubled_numbers = Enum.map(numbers, fn n -> n * 2 end)

IO.puts "Original: #{inspect numbers}"
IO.puts "Doubled: #{inspect doubled_numbers}"

Filtering Lists with Enum.filter

Another handy HOF is Enum.filter/2. It takes an enumerable and a function that should return a boolean (true or false).

It returns a new list containing only the elements for which the function returned true. It's perfect for selecting specific items from a collection.

numbers = [1, 2, 3, 4, 5, 6]
even_numbers = Enum.filter(numbers, fn n -> rem(n, 2) == 0 end)

IO.puts "Original: #{inspect numbers}"
IO.puts "Even: #{inspect even_numbers}"

Aggregating with Enum.reduce

Enum.reduce/3 is perhaps the most powerful HOF for working with enumerables. It takes an enumerable, an initial accumulator value, and a function.

It iterates through the collection, applying the function to each element and the current accumulator, eventually reducing the entire collection to a single value.

numbers = [1, 2, 3, 4]
sum = Enum.reduce(numbers, 0, fn n, acc -> n + acc end)
product = Enum.reduce(numbers, 1, fn n, acc -> n * acc end)

IO.puts "Numbers: #{inspect numbers}"
IO.puts "Sum: #{sum}"
IO.puts "Product: #{product}"

Anonymous Functions and HOFs

You've seen fn n -> n * 2 end. These are anonymous functions (or lambdas). Elixir provides a shorthand for simple anonymous functions:

&1 refers to the first argument.
&2 refers to the second argument, and so on.
&(&1 + &2) is equivalent to fn a, b -> a + b end.

This makes HOF calls even more concise!

numbers = [1, 2, 3, 4]
doubled_short = Enum.map(numbers, &(&1 * 2))
even_short = Enum.filter(numbers, &(rem(&1, 2) == 0))

IO.puts "Doubled (short): #{inspect doubled_short}"
IO.puts "Even (short): #{inspect even_short}"

Test Your HOF Knowledge

Higher-Order Functions are a cornerstone of functional programming in Elixir. Let's check your understanding.

Recursion & HOFs: Key Takeaways

Great job! In this lesson, you've grasped two fundamental concepts in functional Elixir:

Recursion: A function calling itself, defined by a base case and a recursive step.
Tail Call Optimization (TCO): An important Elixir feature for efficient, stack-safe recursion, often achieved with an accumulator.
Higher-Order Functions (HOFs): Functions that take other functions as arguments or return them, like Enum.map, Enum.filter, and Enum.reduce.
Anonymous Functions: Concise ways to define functions inline, often used with HOFs, including the &1 shorthand.

These tools are essential for writing expressive and powerful Elixir code. Keep practicing!

Frequently asked questions

Is the “Recursion and Higher-Order Functions” lesson free?

Yes — the full text of “Recursion and Higher-Order Functions” is free to read here on the web, and the Elixir & Phoenix: Scalable Backend Development course includes 4 lessons in total. To practise it interactively (a built-in code editor and a 24/7 AI tutor) and unlock the rest of the Elixir & Phoenix: Scalable Backend Development course, upgrade to CoddyKit PRO.

What will I learn in “Recursion and Higher-Order Functions”?

Understand recursion as a fundamental functional concept and explore higher-order functions for abstracting behavior. You practise Elixir & Phoenix: Scalable Backend Development with hands-on code you run directly in the browser, and a 24/7 AI tutor answers your questions as you work through the lesson.

Do I need any experience to start Elixir & Phoenix: Scalable Backend Development?

No prior experience is required. Elixir & Phoenix: Scalable Backend Development on CoddyKit is structured for beginners through advanced learners; this is — lesson 3 of 4, so you can start here or from the beginning and move at your own pace.

How long does the “Recursion and Higher-Order Functions” lesson take?

Most CoddyKit lessons take about 5–10 minutes. Each one is bite-sized and interactive, so you make steady progress and pick up exactly where you left off across the web and the app.

Can I write and run code in this Elixir & Phoenix: Scalable Backend Development lesson?

Yes. Every Elixir & Phoenix: Scalable Backend Development lesson includes a built-in code editor, so you write and run real code right in your browser and get instant AI feedback — no local setup required.

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