Recreating Elixir's Enum.sum

June 30, 2016

I’m learning Elixir, and today as an exercise, I was challenged to recreate Enum.sum using recursion. Here’s my solution.

For those new to the language, Enum.sum iterates over a list and returns the sum. Here’s an example:

iex(0)> Enum.sum([1,2,3,4,5])
15

Here is my implementation:

defmodule MyList do
  @moduledoc """
  Defines the list struct and functions.
  """

  @doc """
  Add all the items in a list together.
  """
  def sum(list) do
    sum(list, 0)
  end

  defp sum([], count) do
    count
  end

  defp sum([h|t], count) do
    sum(t, count + h)
  end
end

And the execution:

iex(1)> MyList.sum([1,2,3,4,5])
15

Explanation

Elixir doesn’t have loops, so sum must be written using recursion. When MyList.sum([1,2,3,4,5] is called, here’s what happens.

On the first call, the argument is a non-empty list, so the first (public) sum function is invoked. This calls sum again, with a count of 0 as an argument.

On the second call, the argument is a non-empty list plus a count, so the last (private) sum function is invoked. Here’s where the recursion happens. This function calls itself with the tail (indexes 1 to -1) of the list, and adds the head of the list to the count.

This recursion continues until the list is empty, when the second (private) sum method is called. This method just returns the count.

A nice side effect of this implementation is that MyList.sum([]) returns zero, just like Enum.sum([]).

Thanks to Josh Branchaud for pointing out some optimizations I have included in this example.

Elixir