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Joseph Kain

Professional Software Engineer learning Elixir.

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In this week’s post I want to take a look at the Enum.reduce function. The documentation can explain the basics of how reduce works. And there are many other blogs on functional programming that can explain the basics of how it can be used. But in this post I want to look at examples of reduce from real, production, Elixir code. And from these examples I want to extract some useful patterns that you can be used in new code that we will write in the future.

The first example comes from the ExDoc project:


# From ex_doc/lib/ex_doc/retriever.ex
# Returns a dict of { name, arity } -> [ behaviour_module ].
defp callbacks_implemented_by(module) do
  |> behaviour ->, &{ &1, behaviour }) end)
  |> Enum.reduce(%{}, &Enum.into/2)

There is a lot going on here. The high level pattern is: |> Enum.reduce(f2)

which is the famous map-reduce pattern. In ExDoc’s particular case, Enum.reduce is used to reduce into a map which leads to the second pattern - the reduce into pattern:

Enum.reduce(enumerable, collectable, &Enum.into/2)

This pattern uses reduce to fill some collectable like a list, stream or map. The pattern can be used, as in ExDoc, to convert a keyword list into a map. The full pattern used by ExDoc, list_to_keyword_function) |> Enum.reduce(collectable, &Enum.into/2)


Mix has many great examples of reduce. In the following section I’ll describe them starting with the simplest and working up to more complex examples.

The first is a classic usage of reduce:

max = Enum.reduce docs, 0, fn({task, _}, acc) ->
  max(byte_size(task), acc)

The reduce considers all elements of docs and computes a single value - in this case the maximum byte size. This is the basic usage. More complex uses can build up a more complex data structure than just a value. In this next example we see an Enum.reduce call that builds up a list:

# From elixir/lib/mix/lib/mix/archive.ex:
Enum.reduce evsn ++ ebin ++ priv, [], fn(f, acc) ->
  case do
    {:ok, bin} ->
      [{Path.join(dir, f) |> String.to_char_list, bin}|acc]
    {:error, _} ->

In this case Enum.reduce is being used in a very similar way to; to convert one array into another using a mapping function. A simplified or generalized version might look like this:

Enum.reduce list, [], fn(element, acc) ->
  case some_test(f) do
    :ok    -> [ f(element) | acc ]
    :error -> acc

With the details removed it is easier to see that the major difference between and this use of Enum.reduce is that reduce allows Mix to skip the elements that fail the test. The :ok case is the same as map while the :error case leaves the accumulated value unchanged.

So, the pattern here is

Enum.reduce can be used as a filtering version of

Our final example is another function from Mix which uses a beautiful, nested reduce:

# From elixir/lib/mix/lib/mix/compilers/elixir.ex
defp mtimes(entries) do
  Enum.reduce(entries,, fn {_b, _m, source, _d, files, _bin}, dict ->
    Enum.reduce([source|files], dict, fn file, dict ->
      if HashDict.has_key?(dict, file) do
        HashDict.put(dict, file, Mix.Utils.last_modified(file))

To understand this function let’s look first at the inner reduce:

Enum.reduce([source|files], dict, fn file, dict ->
  if HashDict.has_key?(dict, file) do
    HashDict.put(dict, file, Mix.Utils.last_modified(file))

This Enum.reduce builds up dict by inserting files as keys but only if the file is not already present in dict. This is like the reduce into pattern we saw before but uses a filter (which seems to be a common practice in Mix).

The outer reduce does two things

  1. It creates a new HashDict to be used as the initial dict in the inner reduce.
  2. It reduces over all entries which must be a list (or stream) of tuples which contain the files. The structure of the tuple is not important to the underlying pattern.

So the two patterns I see here are:

Enum.reduce can be used as a filtering version of Enum.into


Nested Enum.reduce calls can be used to accumulate a data structure over multiple steps.

If you like this post then you should know that I'm writing a book full of patterns like these called Idiomatic Elixir.

If you inerested in the book then sign up below to follow its progress and be notified when it is launched. You'll also receive two free chapters as a sample of what the book will contain.

Along the way you will also receive Elixir tips based on my research for the book.

Next Steps

When I started writing this post I alredy knew how to use reduce but in looking over these examples and extracting generalized patterns I’ve learned many new ways of using reduce that I hope to be able to apply in my own code in the future. I hope you have also learned something from reading my analysis and description of the patterns.

Next week I plan to continue with this style of analysis and will look for patterns with Streams in Elixir.