The Power of Map.take/2 in Elixir

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After working exclusively in elixir for 3 years now, I’ve been floored by the breadth of its standard library. Simply put, it’s delightful.

I’ve developed a loving connection to certain utilities in the standard library; Enum.any?/2, Map.pop/3, Enum.zip/1 to name a few. But there’s one you may not have used before that has made me very happy over the years. Map.take/2.

It’s a simple little function that doesn’t do much on the surface. The documentation is fairly unassuming. But, I’ll bet you can find a way to make your code more readable after seeing these examples.

Starting out in elixir I would commonly take some source map and put its values on another map conditionally. Usually, that condition would just be whether or not the source map had the key. I would find myself writing these kinds of functions:

def call do
  source = get_some_map()

  %{}
  |> maybe_add_foo()
  |> maybe_add_bar()
end

defp maybe_add_foo(map, %{foo: foo}) do
  Map.put(map, :foo, foo)
end

defp maybe_add_foo(map, _), do: map

defp maybe_add_bar(map, %{bar: bar}) do
  Map.put(map, :bar, bar)
end

defp maybe_add_bar(map, _), do: map

Now, there’s nothing wrong with the code above. But you could imagine that if you needed 2-3 more maybe_add_* functions, it could get pretty unwieldy.

Using Map.take/2 is so much simpler when you just need to check if a key exists.

def call do
  source = get_some_map()

  Map.take(source, [:foo, :bar])
end

When doing data transformations I’ll sometimes need to split a source map into 2 or more sub-maps. Without Map.take/2 your code might look like:

def call(args) do
  contact = split_contact_info(args)
  address = split_address_info(args)

  {contact, address}
end

defp split_contact_info(map) do
  Enum.reduce(map, %{}, fn
    {:phone, phone}, acc -> Map.put(acc, :phone, phone)
    {:email, email}, acc -> Map.put(acc, :email, email)
    _, acc -> acc
  end)
end

defp split_address_info(map) do
  Enum.reduce(map, %{}, fn
    {:line1, line1}, acc -> Map.put(acc, :line1, line1)
    {:line2, line2}, acc -> Map.put(acc, :line2, line2)
    {:city, city}, acc -> Map.put(acc, :city, city)
    {:state, state}, acc -> Map.put(acc, :state, state)
    {:postal_code, postal_code}, acc -> Map.put(acc, :postal_code, postal_code)
    {:country, country}, acc -> Map.put(acc, :country, country)
    _, acc -> acc
  end)
end

Again, using Map.take/2 is so much simpler.

def call(args) do
  contact = Map.take(args, [:phone, :email])
  address = Map.take(args, [:line1, :line2, :city, :state, :postal_code, :country])

  {contact, address}
end

Often I’ve found myself writing a function where I want to be ultra-defensive about what I allow to be passed in. When there’s only a few arguments, passing them positionally works well:

def call(name, email) do
  # Process something with name and email
end

With a few more arguments, I’ll pass a map to the function and use Map.take/2 to only allow the values I want. I’ve found this especially helpful when talking to external APIs.

Without Map.take/2 the reader has to keep the context of a reduce loop in their head:

def call(params) do
  params = allowed_params(params)

  # Process something with params now sanitized
end

def allowed_params(params) do
  Enum.reduce(map, %{}, fn
    {:name, name}, acc -> Map.put(acc, :name, name)
    {:email, email}, acc -> Map.put(acc, :email, email)
    {:phone, phone}, acc -> Map.put(acc, :phone, phone)
    {:country, country}, acc -> Map.put(acc, :country, country)
    {:message, message}, acc -> Map.put(acc, :message, message)
    _, acc -> acc
  end)
end

Map.take/2 shines again with its readability. Making the code declarative like this removes overhead from the reader, freeing their mind from the reduce loop logic.

def call(params) do
  params = Map.take(params, [:name, :email, :phone, :country, :message])

  # Process something with params now sanitized
end

Hopefully, you can start to appreciate the readability that Map.take/2 can provide. For me, it sparks a little joy each time I use it because I know the code it’s saving me from writing, documenting, and testing ❤️.

elixir