Rename all map keys

Hi! I have a list of maps and I want to rename the keys.

What I have:

[info] entities: [ok: %{"Email Address [Required]" => "", "First Name [Required]" => "Blue", "Last Name [Required]" => "First"}, ok: %{"Email Address [Required]" => "", "First Name [Required]" => "Red", "Last Name [Required]" => "Second"}, ok: %{"Email Address [Required]" => "", "First Name [Required]" => "Yellow", "Last Name [Required]" => "Third"}]

What I want to achieve:

[info] entities: [ok: %{"email" => "", "first_name" => "Blue", "last_name" => "First"}, ok: %{"email" => "", "first_name" => "Red", "last_name" => "Second"}, ok: %{"email" => "", "first_name" => "Yellow", "last_name" => "Third"}]

First Name [Required] to first_name
Last Name [Required] to last_name
Email Address [Required] to email

I am not sure how will I work on the keys with all the white space and the [] brackets.

You might remove [Required] with String.replace + trim. You might even transform “LastName” to “last_name” with Macro.underscore.

But how do You want him to translate “Email Address” to “email”?

Unless You use some custom rules…


for "Last Name [Required]""last_name"

key |> String.trim(" [Required]") |> String.downcase() |> String.replace(" ", "_")

for "Email Address [Required]""email", you might just pattern match on the special case in a multiple function clause

defp rename_key({"Email Address [Required]", val}), do: {"email", val}
defp rename_key({key, val}) do
  # operate on the key
  {new_key, val}

Here is a complete code:

defmodule Example do
  # First of all we call map on our keyword
  def sample(keyword) when is_list(keyword), do:, &rename/1)

  # When key is ok atom and value is a map call rename function
  defp rename({key = :ok, map}) when is_map(map), do: {key, rename(map)}

  # Rename for maps is using for comprehension changing only key and keeping value as is
  defp rename(map) when is_map(map) do
    for {key, value} <- map, into: %{}, do: {rename(key), value}

  # Rename for binary is removing trailing string and calling rename_key
  defp rename(binary) when is_binary(binary) do
    binary |> String.trim_trailing(" [Required]") |> rename_key()

  # A simple pattern-match in rename_key for a special email case
  defp rename_key("Email Address"), do: "email"

  # rename_key for any other string
  defp rename_key(key) when is_binary(key) do
    key |> String.replace(" ", "") |> Macro.underscore()

expected_output = [
  ok: %{
    "email" => "",
    "first_name" => "Blue",
    "last_name" => "First"
  ok: %{
    "email" => "",
    "first_name" => "Red",
    "last_name" => "Second"
  ok: %{
    "email" => "",
    "first_name" => "Yellow",
    "last_name" => "Third"

input = [
  ok: %{
    "Email Address [Required]" => "",
    "First Name [Required]" => "Blue",
    "Last Name [Required]" => "First"
  ok: %{
    "Email Address [Required]" => "",
    "First Name [Required]" => "Red",
    "Last Name [Required]" => "Second"
  ok: %{
    "Email Address [Required]" => "",
    "First Name [Required]" => "Yellow",
    "Last Name [Required]" => "Third"

result = Example.sample(input)
IO.puts(result == expected_output)
# true

Helpful resources:

  1. Kernel.is_list/1
  2. Kernel.is_map/1
  4. Kernel.SpecialForms.for/1
  5. String.replace/4
  6. String.trim_trailing/2
  7. Macro.underscore/1
  8. Patterns and Guards

The simplest way to accomplish what you’ve written is to look up replacement keys in a supplied map. For instance:

defmodule KeyRenamer do
  @key_replacements %{
    "First Name [Required]" => "first_name",
    "Last Name [Required]" => "last_name",
    "Email Address [Required]" => "email"

  def rename_keys(map) do, fn k, v ->
      new_key = Map.get(@key_replacements, k, k)
      {new_key, v}

An alternative approach would be to fix the code that’s generating these keys - for instance, if it’s extracting a value from an HTML label maybe it should be using the input's name or something instead…


If the keys are known and static, and you don’t need to make this logic reusable, you could also use plain pattern-matching:

  def rename_keys(%{
    "First Name [Required]" => first_name,
    "Last Name [Required]" => last_name,
    "Email Address [Required]" => email
  }) do
    %{first_name: first_name, last_name: last_name, email: email}

I don’t mind being that guy that comments on OLD threads. I ran into a sexy way to rename some keys (or all if you want). I tried googling for it later and couldn’t find it. Google just keep sending me back to this thread. I eventually found the solution in some old code. So, I’m going to dump this solution here so next time I’ll find it faster. (and maybe it will help someone else)

To rename SOME of the keys:

bad_data = %{
  "lat" => 12.43244,
  long: -5.342,
  name: "something",
  age: 91

good_data =, fn
  {"lat", lat} -> {"latitude", lat}
  {:long, long} -> {:longitude, long}
  anything -> anything


:age => 91,
:longitude => -5.342,
:name => "something",
"latitude" => 12.43244

I mixed :keys and “keys” in the example to show it works with both.

good_data should now have the keys latitude, longitude, name, age. It might be cool if Hexdoc had a section showing some common uses that aren’t functions, but indexed like functions. Example “rename_key()” Don’t need the function because you can easily do it like (copy and paste example above)