Prettify Ecto errors

ecto_helper.ex

defmodule EctoHelper do
  @moduledoc """
  Provides helper functions
  """

  @doc """
  Prettifies changeset error messages.
  By default `changeset.errors` returns errors as keyword list, where key is name of the field
  and value is part of message. For example, `[body: "is required"]`.
  This method transforms errors in list which is ready to pass it, for example, in response of
  a JSON API request.

  ## Example of basic usage

  ```elixir
  EctoHelper.pretty_errors([body: "is required"]) # => ["Body is required"]
  ```

  ## Example of usage with interpolations

  ```elixir
  EctoHelper.pretty_errors([login: {"should be at most %{count} character(s)", [count: 10]}])
  # => ["Login should be at most 10 character(s)"]
  ```
  """
  @spec pretty_errors(Map.t) :: [String.t]
  def pretty_errors(errors) do
    errors
    |> Enum.map(&do_prettify/1)
  end

  defp do_prettify({field_name, message}) when is_bitstring(message) do
    human_field_name = field_name
                        |> Atom.to_string
                        |> String.replace("_", " ")
                        |> String.capitalize
    human_field_name <> " " <> message 
  end
  defp do_prettify({field_name, {message, variables}}) do
    compound_message = do_interpolate(message, variables)
    do_prettify({field_name, compound_message})
  end

  defp do_interpolate(string, [{name, value} | rest]) do
    n = Atom.to_string(name)
    msg = String.replace(string, "%{#{n}}", do_to_string(value))
    do_interpolate(msg, rest)
  end
  defp do_interpolate(string, []), do: string

  defp do_to_string(value) when is_integer(value), do: Integer.to_string(value)
  defp do_to_string(value) when is_bitstring(value), do: value
  defp do_to_string(value) when is_atom(value), do: Atom.to_string(value)
end

ecto_helper_spec.ex

defmodule EctoHelperTest do
  use ExUnit.Case, async: true

  test "prettify simple errors" do
    res = EctoHelper.pretty_errors([body: "is required"])
    assert ["Body is required"] = res
  end

  test "prettify errors with variables" do
    res = EctoHelper.pretty_errors([
      login: {"should be at most %{count} character(s)", [count: 10]},
      message_body: "should not be blank"
    ])
    assert String.equivalent?(Enum.at(res, 0), "Login should be at most 10 character(s)")
    assert String.equivalent?(Enum.at(res, 1), "Message body should not be blank")
  end
end