Here’s what I have… useful for formatting the changeset errors into a string for logging etc.

def changeset_error_to_string(changeset) do
  Ecto.Changeset.traverse_errors(changeset, fn {msg, opts} ->
    Enum.reduce(opts, msg, fn {key, value}, acc ->
      String.replace(acc, "%{#{key}}", to_string(value))
    end)
  end)
  |> Enum.reduce("", fn {k, v}, acc ->
    joined_errors = Enum.join(v, "; ")
    "#{acc}#{k}: #{joined_errors}\n"
  end)
end

It’ll return a string like

"password: Not enough special characters (only 0 instead of at least 1); Not enough numbers characters (only 0 instead of at least 1); Not enough upper_case characters (only 0 instead of at least 1)\n"

I had to add a case for when the traversed error value came as an error:

defp _to_string(val) when is_list(val) do
    Enum.join(val, ",")
  end
  defp _to_string(val), do: to_string(val)

Full implementation:

defmodule Yourapp.Ecto.Errors do

  def mapper(%Ecto.Changeset{} = changeset) do
    IO.inspect(changeset)
    Ecto.Changeset.traverse_errors(changeset, fn {msg, opts} ->
      Enum.reduce(opts, msg, fn {key, value}, acc ->
        String.replace(acc, "%{#{key}}", _to_string(value))
      end)
    end)
    |> Enum.reduce("", fn {k, v}, acc ->
      joined_errors = Enum.join(v, "; ")
      "#{acc} #{k}: #{joined_errors}"
    end)
  end
  def mapper(changeset), do: changeset

  defp _to_string(val) when is_list(val) do
    Enum.join(val, ",")
  end
  defp _to_string(val), do: to_string(val)
end

traverse_errors/2 might help out