pleroma/lib/pleroma/web/activity_pub/mrf.ex

defmodule Pleroma.Web.ActivityPub.MRF do
  @callback filter(Map.t()) :: {:ok | :reject, Map.t()}

  def filter(object) do
    get_policies()
    |> Enum.reduce({:ok, object}, fn
      policy, {:ok, object} ->
        policy.filter(object)

      _, error ->
        error
    end)
  end

  def get_policies() do
    Application.get_env(:pleroma, :instance, [])
    |> Keyword.get(:rewrite_policy, [])
    |> get_policies()
  end

  defp get_policies(policy) when is_atom(policy), do: [policy]
  defp get_policies(policies) when is_list(policies), do: policies
  defp get_policies(_), do: []
end