download.ex

defmodule Download do
  use Pipe

  def start(api_key) do
    api_key
    ~> download()
    ~> parse()
  end

  defp download(123), do: {:ok, "valid result"}
  defp download(_), do: {:error, "invalid API key"}

  defp parse({:ok, message}), do: %{message: message, downloaded_at: Time.utc_now()}
end

Download.start(123)
#=> %{downloaded_at: ~T[10:53:31.643047], message: "valid result"}

Download.start("invalid") |> IO.inspect()
#=> {:error, "invalid API key"}

pipe.ex

defmodule Pipe do
  defmacro __using__(_) do
    quote do
      require unquote(__MODULE__)

      import unquote(__MODULE__)
    end
  end

  defmacro left ~> right do
    quote do
      case unquote(left) do
        {:error, error} ->
          {:error, error}

        result ->
          result |> unquote(right)
      end
    end
  end
end