ok_then

my_register.exs

defmodule MyRegister do
  @enforce_keys [:email]

  defstruct [:email, valid?: true, errors: nil]

  def register_user_email(email) do
    %__MODULE__{email: email}
    |> ok_then(&ensure_not_nil/1)
    |> ok_then(&downcase/1)
    |> ok_then(&validate_format/1)
    |> ok_then(&validate_available/1)

    # The list can goes on:
    # |> ok_then(&do_register_user/1)
    # |> ok_then(&send_onboard_gift/1)
    # |> ok_then(&create_onboard_tasks/1)
  end

  defp ok_then(%{valid?: true} = ctx, f), do: f.(ctx)
  defp ok_then(ctx, _f), do: ctx

  def ensure_not_nil(%{email: email} = ctx) when is_binary(email) and email != "",
    do: ctx

  def ensure_not_nil(ctx),
    do: %{ctx | errors: "email is nil", valid?: false}

  def downcase(%{email: email} = ctx),
    do: %{ctx | email: String.downcase(email)}

  def validate_format(%{email: email} = ctx) do
    if email =~ ~r/.@.+\..+$/ do
      ctx
    else
      %{ctx | errors: "invalid email format", valid?: false}
    end
  end

  def validate_available(%{email: email} = ctx) do
    if unused_email?(email) do
      ctx
    else
      %{ctx | errors: "email has already been taken", valid?: false}
    end
  end

  defp unused_email?(_), do: true
end

MyRegister.register_user_email("foo") |> IO.inspect()
MyRegister.register_user_email("foo@zubale.com") |> IO.inspect()