A screen in a USSD flow.

defmodule MyApp.States.Welcome do
  use Ussd.State, initial: true

  alias Ussd.Decisions.Equal

  transition Equal.new("1"), to: MyApp.States.Airtime
  transition Equal.new("2"), to: MyApp.States.DataBundle

  @impl Ussd.State
  def render(_context) do
    Ussd.Menu.build()
    |> Ussd.Menu.line("Welcome")
    |> Ussd.Menu.listing(["Airtime", "Data Bundle"])
  end
end

use Ussd.State accepts:

• :initial - advisory metadata (default false) read by mix ussd.lint to auto-discover flow entry points when none are given explicitly. It is not enforced by Ussd.use_initial_state/2.
• :continue - when true, also requires @behaviour Ussd.ContinueState (a confirm/0 callback), and marks this state eligible as the continuing_state argument to Ussd.use_continuing_state/4.

Inside the module body, five macros build the state's routing:

• transition(decision, to: Module, callback: fun \\ nil) - repeatable; the first one whose decision matches the input wins.
• back(decision, callback: fun \\ nil) - returns to whatever state pushed this one onto the session's back-stack.
• paginate(next: decision, previous: decision \\ nil, callback: fun \\ nil) - requires use Ussd.Pagination in the same module.
• truncate(limit: n, ending: string, more: decision) - caps how much of render/1's output is returned per screen.
• terminate() - marks this state as ending the session once reached with no further input expected.

A state with none of these raises Ussd.Exceptions.NextStateNotFoundError the moment it's reached with input that doesn't match anything - mix ussd.lint finds these before your users do.