Elm-Style TUI

record Model {value: Int, running: Bool}


union Msg:
	Increment
	Decrement
	Reset
	Quit
	Ignore


func run_app[State, Message](
	initial: State,
	parse_event: pure (String) -> Message,
	step: pure (State, Message) -> State,
	render: pure (State) -> String,
	active: pure (State) -> Bool,
) -> Void:
	-- This shell owns all terminal I/O.
	var state: State = initial
	var eof: Bool = False
	print(render(state))
	while active(state) and not eof:
		match input("cmd> "):
			Some(raw):
				-- The application callbacks are pure, so input and output cannot leak into update or view.
				state = step(state, parse_event(raw))
				print(render(state))
			None:
				eof = True


pure func init() -> Model:
	{value = 0, running = True}


pure func parse_msg(raw: String) -> Msg:
	match raw.trim():
		"+": Increment
		"-": Decrement
		"0": Reset
		"q": Quit
		_: Ignore


pure func update(model: Model, msg: Msg) -> Model:
	-- Every message returns the next model instead of mutating global state.
	match msg:
		Increment:
			{ model | value = model.value + 1 }
		Decrement:
			{ model | value = model.value - 1 }
		Reset:
			{ model | value = 0 }
		Quit:
			{ model | running = False }
		Ignore:
			model


pure func is_running(model: Model) -> Bool:
	model.running


pure func view(model: Model) -> String:
	status: String = if model.running:
		"running"
	else:
		"done"
	lines: List[String] = [
		"Counter TUI (" + status + ")",
		"value: " + model.value.to_string(),
		"+ increment | - decrement | 0 reset | q quit",
	]
	lines.join("\n")


func main(args: List[String]) -> Void:
	run_app(init(), parse_msg, update, view, is_running)

printf '+\n+\n-\nq\n' | blorp run todo-tui.brp