// Todo service backed by a typed state machine (sum type TodoStatus).
// Each todo transitions Pending -> InProgress -> Done, or * -> Cancelled.

cell type TodoStatus {
    variants { Pending  InProgress  Done  Cancelled }
}

cell TodoService {
    face {
        signal request(method: String, path: String, body: String) -> Map
        signal add(text: String) -> Map
        signal start(id: String) -> Map
        signal finish(id: String) -> Map
        signal cancel(id: String) -> Map
        signal show(id: String) -> Map
        promise all_persistent
    }

    memory {
        todos: Map<String, Map> [persistent, consistent]
    }

    state todo: TodoStatus {
        initial: Pending
        Pending    -> InProgress
        InProgress -> Done
        *          -> Cancelled
    }

    on add(text: String) {
        let id = to_string(next_id())
        todos.set(id, map("id", id, "text", text))
        return map("id", id, "status", get_status(id))
    }

    on start(id: String) {
        if todos.get(id) == () { return response(404, map("error", "not found")) }
        transition(id, InProgress)
        return map("id", id, "status", get_status(id))
    }

    on finish(id: String) {
        if todos.get(id) == () { return response(404, map("error", "not found")) }
        transition(id, Done)
        return map("id", id, "status", get_status(id))
    }

    on cancel(id: String) {
        if todos.get(id) == () { return response(404, map("error", "not found")) }
        transition(id, Cancelled)
        return map("id", id, "status", get_status(id))
    }

    on show(id: String) {
        let t = todos.get(id)
        if t == () { return response(404, map("error", "not found")) }
        return map("id", id, "text", t.text, "status", get_status(id))
    }

    on request(method: String, path: String, body: String) {
        let req = map("method", method, "path", path)
        match req {
            {method: "POST", path: "/todos"}            -> add(from_json(body).text)
            {method: "POST", path: "/start/" + id}      -> start(id)
            {method: "POST", path: "/finish/" + id}     -> finish(id)
            {method: "POST", path: "/cancel/" + id}     -> cancel(id)
            {method: "GET",  path: "/todos/" + id}      -> show(id)
            _                                           -> response(404, map("error", "no route"))
        }
    }
}

cell test TodoServiceTest {
    rules {
        assert add("write tests").status == "Pending"
        assert start("1").status == "InProgress"
        assert finish("1").status == "Done"
        assert show("1").text == "write tests"
        assert cancel(add("scrap").id).status == "Cancelled"
    }
}