Guard Composition

Sometimes it can be useful to call guards from another guard. For example, these two guards:

constexpr auto is_small = maki::guard_e([](const some_event& evt)
{
    return evt.size < 10;
});
 
constexpr auto is_large = maki::guard_e([](const some_event& evt)
{
    return evt.size > 1000;
});

... can be composed like so:

constexpr auto is_average = maki::guard_e([](const some_event& evt)
{
    return !is_small.callable(evt) && !is_large.callable(evt);
});

But this is a bit verbose and inconvenient.

Fortunately, Maki allows you to compose guards using boolean operators (!, &&, || and !=). For example:

constexpr auto is_average_2 = !is_small && !is_large;

You can also inline such expressions in transition tables:

constexpr auto transition_table = maki::transition_table{}
    (maki::init, some_state)
    (some_state, some_other_state, maki::event<some_event>, some_action, !is_small && !is_large)
;