Chapter 21

Records

Tour.step_back makes a new tour with a record update, and record updates run all through the ride. A Roc record stays as it was made, so a field with a new value means a new record.

The motorcycle is a record of four fields: its place on the route, which way it's turned, its speed and its lean. The rider's step uses this function to hold the bike at the end of the road:

Motorcycle.roc

## This motorcycle stopped dead at `at` on its segment.
stopped_at : Motorcycle, Along -> Motorcycle
stopped_at = |motorcycle, at| {
	place = motorcycle.route_place
	{
		..motorcycle,
		route_place: { ..place, road_point: { ..place.road_point, along: at } },
		speed: Speed.at_rest,
	}
}

Here's the function, piece by piece:

Code	Term	What it says
`{ ..motorcycle, … }`	record update	A new record with every field of `motorcycle`, except the ones named after it.
`route_place: { ..place, … }`	record update	The new `route_place` is an update of the old place in turn, and its `road_point` is an update of the old point. So the new motorcycle's `along` is `at`, three records deep.
`speed: Speed.at_rest`	field	A second field with a new value.

motorcycle itself stays as it was, and so do place and its road point. Every step of the ride builds its next moment as a new record, out of the old moment's fields.