🚀 go-pugleaf

In article <2371@skye.ed.ac.uk>, ken@aiai.ed.ac.uk (Ken Johnson) writes:
> THIS IS an interesting data set if you want to give your students a
> ``real'' programming problem.

Ken Johnson is one of my favourite people, and it's nice to have good
examples to play with, _BUT_

The tuples in his data base have the form
	> % station(Name, X_coord, Y_coord, Lines)
 
	> % X_coord and Y_coord are centimetres from the bottom left hand
	> % corner of the London Underground map and have little
	> % geographical significance.

	> % Lines is a list of London Underground lines that pass through
	> % the Station.

The first thing that hits a Codd fan in the eye is that this relation is
not in 1st Normal Form:  the Lines attribute is not atomic with respect to
likely applications (e.g. if one wants to know whether two stations are on
the same line, and if so which).

But that was actually the second thing that hit me.  The first thing that
_really_ hit was "I can't use this".  I have often travelled on the London
underground, and the two questions I have been vitally concerned with are
    (1) how many stops before I get off?
    (2) where should I change?
Basically, the kind of information you need for this is "how are the
stations connected".

It seems to me that a better structure for this data base would be

	station(StationName, X, Y).
	-- StationName is the name of a station at (X,Y) on the map.

	line(FromStation, ToStation, LineName)
	-- you can get from FromStation to ToStation in one step
	-- by travelling along the line called LineName

Then one could define

	station_line(Station, Line) :-
		line(Station, _, Line).
	station_line(Station, Line) :-
		line(_, Station, Line).

	kj_station(Station, X, Y, Lines) :-
		station(Station, X, Y),
		setof(Line, station_line(Station,Line), Lines).

A richer data base for the London underground would also include
the fares, or rather the zoning structure underlying the fares.
I've forgotten how that would go for London.  In Melbourne the
fare structure for the Met would go something like

	station_zone(Station, /* -> */ Zone /* 1, 2, or 3 */).

	zone_card(FromZone, ToZone, CardType).
	-- you can travel from FromZone to ToZone if you hold a
	-- current card of type CardType (e.g. '2/1 weekly')
	-- This is symmetric in FromZone, ToZone

	card_price(CardType, /* -> */ Price).