§13.12. Relations which express conditions

One last way to create a new relation and, in many ways, the easiest of all. If we write:

Contact relates a thing (called X) to a thing (called Y) when X is part of Y or Y is part of X. The verb to be joined to means the contact relation.

then we would be able to talk about a handle being joined to a door, and a door being joined to a handle, and so on. We are not allowed to declare:

The hook is joined to the line.

because the question of whether they are joined is not for us to decide: that will be for the condition to determine, whenever we test it. Similarly, we cannot meaningfully write

now the hook is joined to the line;

(and Inform will not let us) because this relation is not something we can force either way: we can make it come true by other means, maybe, but we cannot simply make it true by saying so. Lastly, this kind of relation is restricted in that we are not allowed to find paths or calculate numbers of steps through it.

So this way to define relations is, on the face of it, just a sort of verbal trick to write conditions in a more attractive way. The more flexible, changeable relations in previous sections have much greater expressive power. All the same, it is nice to be able to write -

Nearness relates a room (called A) to a room (called B) when the number of moves from B to A is less than 3. The verb to be near means the nearness relation.

and then to be able to write rules like:

Instead of listening when the location is near the Sundial: say "You hear a splashing of water."

As with other relations, there's no reason why we have to use objects. For example:

Material is a kind of value. The materials are wood and metal. A thing has a material.

Materiality relates a thing (called X) to a material (called Y) when Y is the material of X. The verb to be made of means the materiality relation.

which enables us to write:

if the cube is made of wood, ...
say "The carpenter looks at [the list of things which are made of wood].";

And here is a mathematical one:

Divisibility relates a number (called N) to a number (called M) when the remainder after dividing M by N is 0. The verb to divide means the divisibility relation. The verb to be a factor of means the divisibility relation.

We now find that "2 divides 12", "5 is not a factor of 12" and "12 is divisible by 3" are all true. Again, we are only really gaining a nice form of words, but improving the clarity of the source text is never a bad thing.

 ExampleWainwright ActsA technical note about checking the location of door objects when characters other than the player are interacting with them.

 ExampleA Humble Wayside FlowerRelations track the relationships between one character and another. Whenever the player meets a relative of someone he already knows, he receives a brief introduction.