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Start of Chapter 13: Relations |
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Back to §13.7. Relations in groups |
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Onward to §13.9. Defining new assertion verbs |
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§13.8. The built-in verbs and their meanings
It is all very well to define new relations, but this does nothing if there is no way to assert that they are true, or to ask whether they are true or false. That requires a verb: in fact, a relation is nothing more than what Inform uses as the "meaning" of a verb. The assertion verbs built in to Inform have the following built-in relations as their meanings:
Verb - Relation
to be - equality relation
to have - possession relation
to contain - containment relation
to support - support relation
to carry - carrying relation
to wear - wearing relation
to incorporate - incorporation relation
Two of Inform's built-in relations are expressed using prepositions instead:
Preposition - Relation
to be part of - (reversed) incorporation relation
to be adjacent to - adjacency relation
It would be easy to make verbs for these if we wanted ("to adjoin", say) using the techniques of the next section.
The verb to be is grammatically different from any other, and its meaning is too complicated to be fully expressed by any one relation. A great deal of the Inform program is given over to its "meaning", which we are not allowed to change or imitate. The "equality relation" is simple enough, and is the one implied by conditions like
but to be can have more complicated implications - "if Mr Wickham is hungry" clearly doesn't test whether two quantities are equal. Fortunately the other verbs are much simpler.
There are a few other built-in verbs, as can be seen in the Index, but these are mostly for experts only. For example:
Verb - Relation
to mean - meaning relation
to provide - provision relation
"To mean" can be used to make new verbs, as we'll soon see. Provision is to do with whether something can have a given property: for example, "if R provides the property lighted" tests whether R is able to have this property, not whether it actually has it at the moment.