3.2.1 The rules

Two ways

In Prolog+CG, we have two ways of saying that a type has an instance:

Like types, instances must be identifiers.

How to write a catalog of individuals

A catalog of individuals for a given type is written like this:

Type = Instance1, Instance2, ..., InstanceN.

Here, a number of instances (on the right-hand side) are declared to be instances of a specific type (on the left-hand side).

This rule is then repeated for each type that has instances.

This is similar to type-hierarchies

Notice how this is similar to the recipe for type-hierarchies.

The differences include:

  • There is an "=" (equal sign) rather than a ">" (greater-than) after the first type.
  • The list on the right-hand-side lists instances of the type on the left-hand-side, not subtypes of it.


Next, we give some examples.

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