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predicate_lookups

This example illustrates the predicate declaration and predicate definition lookup algorithms used when sending a message to an object. For full details, see the Handbook section on inheritance.

The lookup algorithms differ for instances and for prototypes and also depend if the lookup is for a predicate declaration or for a predicate definition. Sending a message to an object, requires two predicate lookups (performed at compile-time, when possible):

Lookup the predicate declaration to check that the predicate in within the scope of the sender. In the most common cases, this means that the predicate is declared public.
Assuming that the predicate exists and is within scope, lookup the predicate definition to answer the message. If none found, the message simply fails as per the Closed World Assumption (CWA).
See the comments in the prototypes.lgt and classes.lgt source files for further details.

Print Logtalk, Prolog backend, and kernel versions (if running as a notebook):
```
%versions
```
Start by loading the example:
```
logtalk_load(predicate_lookups(loader)).
```
When sending a message to a prototype, the lookup for the predicate declaration starts at the prototype itself; therefore, the following message is valid:
```
bike::frame(Material).
```


When a prototype doesn't contain a declaration for the predicate in the message, the lookup continues in the prototype parent(s):
```
bike::where(Where).
```


When sending a message to a prototype, the lookup for the predicate definition starts at the prototype itself:
```
mountain_bike::frame(Material).
```


When a prototype doesn't contain a definition for the predicate in the message, the lookup continues in the prototype parent(s):
```
mountain_bike::crewed.
```
We can use the built-in reflection predicates to query about predicate declarations and definitions.

Public predicates of the mountain_bike object:
```
%%table
mountain_bike::current_predicate(Predicate).
```


Properties of the mountain_bike object frame/1 predicate:
```
%%table
mountain_bike::predicate_property(frame(_), Property).
```


When sending a message to an instance, the lookup for the predicate declaration starts at the instance class; therefore, the following message is valid:
```
paraglider::structure(Structure).
```


Note that the lookup for the predicate definition starts in the instance itself, resulting in the structure(soft) answer;

When the instance class doesn't contain a declaration for the predicate in the message, the lookup continues in the class superclasses; therefore, the following message is also valid:
```
sailplane::purpose(Purpose).
```


When the instance doesn't contain a definition for the predicate in the message, the lookup continues in the class and, if not found there, in the class superclasses:
```
sailplane::structure(Structure).
```


Public predicates of the sailplane object:
```
%%table
sailplane::current_predicate(Predicate).
```


Properties of the sailplane object purpose/1 predicate:
```
%%table
sailplane::predicate_property(purpose(_), Property).
```