2.7.2 Semantics

Method lookup in Cecil uses a form of Touretzky's inferential distance heuristic [Touretzky 86], where children override parents. The method lookup rules interpret a program's inheritance graph as a partial ordering on objects, where being less in the partial order corresponds to being more specific: an object A is less than (more specific than) another object B in the partial order if and only if A is a proper descendant of B. This ordering on objects in turn induces an analogous ordering on the set of methods specialized on the objects, reflecting which methods override which other methods. In the partial ordering on methods with a particular name and number of arguments, one method M is less than (more specific than) another method N if and only if each of the argument specializers of M is equal to or less than (more specific than) the corresponding argument specializer of N. Since two methods cannot have the same argument specializers, at least one argument specializer of M must be strictly less than (more specific than) the corresponding specializer of N. An unspecialized argument is considered specialized on the any object which is an ancestor of all other objects; a specialized argument therefore is strictly less than (more specific than) an unspecialized argument. The ordering on methods is only partial since ambiguities are possible.

Given the partial ordering on methods, method lookup is straightforward. For a particular message send, the system constructs the partial ordering of methods with the same name and number of arguments as the message. The system then throws out of the ordering any method that has an argument specializer that is not equal to or an ancestor of the corresponding actual argument passed in the message; such a method is not applicable to the actual call. Finally, the system attempts to locate the single most-specific method remaining, i.e., the method that is least in the partial order over applicable methods. If no methods are left in the partial order, then the system reports a "message not understood" error. If more than one method remains in the partial order, but there is no single method that overrides all others, then the system reports a "message ambiguous" error. Otherwise, there is exactly one method in the partial order that is strictly more specific than all other methods, and this method is returned as the result of the message lookup.