18.4 Multiple inheritance and method dispatch

Now that we have seen how Dylan constructs the class precedence list, we return to the issue of how multiple inheritance affects method dispatch. Recall that, when a generic function is called, Dylan chooses the most specific applicable method to call. For simplicity, let's consider a generic function that has one specialized parameter. As we have seen, Dylan chooses which method to dispatch by comparing the type of the required argument to the generic function with the type of the corresponding specialized parameter for each method, using the following procedure:

1. Find all the applicable methods. A method is applicable if the required argument is an instance of the type of the specialized parameter.

2. Sort the applicable methods in order of specificity. A method is more specific than another if the type of its specialized parameter is a proper subtype of the type of the other method's specialized parameter.

3. Call the most specific method.

In the presence of multiple inheritance, it is possible to have two or more methods that are applicable, but that cannot be sorted by specificity because neither parameter type is a subtype of the other. By following only the rules that we have seen so far, Dylan cannot choose either method to call.