Overview

This chapter describes techniques for sealing portions of a Dylan program by declaring that classes or functions will never be used in particular ways, or will never be extended in particular ways. These sealing directives enable a range of compiler optimizations, and also clarify the programmer's intent.

The sealing directives include:

Declaring a class to be sealed or open. This controls whether a class can be directly subclassed outside the library in which it is defined.
Declaring a class to be abstract or concrete. This controls whether a class can have direct instances.
Declaring a class to be primary or free. This controls how a class can be used for multiple inheritance.
Declaring a generic function to be sealed or open. This controls whether methods can be added to the generic function from outside the library in which the generic function is defined.
Using define sealed domain, or using the abbreviations define sealed method and sealed slot. These disallow the addition of some methods from outside the library in which the generic function is defined.

With the exception of define sealed domain, these directives are expressed as adjectives on the generic function definition, class definition, method definition, or slot specification.