User-Level Language Crafting: Introducing the CLOS Metaobject Protocol

Introduction The idea of open and modular systems is becoming more and more popular in the areas of networking and operating systems. In the former, services like packet transfer may be implemented in different ways without affecting the rest of the system [1]. In operating systems, attempts are made to open functions such as memory paging up to change [2]. CLOS carries this idea into the realm of language design which has traditionally been almost as closed as database implementations. There are many reasons why language implementations should be open. One important reason is the ever increasing complexity of software development. Its management requires correspondingly more sophisticated tools which must obtain detailed language-internal information, such as class structure or information about methods. Traditionally designed languages often require implementation-specific modifications to compilers or run-time environments which are non-portable because that information is