Abstract
Out of a concern for focus and concision, domain-specific languages (DSLs) are usually very different from general purpose programming languages (GPLs), both at the syntactic and the semantic levels. One approach to DSL implementation is to write a full language infrastructure, including parser, interpreter or even compiler. Another approach however, is to ground the DSL into an extensible GPL, giving you control over its own syntax and semantics. The DSL may then be designed merely as an extension to the original GPL, and its implementation may boil down to expressing only the differences with it. The task of DSL implementation is hence considerably eased. The purpose of this chapter is to provide a tour of the features that make a GPL extensible, and to demonstrate how, in this context, the distinction between DSL and GPL can blur, sometimes to the point of complete disappearance.