Language Composition Untangled

In language-oriented programming and modeling, software developers are largely concerned with the definition of domain-specific languages (DSLs) and their composition. While various implementation techniques and frameworks exist for defining DSLs, language composition has not obtained enough attention and is not well-enough understood. In particular, there is a lack of precise terminology for describing observations about language composition in theory and in existing language-development systems. To clarify the issue, we specify five forms of language composition: language extension, language restriction, language unification, self-extension, and extension composition. We illustrate this classification by various examples and apply it to discuss the performance of different language-development systems with respect to language composition. We hope that the terminology provided by our classification will enable more precise communication on language composition.