Design principles for internal domain-specific languages: a pattern catalog illustrated by Ruby

Some programming languages, especially dynamic programming languages, offer suitable mechanisms for the construction of internal domain-specific languages (DSL). Internal DSLs inherit the facilities of their host language such as the availability of libraries, frameworks, tool support, and other DSLs. When developing an internal DSL, there are two challenges. First, to cope with the host language's syntactic and semantic restrictions. Careful and thoughtful extensions and modifications of the host language are crucial to overcome these restrictions without reverting to poor language design. Second, to support several design principles that are genuine for a DSL. Although there is an extensive body of knowledge about DSL design principles and desirable quality properties, it remains difficult to apply them, or to reason about whether a particular DSL exhibits specific desirable principles.

Our objective is to put the two perspectives together. We research the most important design principles of a DSL and show how different patterns can be used to support these principles. This allows us to produce an extensive pattern catalog which is the foundation of principled approach for designing internal DSLs. The patterns can be used to assess the design quality of a DSL and structure its implementation. In particular, we show a complex DSL example illustrating each principle and the corresponding patterns. While we stick to Ruby for the explanation and application of the patterns, we also name known uses in Python, Scala, and Smalltalk. Patterns are explained with their context, problem, forces, solution, and consequences. We also explain the patterns with the classical structure of intent, motivation, forces, implementation and their consequences. Finally we reflect upon the pattern utilization by discussing their strengths and weaknesses.