Polymorphic Templates: A design pattern for implementing agile model-to-text transformations

Model-to-text transformations are often used to produce source code, documentation or other textual artefacts from models. A common way of implementing them is using template languages. Templates are easy to read and write, however, they tend to become long and complex as the complexity of the metamodel grows. This paper proposes a design pattern that allows for the decomposition of complex templates with branching and conditions inside into simpler ones. Its main idea is that the code generator does not know about the concrete templates that are called: they are determined by the objects of the model being traversed. The concrete template is selected through object-oriented polymorphism. The pattern results in a flexible code generator with simple templates, good extensibility and separation of concerns. This agility facilitates the design for extension and changes, which is paramount nowadays.