Compositional Language Engineering Using Generated, Extensible, Static Type-Safe Visitors

Language workbenches usually produce infrastructure to represent models as abstract syntax trees (AST) and employ processing infrastructure largely based on visitors. The visitor pattern suffers from the expression problem regarding extensibility and reuse. Current approaches either forsake static type safety, require features unavailable in popular object-oriented languages (e.g., open classes), or rely on procedural abstraction and thereby give up the object-oriented data encapsulation (the AST) itself. Our approach to visitors exploits knowledge about the AST and generation of statically type-safe external visitor interfaces that support extensibility in two dimensions: (1) defining new operations by implementing the interface and (2) extending the underlying data structure, usually without requiring adaptation of existing implemented visitors. We present a concept of visitor development for language engineering that enables an adaptable traversal and provides hook points for implementing concrete visitors. This approach is applicable to single DSLs and to language composition. It thus enables a transparent, easy to use, and static type-safe solution for the typical use cases of language processing.