Reflective program generation with patterns

Runtime reflection facilities, as present in Java and.NET, are powerful mechanisms for inspecting existing code and metadata, as well as generating new code and metadata on the fly. Such power does come at a high price though. The runtime reflection support in Java and.NET imposes a cost on all programs, whether they use reflection or not, simply by the necessity of keeping all metadata around and the inability to optimize code because of future possible code changes. A second—often overlooked—cost is the difficulty of writing correct reflection code to inspect or emit new metadata and code and the risk that the emitted code is not well-formed. In this paper we examine a subclass of problems that can be addressed using a simpler mechanism than runtime reflection, which we call compile-time reflection. We argue for a high-level construct called a transform that allows programmers to write inspection and generation code in a pattern matching and template style, avoiding at the same time the complexities of reflection APIs and providing the benefits of staged compilation in that the generated code and metadata is known to be well-formed and type safe ahead of time