Truly abstract interfaces for algebraic data types: the extractor typing problem

Pattern matching enables inspecting algebraic data types, but typically prevents hiding the implementation of the matched algebraic data type. In Scala, instead, extractors also allow pattern matching on non-algebraic data types and invoking methods on the obtained objects, while partially decoupling API consumers from the API implementation. But as we show in this paper, pattern matching using extractors is restricted compared to matching against case classes. We argue this violates the appropriate variant of the uniform access principle. To address this problem, we propose a small language extension, which enables defining truly abstract interfaces and freely evolve their implementation.