Various Java APIs depend on extension of abstract base classes rather than interfaces
"proxy" has limitations (no access to protected fields or super)
"proxy" has performance overhead because of an extra layer of functions / parameter boxing etc.
"gen-class" is complex and is complected with compilation / bytecode generation
In summary: Clojure does not currently have a good / convenient way to extend a Java abstract base class dynamically.
The proposal is to create a variant of "reify" that allows the extension of a single abstract base class (optionally also with interfaces/protocols). Code generation would occur as if the abstract base class had been directly extended in Java (i.e. with full access to protected members and with fully type-hinted fields).
Since this is a JVM-only construct, it should not affect the portable extension methods in Clojure (deftype etc.). We propose that it is placed in an separate namespace that could become the home for other JVM-specific interop functionality, e.g. "clojure.java.interop"
Proposed solution: The attached patch proposes an implementation for this feature, providing a new `clojure.interop` namespace, containing the `defclass` and `extend-class` macros.