Expressive and safe static reflection with MorphJ

Abstract

Recently, language extensions have been proposed for Java and C# to support pattern-based reflective declaration. These extensions introduce a disciplined form of meta-programming and aspectoriented programming to mainstream languages: They allow members of a class (i.e., fields and methods) to be declared by statically iterating over and pattern-matching on members of other classes. Such techniques, however, have been unable to safely express simple, but common, idioms such as declaring getter and setter methods for fields.In this paper, we present a mechanism that addresses the lack of expressiveness in past work without sacrificing safety. Our technique is based on the idea of nested patterns that elaborate the outer-most pattern with blocking or enabling conditions. We implemented this mechanism in a language, MorphJ. We demonstrate the expressiveness of MorphJ with real-world applications. In particular, the MorphJ reimplementation of DSTM2, a software transactional memory library, reduces 1,107 lines of Java reflection and bytecode engineering library calls to just 374 lines of MorphJ code. At the same time, the MorphJ solution is both high level and safer, as MorphJ can separately type check generic classes and catch errors early. We present and formalize the MorphJ type system, and offer a type-checking algorithm.

A variability-aware module system

“…• Finally, we can use any other implementation strategy, including runtime variability [44], sophisticated metaprogramming systems [26,31], and configuration management systems [10,32]. For most of these implementation mechanisms, no efficient means to type check all configurations is available yet.…”

Marco: Safe, Expressive Macros for Any Language

“…C++ concepts add contracts to templates [7]. MorphJ statically verifies some contracts so that expanded code will not have name-resolution conflicts [11]. Quail checks types between SQL queries and the database system [24].…”

Object reuse and behavior adaptation in Java-like languages

“…In [20] the language MorphJ offers the mechanism of nested patterns to implement a form of "class morphing", which enables safe static reflection over members of a type, in order to generate new classes in a meta-programming style. Generic classes are used to abstract over the structure of methods in other classes and they can be type checked separately from the classes that instantiate it, in a type safe way.…”

“…Generic classes are used to abstract over the structure of methods in other classes and they can be type checked separately from the classes that instantiate it, in a type safe way. The main point of similarity between [20] and our approach is the aim of integrating abstraction linguistic constructs into mainstream languages with static typing. However, such constructs are at the class level in MorphJ, whereas they work dynamically at object composition level in CompObJ.…”