Extensible records in the System E Framework and a new approach to object-oriented type inference

University of Technology, Sydney. Faculty of Engineering and Information Technology.Extensible records were proposed by Wand as a foundation for studying object-oriented type inference. One of their key benefits is that they allow for an elegant encoding of object-oriented inheritance, where one class of objects may be defined as an extension of another class of objects. However, every system of type inference designed for extensible records to date has been developed in the Hindley/Milner-style, a consequence being that polymorphism in these systems is not first-class and analysis is not strictly compositional. We argue that both of these features are necessary to retain the modelling and engineering benefits of traditional object-oriented languages such as Java: 1. Object-oriented modelling depends on the treatment of objects as first-class citizens, and this demands a type inference system capable of handling first-class polymorphism. 2. Object-oriented engineering encourages the separate development of software modules, and this should be supported by the type inference system with compositional analysis. Both of these features are present in a type system for the λ-calculus called System E, which supports first-class polymorphism via intersection types, and compositional type inference via expansion variables. However, research into System E has so far focused on refining and simplifying the formulation of expansion variables and exploring type inference algorithms with various properties. Meanwhile, the system has not yet been extended beyond the terms of the pure λ-calculus and it lacks many features that would be needed in a practical object-oriented language. In this dissertation, we combine System E with Wand’s extensible records resulting in a new approach to type inference for extensible records that better preserves the modelling and engineering benefits of object orientation stated above. The resulting system, called System Evcr, is significant because previous type inference systems, both for extensible records in particular, and also for object orientation in general, have at best preserved only one or the other of these two benefits, but never both of them simultaneously. System Evcr also makes a significant contribution to the work on System E, since it demonstrates for the first time that the System E’s expansion variables can be adapted to analyse programs whose term language extends beyond the pure λ-calculus. To demonstrate the potential use of System Evcr in object-oriented type inference, an implementation of our type inference algorithm was created and is shown to succeed on problem examples that previous systems either fail to analyse, or else fail to analyse compositionally

Less-Java, more type safety: Type inference and static analysis in Less-Java

Less-Java is an object-oriented programming language whose primary goal is to help new programmers learn programming. Some of the features of Less-Java that might make it better for beginners are static typing, implicit typing, low verbosity, and built-in support for unit testing. The primary focus of this project is on improving type inference (especially with regards to object-oriented programming) and adding static analysis in the Less-Java compiler

Coinductive subtyping for abstract compilation of object-oriented languages into Horn formulas

In recent work we have shown how it is possible to define very precise type systems for object-oriented languages by abstractly compiling a program into a Horn formula f. Then type inference amounts to resolving a certain goal w.r.t. the coinductive (that is, the greatest) Herbrand model of f. Type systems defined in this way are idealized, since in the most interesting instantiations both the terms of the coinductive Herbrand universe and goal derivations cannot be finitely represented. However, sound and quite expressive approximations can be implemented by considering only regular terms and derivations. In doing so, it is essential to introduce a proper subtyping relation formalizing the notion of approximation between types. In this paper we study a subtyping relation on coinductive terms built on union and object type constructors. We define an interpretation of types as set of values induced by a quite intuitive relation of membership of values to types, and prove that the definition of subtyping is sound w.r.t. subset inclusion between type interpretations. The proof of soundness has allowed us to simplify the notion of contractive derivation and to discover that the previously given definition of subtyping did not cover all possible representations of the empty type

A Simple CPS Transformation of Control-Flow Information

We build on Danvy and Nielsen's first-order program transformation into continuation-passing style (CPS) to design a new CPS transformation of flow information that is simpler and more efficient than what has been presented in previous work. The key to simplicity and efficiency is that our CPS transformation constructs the flow information in one go, instead of first computing an intermediate result and then exploiting it to construct the flow information. More precisely, we show how to compute control-flow information for CPS-transformed programs from control-flow information for direct-style programs and vice-versa. As a corollary, we confirm that CPS transformation has no effect on the control-flow information obtained by constraint-based control-flow analysis. The transformation has immediate applications in assessing the effect of the CPS transformation over other analyses such as, for instance, binding-time analysis

Class Analysis of Object-Oriented Programs through Abstract Interpretation

We use abstract interpretation to define a uniform formalism for presenting and comparing class analyses for object-oriented languages. We consider three domains for class analysis derived from three techniques present in the literature, viz., rapid type analysis, a simple dataflow analysis and constraint-based 0-CFA analysis. We obtain three static analyses which are provably correct and whose abstract operations are provably optimal. Moreover, we prove that our formalisation of the 0-CFA analysis is more precise than that of the dataflow analysis

Precise set sharing and nullity analysis for java-style program

Finding useful sharing information between instances in object- oriented programs has been recently the focus of much research. The applications of such static analysis are multiple: by knowing which variables share in memory we can apply conventional compiler optimizations, find coarse-grained parallelism opportunities, or, more importantly,erify certain correctness aspects of programs even in the absence of annotations In this paper we introduce a framework for deriving precise sharing information based on abstract interpretation for a Java-like language. Our analysis achieves precision in various ways. The analysis is multivariant, which allows separating different contexts. We propose a combined Set Sharing + Nullity + Classes domain which captures which instances share and which ones do not or are definitively null, and which uses the classes to refine the static information when inheritance is present. Carrying the domains in a combined way facilitates the interaction among the domains in the presence of mutivariance in the analysis. We show that both the set sharing part of the domain as well as the combined domain provide more accurate information than previous work based on pair sharing domains, at reasonable cost