Language's Type System Research Articles

Effects and Coeffects in Call-by-Push-Value

Effect and coeffect tracking integrate many types of compile-time analysis, such as cost, liveness, or dataflow, directly into a language's type system. In this paper, we investigate the addition of effect and coeffect tracking to the type system of call-by-push-value (CBPV), a computational model useful in compilation for its isolation of effects and for its ability to cleanly express both call-by-name and call-by-value computations. Our main result is effect-and-coeffect soundness, which asserts that the type system accurately bounds the effects that the program may trigger during execution and accurately tracks the demands that the program may make on its environment. This result holds for two different dynamic semantics: a generic one that can be adapted for different coeffects and one that is adapted for reasoning about resource usage. In particular, the second semantics discards the evaluation of unused values and pure computations while ensuring that effectful computations are always evaluated, even if their results are not required. Our results have been mechanized using the Coq proof assistant.

Intrinsically-typed definitional interpreters for imperative languages

A definitional interpreter defines the semantics of an object language in terms of the (well-known) semantics of a host language, enabling understanding and validation of the semantics through execution. Combining a definitional interpreter with a separate type system requires a separate type safety proof. An alternative approach, at least for pure object languages, is to use a dependently-typed language to encode the object language type system in the definition of the abstract syntax. Using such intrinsically-typed abstract syntax definitions allows the host language type checker to verify automatically that the interpreter satisfies type safety. Does this approach scale to larger and more realistic object languages, and in particular to languages with mutable state and objects? In this paper, we describe and demonstrate techniques and libraries in Agda that successfully scale up intrinsically-typed definitional interpreters to handle rich object languages with non-trivial binding structures and mutable state. While the resulting interpreters are certainly more complex than the simply-typed λ-calculus interpreter we start with, we claim that they still meet the goals of being concise, comprehensible, and executable, while guaranteeing type safety for more elaborate object languages. We make the following contributions: (1) A dependent-passing style technique for hiding the weakening of indexed values as they propagate through monadic code. (2) An Agda library for programming with scope graphs and frames , which provides a uniform approach to dealing with name binding in intrinsically-typed interpreters. (3) Case studies of intrinsically-typed definitional interpreters for the simply-typed λ-calculus with references (STLC+Ref) and for a large subset of Middleweight Java (MJ).

Design and Implementation of Concurrent C0

We describe Concurrent C0, a type-safe C-like language with contracts and session-typed communication over channels. Concurrent C0 supports an operation called forwarding which allows channels to be combined in a well-defined way. The language's type system enables elegant expression of session types and message-passing concurrent programs. We provide a Go-based implementation with language based optimizations that outperforms traditional message passing techniques.

Compiling a high-level language for GPUs

Languages such as OpenCL and CUDA offer a standard interface for general-purpose programming of GPUs. However, with these languages, programmers must explicitly manage numerous low-level details involving communication and synchronization. This burden makes programming GPUs difficult and error-prone, rendering these powerful devices inaccessible to most programmers. We desire a higher-level programming model that makes GPUs more accessible while also effectively exploiting their computational power. This paper presents features of Lime, a new Java-compatible language targeting heterogeneous systems, that allow an optimizing compiler to generate high quality GPU code. The key insight is that the language type system enforces isolation and immutability invariants that allow the compiler to optimize for a GPU without heroic compiler analysis. Our compiler attains GPU speedups between 75% and 140% of the performance of native OpenCL code.

Software Tracks

Railroad tracks offer guidance and support. There are various tools that can give our software the same handling. The main tool for guiding the code's direction is the language's type system. For values, the type system can help us by establishing a separate type for each distinct class; for code, interfaces and abstract classes ensure that we won't forget some crucial methods when we add functionality through a new class. With domain-specific languages or even suitably initialized data structures we can efficiently express exactly what the designer intended and nothing more. At a higher level, architectures that enforce a particular open-ended but well-defined interface will also guide a software's progress. Finally, the most flexible track-laying approach is a tool-supported software development process.

Type-Safe Code Transformations in Haskell

The use of typed intermediate languages can significantly increase the reliability of a compiler. By type-checking the code produced at each transformation stage, one can identify bugs in the compiler that would otherwise be much harder to find. We propose to take the use of types in compilation a step further by verifying that the transformation itself is type correct, in the sense that it is impossible that it produces an ill typed term given a well typed term as input.We base our approach on higher-order abstract syntax (HOAS), a representation of programs where variables in the object language are represented by meta-variables. We use a representation that accounts for the object language's type system using generalized algebraic data types (GADTs). In this way, the full binding and type structure of the object language is exposed to the host language's type system. In this setting we encode a type preservation property of a CPS conversion in Haskell's type system, using witnesses of a type correctness proof encoded in a GADT.

Type hierarchies and Semantic Data Models

The basic abstraction mechanisms of Semantic Data Models - aggregation, classification and generalization - are considered the essential features to overcome the limitations of traditional data models in terms of semantic expressiveness. An important issue in database programming language design is which features should a programming language have to support the abstraction mechanisms of Semantic Data Models. This paper shows that when using a strongly typed programming language, that language should support the notion of type hierarchies to achieve a full integration of Semantic Data Models abstraction mechanisms within the language's type system. The solution is presented using the language Galileo, a strongly typed, interactive programming language specifically designed for database applications.