The π-calculus offers an attractive basis for concurrent programming. It is small, elegant, and well studied, and supports (via simple encodings) a wide range of high-level constructs including data structures, higher-order functional programming, concurrent control structures, and objects. Moreover, familiar type systems for the -calculus have direct counterparts in the π-calculus, yielding strong, static typing for a high-level language using the π-calculus as its core. This paper describes Pict, a strongly-typed concurrent programming language constructed in terms of an explicitly-typed-calculus core language.

ly, the remote procedure call problem, which an RPC protocol undertakes to solve, consists of emulating LPC using message passing. LPC has a number of "properties" -- a single procedure invocation results in exactly one execution of the procedure body, the result returned is reliably delivered to the invoker, and exceptions are raised if (and only if) an error occurs. Given a completely reliable communication environment, which never loses, duplicates, or reorders messages, and given client and server processes that never fail, RPC would be trivial to solve. The sender would merely package the invocation into one or more messages, and transmit these to the server. The server would unpack the data into local variables, perform the desired operation, and send back the result (or an indication of any exception that occurred) in a reply message. The challenge, then, is created by failures. Were it not for the possibility of process and machine crashes, an RPC protocol capable of overcomi...

Standard ML is one of a number of new programming languages developed in the 1980s that are seen as suitable vehicles for serious systems and applications programming. It offers an excellent ratio of expressiveness to language complexity, and provides competitive efficiency. Because of its type and module system, Standard ML manages to combine safety, security, and robustness with much of the flexibility of dynamically typed languages like Lisp. It is also has the most well-developed scientific foundation of any major language. Here I review the strengths and weaknesses of Standard ML and describe some of what we have learned through the design, implementation, and use of the language.

We investigate whether the π-calculus is able to serve as a good foundation for the design and implementation of a strongly-typed concurrent programming language. The first half of the dissertation examines whether the π-calculus supports a simple type system which is flexible enough to provide a suitable foundation for the type system of a concurrent programming language. The second half of the dissertation considers how to implement the π-calculus efficiently, starting with an abstract machine for π-calculus and finally presenting a compilation of π-calculus to C. We start the dissertation by presenting a simple, structural type system for π-calculus, and then, after proving the soundness of our type system, show how to infer principal types for π-terms. This simple type system can be extended to include useful type-theoretic constructions such as recursive types and higherorder polymorphism. Higher-order polymorphism is important, since it gives us the ability to implement abstract datatypes in a type-safe manner, thereby providing a greater degree of modularity for π-calculus programs. The functional computational paradigm plays an important part in many programming languages. It is well-known that the π-calculus can encode functional computation. We go further and show that the type structure of λ-terms is preserved by such encodings, in the sense that we can relate the type of a λ-term to the type of its encoding in the π-calculus. This means that a π-calculus programming language can genuinely support typed functional programming as a special case. An efficient implementation of π-calculus is necessary if we wish to consider π-calculus as an operational foundation for concurrent programming. We first give a simple abstract machine for π-calculus and prove it correct. We then show how this abstract machine inspires a simple, but efficient, compilation of π-calculus to C (which now forms the basis of the Pict programming language implementation).

We present a set of concurrency primitives for Standard ML. We define these by giving the transitional semantics of a simple language. We prove that our semantics preserves the expected behaviour of sequential programs. We also show that we can define stores as processes, such that the representation has the same behaviour as a direct definition. These proofs are the first steps towards integrating our semantics with the full definition of Standard ML. 1 Background and Motivation There have been several attempts to add concurrency primitives to Standard ML (SML) and related languages [Hol83, Mat91, Rep91a, CM90, Ber89]. However, when we began this work none of these implementations had a published formal definition. The formal definition of SML is an integral part of the development of the language. If we are to add concurrency to the language, it is essential that we have a formal semantics for the new constructs that is compatible with the existing definition. In this paper we prese...

Recent attempts at incorporating concurrency into functional languages have identified synchronous communication via shared channels as a promising primitive. An additional useful feature found in many proposals is a nondeterministic choice operator. Similar in nature to the CSP alternative command, this operator allows different possible actions to be guarded by sends or receives. Choice is difficult to implement in a distributed environment because it requires offering many potential communications but closing only one. In this paper we present the first distributed, deadlock-free algorithm for choice. Keywords: Distributed protocols, channels, synchronous communication, choice operator, CSP alternative command. 1. Introduction In 1978, C.A.R. Hoare introduced a now-classic paradigm for parallel programming called communicating sequential processes, or CSP [12]. CSP presented a new approach to concurrent programming, and its legacy has continued to influence wellfounded pr...

We describe a variant of the Standard ML programming language which incorporates a facility for the replacement of modular components during program execution. This useful extension of the language builds upon existing compiler technology which permits the separate compilation of modular units of a Standard ML program. Defined naively, this extension would incur performance overheads due to the need to retain typing information at run-time. Here we explain how this cost can be significantly lessened and in some cases eliminated entirely. The essential technical device which we employ for implementation of our extension to the language is a modification of two-space copying garbage collection. Keywords: languages modular programming; static type checking; dynamic 1

LCS is an experimental high level asynchronous parallel programming language primarily aimed at exploring design, implementation and use of programming languages based upon the behavioral paradigms introduced by CSP and CCS. The language extends Standard ML with primitives for concurrency and communication based upon a higher order extension of the CCS formalism. Typechecking enforces consistency of communications. An abstract operational semantics of the language is given in terms of a transition system.

The -calculus offers an attractive basis for concurrent programming languages. It is small, elegant, and well understood, and it supports, via simple encodings,a wide range of high-level constructs such as structured data, higher-order programming, concurrent control structures, and objects. Moreover, familiar type systems for the -calculus have direct counterparts in the -calculus, yielding strong, static typing for high-level languages defined in this way. 1 Introduction Though it originated some years before computer science itself, the -calculus has come to be regarded as a canonical calculus capturing the notion of sequential computation in an elegant, mathematically tractable presentation. Many of the fundamental issues of sequential programming languages can profitably be studied by considering them in the more abstract setting of the -calculus. Conversely, the -calculus has strongly influenced the design of many programming languages, notably McCarthy's LISP [McC78]. Mil...

This paper describes the platform's design, implementation, and performance.