Types in programming languages are commonly thought of as a way of preventing certain bad things from happening, such as multiplying a number by a string. But this is only half of the benefit of types: it is what types are against. Types in programming languages are also what enable some good things to happen, such as selecting the right implementation of a heterogeneous operation like comparison or printing based on type information; this is what are types for. This ability is surprisingly powerful, and gives rise to a variety of highly expressive generic programming techniques. Jeremy illustrates with some examples based on the rank-polymorphic array operations introduced in Iverson’s APL: not only does the type information prevent array shape errors, it is what directs the lifting of operations across array dimensions.

Strachey 100: an Oxford Computing Pioneer

What are types for?

In this presentation, Uday brings together two strands of Christopher Strachey’s thought: parametric polymorphism and abstract models of storage. The term parametric polymorphism was introduced in by Strachey who distinguished it from “ad hoc” polymorphism. In the words of John Reynolds, “a parametric polymorphic function is one that behaves the same way for all types,” whereas an ad hoc polymorphic function may have unrelated meanings at different types. A very similar intuition arose in mathematics, some twenty years earlier, for which Eilenberg and MacLane proposed a “General Theory of Natural Equivalences”, what we now call category theory. Relating the two notions allows us to develop a broad view of “parametricity” (the idea of acting the same way for all types), which is applicable not only to programming languages but also to mathematics and perhaps other disciplines. A particularly important application of this broad view is for Strachey’s “mathematical semantics” of programming languages. For modelling imperative programming languages that operate by manipulating a store, Strachey presented a basic model of storage based on “locations”, while promising a further paper on an “abstract model of storage” to appear in future. Unfortunately the latter never appeared. In succeeding work, Reynolds proposed an abstract model of store as well as an “intuitionistic” functor category approach to model the stack discipline of the store. O'Hearn and Tennent made the crucial observation that the model needs to be embellished with parametricity to capture the data abstraction aspects of the store. In this talk, Uday reviews these developments as well as his own recent work on integrating the Reynolds model with parametricity to capture the fact that state changes of the store are irreversible. One way to understand these developments is to view them as a “parametric mathematical semantics” of programming languages.

Parametric Polymorphism and models of storage

Hongseok begins by talking about a program of Strachey’s that wrote “love letters” using the Manchester University computer. He then uses this as a lead in for discussing probabilistic methods of generating algorithms and programs.

Probabilistic Programming

Philip reviews Christopher Strachey’s influence on modern-day functional programming languages.

Christopher Strachey, First-Class Citizen

In this talk, Jane presents about her work on modelling dynamic behaviour of systems using quantative modelling techniques. Particular kinds of modelling diagrams are used and a mathematical approach to looking at their meaning is presented.

A modelling language approach to defining mathematical structures via semantics

The logician Dana Scott played a crucial part in the story of denotational semantics, working for a term with Christopher Strachey in Autumn 1969, when he created a mathematical model for the foundation of the method. In this talk, Dana discusses this period as well as reminiscing about his life and career up to that point.

Greetings to the participants at “Strachey 100”

In this panel discussion, three people who knew Christopher Strachey in different contexts talk about their memories of him. Michael Jackson discusses being taught by Strachey as a boy at Harrow, David Hartley talks about work with Strachey on the programming language CPL, and Roger Penrose remembers working with Strachey at the National Research Development Corporation and introducing him to lambda calculus.

Strachey: school master, language designer, colleague

Christopher Strachey believed that the gap between theory and practice was impeding the development of computing science. In Robert’s talk, he considers how the work he did with Strachey on the essay that ultimately became their book tried to narrow the gap, by formalising, and reasoning about, the implementation concepts for programming languages. A particular focus will be the proof techniques for imperative programs that use storage, which were implicit, but not very easy to discern, in the book.

Semantic relationships: reducing the separation between practice and theory

During Peter’s DPhil studies, supervised by Christopher Strachey, he developed a prototype of a system for executing programs based on their denotational semantics. It involved partial evaluation of lambda-notation, implemented using Wadsworth's call-by-need algorithm. He continued the development of the system as a postdoc at Oxford, and subsequently at Aarhus, Denmark. The system was called SIS: Semantics Implementation System. In this talk, Peter recalls Strachey's influence on his research and on the development of SIS, starting from Strachey’s summer school course on symbolic computation at the PRG in 1969.

SIS, a semantics implementation system

Chrisopher Strachey was the most significant contributor to the design and implementation of the programming language CPL. At the time there was little understanding of the complexities of computer language design and how type systems could cope with lists and the kinds of structures needed to represent, for instance, parse trees. The CPL project cannot be regarded as being successful since it did not result in a usable CPL compiler. The reasons being that the language became too large and complicated, there were insufficient people to implement the compiler and, in the middle of the three year project, all work had to be transferred from Edsac 2 to Titan, a newly designed version of the Ferranti Atlas computer which as yet had no operating system. Even so, we can be proud of the work that went into CPL and its influence on the design of many later languages.

Strachey and the development of CPL

Christopher Strachey’s right-hand man at Oxford talks about Strachey’s time as the head of the Programming Research Group (PRG). The talk covers the foundation of the PRG and the research performed at the (PRG), with an emphasis on the equal importance of theoretical and practical work.

Strachey and the Oxford Programming Research Group

A historian’s perspective on the earlier years of Christopher Strachey’s life. The talk covers his familial connections, his early career as a school master, and his first computing projects.

Strachey: the Bloomsbury Years

Christopher Strachey (1916–1975) was a pioneering computer scientist and the founder of the Programming Research Group, now part of the Department of Computer Science at Oxford University. Although Strachey was keenly interested in the practical aspects of computing, it is in the theoretical side that he most indelibly left his mark, notably by creating with Dana Scott the denotational (or as he called it, ‘mathematical’) approach to defining the semantics of programming languages. Strachey also spent time writing complex programs and puzzles for various computers, such as a draughts playing program for the Pilot ACE in 1951. He developed some fundamental concepts of machine-independent operating systems, including an early suggestion for time-sharing, and was a prime mover in the influential CPL programming language. Strachey came from a notable family of intellectuals and artists, perhaps most famous for Christopher’s uncle Lytton, a writer and member of the Bloomsbury group. We marked the occasion of 100 years since Christopher Strachey's birth on Saturday 19th November 2016, three days after his birthday, with a symposium of invited speakers. The morning looked back at Strachey’s life and works from a historical and technical perspective, and the afternoon concerned continuing research themes in Computer Science inspired by Strachey, at Oxford and elsewhere. This podcast series has recordings of these talks.

hubhopper

Strachey 100: an Oxford Computing Pioneer

hubhopper