In “syntactic control of interference” (POPL, 1978), J.C. Reynolds proposes three design principles intended to constrain the scope of imperative state effects in Algol-like languages. The resulting linguistic framework seems to be a very satisfactory way of combining functional and imperative concepts, having the desirable attributes of both purely functional languages (such as PCF) and simple imperative languages (such as the language of while programs). However, Reynolds points out that the “obvious” syntax for interference control has the unfortunate property that β-reductions do not always preserve typings. Reynolds has subsequently presented a solution to this problem (ICALP, 1989), but it is fairly complicated and requires intersection types in the type system. Here, we present a much simpler solution which does not require intersection types. We first describe a new type system inspired in part by linear logic and verify that reductions preserve typings. We then define a class of “bireflective” models, which provide a categorical analysis of structure underlying the new typing rules; a companion paper “Bireflectivity”, in this volume, exposes wider ramifications of this structure. Finally, we describe a concrete model for an illustrative programming language based on the new type system; this improves on earlier such efforts in that states are not assumed to be structured using locations.

We propose that the phenomenon of local state may be understood in terms of Strachey's concept of parametric (i.e., uniform) polymorphism. The intuitive basis for our proposal is the following analogy: a non-local procedure is independent of locally-declared variables in the same way that a parametrically polymorphic function is independent of types to which it is instantiated. A connection between parametricity and representational abstraction was first suggested by J.C. Reynolds. Reynolds used logical relations to formalize this connection in languages with type variables and user-defined types. We use relational parametricity to construct a model for an Algol-like language in which interactions between local and non-local entities satisfy certain relational criteria. Reasoning about local variables essentially involved proving properties of polymorphic functions. The new model supports straightforward validations of all the test equivalences that have been proposed in the literature for local-variable semantics, and encompasses standard methods of reasoning about data representations. It is not known whether our techniques yield fully abstract semantics. A model based on partial equivalence relations on the natural numbers is also briefly examined.

J. C. Reynolds suggested that Strachey's intuitive concept of “parametric” (i.e., uniform) polymorphism is closely linked to representation independence, and used logical relations to formalize this principle in languages with type variables and user-defined types. Here, we use relational parametricity to address long-standing problems with the semantics of local-variable declarations, by showing that interactions between local and non-local entities satisfy certain relational criteria.

Abstract We compare the American and international standards for Pascal and make suggestions to help implementors and programmers achieve the greatest possible compatibility with both standards.

Abstract This article defends the concept of block structure using two examples that have previously been discussed in a recent article by D. R. Hanson and in textbooks by Wirth and Brinch Hansen. It is shown that effective use of block structuring makes it possible to combine the advantages of the nested versions in the textbooks and Hanson's linear versions, calling into question the disadvantages attributed to the block structure concept by these and other authors.

Abstract Two approaches to type compatibility in Pascal, which were discussed briefly in an earlier paper by Welsh, Sneeringer, and Hoare, are analysed in detail.

This paper analyzes the semantics of the programming language SNOBOL4, following the mathematical approach proposed by D. Scott and C. Strachey. The study aims at clarifying a rather unusual semantic structure, and at demonstrating that the mathematical approach can provide a natural and usable formal specification of a practical programming language.