The Tenth ACM SIGPLAN International Conference on Functional Programming (ICFP) was held in September, 2005, in Tallinn, Estonia; Benjamin Pierce chaired the program committee. After the conference, extended versions of some of the presented papers were solicited for this special issue of JFP. All submitted papers were reviewed following standard JFP procedures and four were ultimately accepted; they form the body of this special issue.

Existential types provide a simple and elegant foundation for understanding generative abstract data types of the kind supported by the Standard ML module system. However, in attempting to extend ML with support for recursive modules, we have found that the traditional existential account of type generativity does not work well in the presence of mutually recursive module definitions. The key problem is that, in recursive modules, one may wish to define an abstract type in a context where a name for the type already exists, but the existential type mechanism does not allow one to do so. We propose a novel account of recursive type generativity that resolves this problem. The basic idea is to separate the act of generating a name for an abstract type from the act of defining its underlying representation. To define several abstract types recursively, one may first ‘forward-declare’ them by generating their names, and then supply each one's identity secretly within its own defining expression. Intuitively, this can be viewed as a kind of backpatching semantics for recursion at the level of types. Care must be taken to ensure that a type name is not defined more than once, and that cycles do not arise among ‘transparent’ type definitions. In contrast to the usual continuation-passing interpretation of existential types in terms of universal types, our account of type generativity suggests a destination-passing interpretation. Briefly, instead of viewing a value of existential type as something that creates a new abstract type every time it is unpacked, we view it as a function that takes as input a pre-existing undefined abstract type and defines it. By leaving the creation of the abstract type name up to the client of the existential, our approach makes it significantly easier to link abstract data types together recursively.

We present a compositional programme logic for call-by-value imperative higher-order functions with general forms of aliasing, which can arise from the use of reference names as function parameters, return values, content of references and parts of data structures. The programme logic extends our earlier logic for alias-free imperative higher-order functions with new operators which serve as building blocks for clean structural reasoning about programms and data structures in the presence of aliasing. This has been an open issue since the pioneering work by Cartwright–Oppen and Morris twenty-five years ago. We illustrate usage of the logic for description and reasoning through concrete examples including a higher-order polymorphic Quicksort. The logical status of the new operators is clarified by translating them into (in)equalities of reference names.

Existing languages provide good support for typeful programming of stand-alone programs. In a distributed system, however, there may be interaction between multiple instances of many distinct programs, sharing some (but not necessarily all) of their module structure, and with some instances rebuilt with new versions of certain modules as time goes on. In this paper, we discuss programming-language support for such systems, focussing on their typing and naming issues. We describe an experimental language, Acute, which extends an ML core to support distributed development, deployment, and execution, allowing type-safe interaction between separately built programs. The main features are (1) type-safe marshalling of arbitrary values; (2) type names that are generated (freshly and by hashing) to ensure that type equality tests suffice to protect the invariants of abstract types, across the entire distributed system; (3) expression-level names generated to ensure that name equality tests suffice for type safety of associated values, for example, values carried on named channels; (4) controlled dynamic rebinding of marshalled values to local resources; and (5) thunkification of threads and mutexes to support computation mobility. These features are a large part of what is needed for typeful distributed programming. They are a relatively lightweight extension of ML, should be efficiently implementable, and are expressive enough to enable a wide variety of distributed infrastructure layers to be written as simple library code above the byte-string network and persistent store APIs. This disentangles the language run-time from communication intricacies. This paper highlights the main design choices in Acute. It is supported by a full language definition (of typing, compilation, and operational semantics), by a prototype implementation, and by example distribution libraries.

The LF logical framework codifies a methodology for representing deductive systems, such as programming languages and logics, within a dependently typed λ-calculus. In this methodology, the syntactic and deductive apparatus of a system is encoded as the canonical forms of associated LF types; an encoding is correct (adequate) if and only if it defines a compositional bijection between the apparatus of the deductive system and the associated canonical forms. Given an adequate encoding, one may establish metatheoretic properties of a deductive system by reasoning about the associated LF representation. The Twelf implementation of the LF logical framework is a convenient and powerful tool for putting this methodology into practice. Twelf supports both the representation of a deductive system and the mechanical verification of proofs of metatheorems about it. The purpose of this article is to provide an up-to-date overview of the LF λ-calculus, the LF methodology for adequate representation, and the Twelf methodology for mechanizing metatheory. We begin by defining a variant of the original LF language, called Canonical LF, in which only canonical forms (long βη-normal forms) are permitted. This variant is parameterized by a subordination relation, which enables modular reasoning about LF representations. We then give an adequate representation of a simply typed λ-calculus in Canonical LF, both to illustrate adequacy and to serve as an object of analysis. Using this representation, we formalize and verify the proofs of some metatheoretic results, including preservation, determinacy, and strengthening. Each example illustrates a significant aspect of using LF and Twelf for formalized metatheory.