. A monad for concurrent programming that is suitable for being built into Haskell is presented. The monad consists of only four primitives with a very simple semantics. A number of examples demonstrate that monads encapsulating other, more sophisticated communication paradigms known from concurrent functional languages such as Concurrent ML, Facile, and Erlang can be naturally and systematically constructed from the built-in monad in a purely functional way. The paper argues that minimizing the number and complexity of the concurrency primitives and maximizing the use of purely functional abstractions in the design of concurrent languages helps to remedy a recurrent dilemma, namely, how to keep the language small and rigorously defined, yet to provide the programmer with all the communication constructs required. An interleaving implementation of the monad has been built by extending Mark Jones's Gofer environment to handle the concurrency primitives. All programs presented ...

Events play an important role in the construction of most software that involves interaction or simulation. Typically, programmers make use of a fixed set of low level events supplied by a window system, possibly augmented with timers and UI components. Event handling generally involves some interpretation of these event occurrences, followed by external actions or modifications to program state. It is possible to extend the event paradigm by using event interpretation to synthesize new kinds of events tailored specifically for a domain or application. In turn, these new events may be used to synthesize yet others, and so on, to an arbitrarily sophisticated degree. This programming paradigm, which we call event-oriented programming, aids in the factoring of programs into understandable and reusable pieces. We propose a declarative approach to event-oriented programming, based on a powerfully expressive event language with a lightweight notation. We illustrate this new approach throug...

Abstract. Taking functional programming to its extremities in search of simplicity still requires integration with other development (e.g. formal) methods. Induction is the key to deriving and verifying functional programs, but can be simplified through packaging proofs with functions, particularly “folds”, on data (structures). “Totally Functional Programming ” avoids the complexities of interpretation by directly representing data (structures) as “platonic combinators ”- the functions characteristic to the data. The link between the two simplifications is that platonic combinators are a kind of partially-applied fold, which means that platonic combinators inherit foldtheoretic properties, but with some apparent simplifications due to the platonic combinator representation. However, despite observable behaviour within functional programming that suggests that TFP is widely-applicable, significant work remains before TFP as such could be widely adopted. 1 Programming is Too Hard There can be little doubt that “programming ” (both as metaphor for and essence of the

The Extensible Markup Language (XML) is extremely popular as a generic markup language for text documents with an explicit hierarchical structure. The different types of XML data found in today’s document repositories, digital libraries, intranets and on the web range from flat text with little meaningful structure to be queried, over truly semistructured data with a rich and often irregular structure, to rather rigidly structured documents with little text that would also fit a relational database system (RDBS). Not surprisingly, various ways of storing and retrieving XML data have been investigated, including native XML systems, relational engines based on RDBSs, and hybrid combinations thereof. Over the years a number of native XML indexing techniques have emerged, the most important ones being structure indices and labelling schemes. Structure indices represent the document schema (i.e., the hierarchy of nested tags that occur in the documents) in a compact central data structure so that structural query constraints (e.g., path or tree patterns) can be efficiently matched without accessing the documents. Labelling schemes specify ways to assign unique identifiers, or labels, to the document nodes so that specific relations (e.g., parent/child)

A new approach is presented for performing concurrent I/O in a functional programming language. A new monad St is introduced which generalizes Haskell's IO monad: A value of type St a represents an imperative program which, at certain times during its execution, will produce a value of type a. In contrast, a value of type IO a represents an imperative program which, at the end of its execution, will produce a value of type a. Not only may values of type St a be used to represent imperative commands, they also serve to create so-called imperative streams. The computer's physical input devices are represented as primitive streams. Composite streams may be constructed and interleaved in a non-preemptive way using, for instance, the monad's bind operation. By way of a series of example programs, the usage of imperative streams is illustrated. Moreover, an implementation in terms of the IO monad is given that is suitable for executing the example programs on the Gofer interpreter. An exte...