Handlers of Algebraic Effects

Abstract. We present an algebraic treatment of exception handlers and, more generally, introduce handlers for other computational effects representable by an algebraic theory. These include nondeterminism, interactive input/output, concurrency, state, time, and their combinations; in all cases the computation monad is the free-model monad of the theory. Each such handler corresponds to a model of the theory for the effects at hand. The handling construct, which applies a handler to a computation, is based on the one introduced by Benton and Kennedy, and is interpreted using the homomorphism induced by the universal property of the free model. This general construct can be used to describe previously unrelated concepts from both theory and practice.

Programming in Prolog

Provided that the reader is equipped with a Prolog implementation that conforms tothe ISO standard, the book Prolog: The Standard almost obviates the need for animplementation-specific reference manual, although the latter would be useful fordocumenting implementation-defined parameters and limits.Like most other programming languages, Prolog exists in a number of differentimplementations, each with its own semantic and syntactic peculiarities. In thisbook we have adopted a core Prolog based on ISO Standard Prolog. Previous editionsconformed to a de facto standard that became known as Edinburgh Prolog. Inturn, Edinburgh Prolog was the main influence on the specification of ISO StandardProlog. The table shown below summarises the main changes that have been madein the use of particular syntactic forms, special atoms and built-in predicates sinceearlier versions of this book in order to conform to the Standard or otherwise reflectmore recent practice. Most of the differences between the Edinburgh and ISO coreversions are of a purely cosmetic nature, though ISO Standard Prolog has gone innew directions in the way that input/output is handled

Flexible Regulation of Distributed Coalitions

This paper considers a coalition C of enterprises {E1 ,..., En}, which is to be governed by a coalition policy PC , and where each memberenterprise E i has its own internal policy P i that regulates its participation in the coalition. The main question addressed in this paper is how can these three policies be brought to bear, on a single transaction--- given that the two internal policies P i and P j may be formulated independently of each other, and may be considered confidential by the respective enterprises. We provide an answer to this question via a concept of policy-hierarchy, introduced into a regulatory mechanism called Law-Governed Interaction (LGI)