Christian Stefansen

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We present a declarative language for compositional specification of contracts governing the exchange of resources. It extends Eber and Peyton Jones's declarative language for specifying financial contracts to the exchange of money, goods and services amongst multiple parties and complements McCarthy's Resources/Events/Agents (REA) accounting model with a(More)
Our aim is to define the kernel of a simple and uniform programming model—the reactor model—suitable for building and evolving internet-scale programs. A reactor consists of two principal components: mutable state, in the form of a fixed collection of relations, and code, in the form of a fixed collection of rules in the style of datalog. A reactor's code(More)
Novel data streams (NDS), such as web search data or social media updates, hold promise for enhancing the capabilities of public health surveillance. In this paper, we outline a conceptual framework for integrating NDS into current public health surveillance. Our approach focuses on two key questions: What are the opportunities for using NDS and what are(More)
User-generated content can assist epidemiological surveillance in the early detection and prevalence estimation of infectious diseases, such as influenza. Google Flu Trends embodies the first public platform for transforming search queries to indications about the current state of flu in various places all over the world. However, the original model(More)
We present a high-level enterprise system architecture that closely models the domain ontology of resource and information flows in enterprises. It is: Process-oriented: formal, user-definable specifications for the expected exchange of resources (money, goods, and services), notably contracts, are represented explicitly in the system state to reflect(More)
Recently, there has been an increasing interest in building ontologies for the enterprise domain, and a number of attempts already exist [4,16]. The Resources/Events/Agents model (REA) is one such ontology. In the following we describe the challenges we have experienced in understanding the REA model's most central ontological pattern, the economic(More)
There is an ongoing debate in the workflow community about the relative merits of Petri nets and π-calculus for workflow modeling. Recently, van der Aalst presented some challenges to model workflow in π-calculus. This paper responds to those challenges by showing how to code the 20 most commonplace workflow patterns in CCS (a subset of π-calculus), and(More)