Apr 6, 2014
Massimo Bartoletti: A theory of agreements and protection
We present a theory of contracts, interpreted as multi-player concurrent games among competitive participants. The two key notions of our model are that of agreement and protection. The agreement property is a game-theoretic generalization of the notion of compliance typically studied in session behaviours: a participant agrees on a contract if she has a strategy to reach her goals (or make another participant chargeable for a violation), whatever the moves of her adversaries. The protection property is relevant when contract brokers are untrusted: a participant is protected by a contract when she has a strategy to defend herself in all possible contexts, even in those where she has not reached an agreement. We show that, in a relevant class of contracts, agreements and protection mutually exclude each other. We then propose a novel formalism for modelling contractual obligations (event structures with circular causality), which allows us to construct contracts which guarantee both agreements and protection. Finally, we establish a correspondence between game-theoretic contracts and Propositional Contract Logic, by showing that winning strategies in game-theoretic contracts are related to proofs in the logic.