Retractable and speculative contracts

Behavioral contracts are abstract descriptions of the communications that clients and servers perform. Behavioral contracts come naturally equipped with a notion of compliance: when a client and a server follow compliant contracts, their interaction is guaranteed to progress or successfully complete. We study two extensions of contracts, dealing respectively with backtracking and with speculative execution. We show that the two extensions give rise to the same notion of compliance. As a consequence, they also give rise to the same subcontract relation, which determines when one server can be replaced by another preserving compliance. Moreover, compliance and subcontract relation are both decidable in polynomial time.