Abstract
We explore different approaches to modelling external choice as a reactive process in a UTP semantics with discrete time. The standard definition of external choice cannot be simply reused in a timed semantics, since it can introduces behaviours which are not prefix-closed and urgent events which occur instantly. We first examine unstable states and urgent events in different semantics for CSP. We present the semantics for a simple timed reactive UTP language and describe the difficulties of adding external choice. We define two new process operators; strict choice, which never engages in urgent events and lazy choice, which can delay urgent events. We briefly discuss two potential modifications to the language model; a lazy semantics, in which termination is not unstable, and a semantics in which unstable states can be observed. Finally, we give a more detailed treatment to strict choice, expressing it as a reactive design and stating its algebraic laws.
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Canham, S., Woodcock, J. (2015). Three Approaches to Timed External Choice in UTP. In: Naumann, D. (eds) Unifying Theories of Programming. UTP 2014. Lecture Notes in Computer Science(), vol 8963. Springer, Cham. https://doi.org/10.1007/978-3-319-14806-9_1
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DOI: https://doi.org/10.1007/978-3-319-14806-9_1
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