A Proposal for Coroutines and Suspend / Resume in E -

The importance of coroutines as a programming paradigm is recognized. This paper proposes to extend LOTOS with a coroutine operator, for which syntax, static semantics, and untimed semantics are provided. We show that this coroutine mechanism generalizes several other op-eratorrs, which exist in LOTOS or which have been proposed for E-LOTOS, including trap, suspend-resume, and hiding. 1 Comments about the existing core behaviour language At present, the deenition of E-Lotos, as provided by the Kansas City output document is far advanced. However, some issues remain unsolved, especially in the behaviour part: The deenition of the core behaviour language, as produced after the Kansas City meeting, has introduced a new class of objects, exceptions, which can be felt as \similar to gates, but diierent". Following the famous Occam razor principle, we must question ourselves about the need for having two concepts: gates and exceptions. If we only consider an \untimed semantics" point of view, it is clear that gates and exceptions are much the same concept, and that we could replace exceptions by gates everywhere without any problem. This approach advocated in GS96] brings simplicity, expressiveness and good algebraic properties. If we consider a \timed semantics" point of view, the major diierence between gates and exceptions is urgency. According to ET-LOTOS principles, urgency is only attached to hidden gates; visible gates are not urgent. This is not the case with exceptions, which are always urgent. We notice that this deenition of exceptions as urgent (although motivated by semantic \tricks") violates the ET-LOTOS principle stating that one cannot enforce urgency on visible events: from the outside of a system, we can observe visible, urgent exceptions, which is is not allowed for gates. If accepted, this design choice will have the following consequence. Practically, when designing a system in E-LOTOS, engineers will associate gates to non-urgent external events and exceptions