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Subject: Issue - 107 - Extension points and opacity
One of the big concerns that came up on today's discussion of issue 107 was how the opacity proposal altered the ability to extend an abstract process. For example, imagine the following abstract process that uses opaque: process opaque receive reply If someone wanted to build an executable process based on this abstract process would it be 'legal' to write something like the following and still have it be considered valid against the abstract process? process ... receive assign reply In other words, could the executable process programmer insert the 'assign' activity between the receive and reply activity and still have their executable process be considered valid against the abstract process definition? In my proposal the answer is - yes. In my opinion one is free to extend an abstract process at absolutely any point one wants. The purpose of validating an executable process against an abstract process is to make sure that the executable process does 'at least' what the abstract process does but it is free to do much more. The purpose of opaque is not to restrict where one can extend an abstract process when creating an executable process. The purpose of opaque is to specify where the abstract process is intentionally leaving out data in situations where the absence of information could be ambiguous. For example, let's say that an abstract process has the following code snippet: process scope eventHandlers onEvent ... ... In this case the onEvent handler in the abstract process has no content. So now the reader of the abstract process is left with a quandary. Is the onEvent handler empty because the definition of the event handler is left up to the implementer or is the event handler empty because the programmer of the abstract process screwed up and forgot to put in the definition for the event handler? One way around this ambiguity is to do the following: process scope eventHandlers onEvent ... empty ... By inserting empty it is clear to the reader that the onEvent handler was intended to not have any definition. The only problem is that empty has its own very specific meaning - do nothing. So now there is another ambiguity. Is the abstract process author telling the executable process author 'Write your own code for this event handler' or are they telling the executable process author 'You are required to catch the messages identified by this event handler but then you should do nothing with those messages'? This is where opaque comes in. process scope eventHandlers onEvent ... opaque ... Now there is no ambiguity. The abstract author is explicitly telling the executable author 'You must catch the messages identified by this event handler but how you handle them is up to you.' It is only necessary, however, to use opaque if its absence would lead to ambiguity. So, going back to the example, that started this mail, there is no need to specify process opaque receive opaque reply In order to tell the executable programmer "You MAY insert activities between the receive and reply". It's fine to just specify: process opaque receive reply The right to insert additional activities between the receive and reply is always implicit and unambiguous so opaque is not needed. I hope this clarifies things, Thanks, Yaron
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