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Subject: Re: [boost] [msm] eUML guard/action location
From: Stewart, Robert (Robert.Stewart_at_[hidden])
Date: 2009-12-07 14:22:10

Christophe Henry wrote:
> >CurrentState() + cool_event()[Guard()]/(action()) = DestState()
> It's a good idea and it surely matches the UML syntax better. I think
> the operator precedence will also work. There are however 2 issues:
> - the current order "CurrentState() + cool_event() = DestState()
> [Guard()]/(action())" has been chosen to closely match the "normal"
> row syntax. Only guard and action have been inverted because of
> operator precedence.

The "normal" and suggested syntax seem consistent, though the guard/action are less visible among transitions with them by not hanging off the right. It also means that viewing the transitions is harder or contains more whitespace. That is, the start/event/next flow is interrupted.

I do like the choice of [] for the condition (guard) and () for the behavior (action).

> - anonymous events. In this case, you have no event, so your syntax
> would become:
> CurrentState() [Guard()]/(action()) = DestState()

What about this:

   CurrentState() any[Guard()]/(action()) = DestState()

Pick another name for "any" if you like.

> But now the guard/action are "attached" to the source state, which
> means 2 grammars to support with the corresponding flow of compiler
> risks and compile-time increases.

By requiring a special type for anonymous events when there's a guard/action, you eliminate that problem.

Rob Stewart robert.stewart_at_[hidden]
Software Engineer, Core Software using std::disclaimer;
Susquehanna International Group, LLP

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