Re: [boost] [msm] eUML guard/action location

Subject: Re: [boost] [msm] eUML guard/action location
From: Stewart, Robert (Robert.Stewart_at_[hidden])
Date: 2009-12-08 08:57:52

Michael Caisse wrote:
> Christophe Henry wrote:
> > Hi Michael,
> >
> > I just remembered the last reason for the current eUML syntax:
> > CurrentState() + cool_event()  = DestState() [Guard()]/(action())
> >
> > It is that eUML row actions/guards can become quite long...
> >
> > CurrentState() + cool_event()  = DestState()
> > [VeeeeeryLoooooooooongGuard()]
> > /(LooooooooooooooooongAction())
> >
> > instead of:
> >
> > CurrentState() + cool_event()
> > [VeeeeeryLoooooooooongGuard()]
> > /(LooooooooooooooooongAction())
> > = DestState()

Both are less than ideal, but as formatted, the former is more readable.

> Ah... very good point. I've been doing a lot of Spirit/Qi work
> this past year and I can tell you the source code formatting makes all
> the difference in the world.
>
> I would likely just use copious white space:
>
> CurrentState() + cool_event()
> [VeeeeeryLoooooooooongGuard()]
> /(LooooooooooooooooongAction())             =    DestState()
>
> Does that look any better or worse than this?
>
> CurrentState() + cool_event()               =    DestState()
> [VeeeeeryLoooooooooongGuard()]
> /(LooooooooooooooooongAction())

With whitespace, I agree that there's little difference between the two.

Here's another suggestion, though it breaks from the logical UML ordering Michael posted previously:

   DestState() = CurrentState() + cool_event()[guard()]/(action())

This is in keeping with Spirit's syntax in that the guard/action expression is positioned like a semantic action.  It also reads better in C++ because we think in terms of evaluating the RHS and assigning the result to the LHS, and the RHS results in the state on the LHS in the above syntax.

_____
Rob Stewart                           robert.stewart_at_[hidden]
Software Engineer, Core Software      using std::disclaimer;
Susquehanna International Group, LLP  http://www.sig.com

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