more-parallel/test0 about scion

[jbeard4]

Just wondering what was the rational for dropping transition types?

Transition types were added to the spec some time after I started SCION in 2010. I think they can be a useful feature, but actually aren't required to have a consistent semantics.

[jbeard4]

I've reviewed the spec, and I think the main problem with SCION's semantics is that it uses an LCA (least common ancestor) as transition scope, as opposed to an LCCA (least common compound ancestor). I think that in most graphical syntaxes, making the transition scope the LCA rather than the LCCA is safe, because the syntax restricts you from creating transitions that explicitly originate from or target orthogonal components; therefore the semantics doesn't need to be as precise (use LCCA instead of LCA).

The implication of making the transition scope the LCCA is that, in cases like test0.scxml, where there is a transition whose source state is an orthogonal component (child of ), is that the parent and all of its orthogonal descendants will also be exited. I think this may be surprising and less intuitive, but may lead to a more regular semantics, and so is an overall better solution than attempting to create a special case in the semantics that prevents the ortho component from being exited. Plus, it appears to be more in line with the semantics described in the spec.

It also means that 'internal' transitions originating from orthogonal components will be more useful, as this will provide a means to create transitions originating from an orthogonal component that targets its descendants, without leaving the parallel parent and all other orthogonal siblings.

SCION doesn't support 'internal' transitions yet, but I think I'm going to add this feature soon.

If you have any ideas about this, I'd enjoy hearing about it.

[jbeard4]

Note that the tests in more-parallel/ were designed specifically to explore these issues.

[jbeard4]

Also, this is off-topic, but we should have a conversation about finding an optimal way to collaborate between scalejs.statechart and SCION.

[jbeard4]

I added support for transition/@type="internal" in the previous commit as well.