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why no triggerless transitions out of non-pseudo states? [message #1753111]

Wed, 01 February 2017 22:05

Juergen Dingel

Messages: 42
Registered: January 2015

Member

I have been asked why Papyrus-RT does not allow transitions that are trigger-less and thus become enabled as soon as the source state has been entered.

My response was that the execution of transitions should be driven by external input only and that otherwise it might be hard to define the notion of a 'stable state' etc.

What is the official take on this?

Thanks

Juergen

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Re: why no triggerless transitions out of non-pseudo states? [message #1753112 is a reply to message #1753111]

Wed, 01 February 2017 22:41

Ernesto Posse

Messages: 438
Registered: March 2011

Senior Member

Bran could give an "official" answer, but as far as I can see, you've got the gist of it: state machines define behaviour that is driven by external stimuli, rather than defining "autonomous" behaviour (cf. tau transitions in process algebras).

It should be noted that you can have trigger-less transitions from pseudo-states. In fact you can only have trigger-less transitions from pseudo-states.

You should also note that you could emulate such autonomous behaviour by "self-messaging": a capsule with a connector to itself (e.g. capsule A with ports p and q and a connector from A.p to A.q). Then you could have a state S with an entry action "p.m().send()" and transition t from S that listens to a message m on port A.q.

I'm not sure if that pattern really makes much sense in practice, but the language does allow it.

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Re: why no triggerless transitions out of non-pseudo states? [message #1753114 is a reply to message #1753112]

Wed, 01 February 2017 23:05

Ernesto Posse

Messages: 438
Registered: March 2011

Senior Member

Actually, a small clarification:

Quote:

In fact you can only have trigger-less transitions from pseudo-states.

Technically you can have a trigger in a transition emanating from an exit point, as long as that is the first transition segment in the transition chain.

By the way, the code generator itself just ignores triggers where they are not expected.

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