Subject: MSC-News: Problem with instance-oriented - shared in-line expression textual
From: UK496 Paul Baker (paul#bae231.comm.mot.com)
Date: Thu Sep 24 1998 - 09:29:00 GMT
The originator of this message is responsible for its content.
-----From UK496 Paul Baker <paul#bae231.comm.mot.com> to mscnews -----
Dear MSC folks,
I think there is a problem regarding the instance-oriented textual
grammar for the shared
inline ‘alt’ and ‘par’ expressions (section 5.3 of Z.120), which:
(1) does not follow the graphical grammar. In the graphical form it is
possible for an
instance included as part of an ‘alt’ and ‘par’ shared inline expression
to be empty i.e.
contain no instance events. This not necessarially the case in the
<shared...> productions,
which insist upon having at least one ‘alt’ or ‘par’ argument containing
at least one MSC
event.
(2) makes it impossible to define the semantics for ‘alt’ and ‘par’
expressions using the
instance-oriented grammar. The reason for this is that it is not
possible (in some cases)
given the information provided to determine which events, existing on
different instances,
are associated with different ‘alt’ or ‘par’ arguments. And in fact
whether events are
indeed associated with the same ‘alt’ or ‘par’ expression.
My suggestion is to change the concrete textual grammar slightly so that
‘alt’
and ‘par’ expression arguments can be empty:
CURRENT TEXTUAL GRAMMAR
-------------------------------------------------
<shared alt expr> ::= alt begin [<inline expr identification>]
<shared> <end>
[inline gate interface>] <instance event
list>
{ empty
| alt <end> [inline gate interface]
<instance event list>}
alt end
<shared par expr> ::= par begin [<inline expr
identification>]<shared> <end>
[inline gate interface>] <instance event
list>
{ empty
| par <end> [inline gate interface]
<instance event list>}
par end
SUGGESTED TEXTUAL GRAMMAR
------------------------------------------------------
<shared alt expr> ::= alt begin [<inline expr identification>]
<shared> <end>
{ empty /* allow empty alternative
arguments */
| [inline gate interface>] <instance
event list>
{
empty
| alt <end> [inline gate interface]
<instance event list>
}
}
alt end
<shared par expr> ::= par begin [<inline expr
identification>]<shared> <end>
{ empty /* allow empty parallel
arguments */
| [inline gate interface>] <instance
event list>
{
empty
| par <end> [inline gate interface]
<instance event list>
}
}
par end
Any comments?
Regards
Paul.
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