Syntax of transitions

"Clive D.W. Feather" <clive@demon.net> Mon, 08 January 2007 16:29 UTC

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Date: Mon, 8 Jan 2007 16:29:00 +0000
From: "Clive D.W. Feather" <clive@demon.net>
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Subject: Syntax of transitions
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The grammar suggests four different syntaxes for transitions:

(1) state1, state2 : message1, message2 -> state3 : action1

(2) message1, message2 : state1, state2 -> state3 : action1

(3) (state1, state2 : message1, message2) -> state3 : action1

(4) message1 : state1 -> state3 : action1; state2 -> state4 : action2;

and this also suggests a fifth:

(5) state1 : message1 -> state3 : action1; message2 -> state4 : action2;

Firstly, if you want to allow states and messages in either order, I
suggest that you should have different delimiters. For example:

    state + message       + suggests "adding this message"
    message @ state       @ suggests "when at this state"

Secondly, is there any reason to forbid multiple actions? It won't make
sense for all uses, but it will for some.

Thirdly, you can then use parentheses to provide grouping, rather than
syntax (4), which is rather hard to parse (the scope of message1 ends at
the next transition with a colon before the arrow). Something like this:

    message1 @ (state1 -> state3 : action1; state2 -> state4 : action2;);
    state1 + (message1 -> state3 : action1; message2 -> state4 : action2;);

These two being equivalent to:

    message1 @ state1 -> state3 : action1;
    message1 @ state2 -> state4 : action2;
and
    state1 + message1 -> state3 : action1;
    state1 + message2 -> state4 : action2;

(Note: the semicolon before the ")" simplifies the grammar, but it would be
easy enough to make it optional.)

Common trailing actions could be put outside the parentheses:

    message1 @ (state1 -> state3 : action1; state2 -> state4;) : action2;

would be equivalent to:

    message1 @ state1 -> state3 : action1, action2;
    message1 @ state2 -> state4 : action2;

So the overall grammar for this would be:

    transition = m-transition / s-transition
    m-transition = message-list "@" body-with-s
    s-transition = state-list   "+" body-with-m
    body-with-s = simple-body-with-s / grouped-body-with-s
    body-with-m = simple-body-with-m / grouped-body-with-m
    grouped-body-with-s = "(" 1*(simple-body-with-s ";") ")" [":" action-list]
    grouped-body-with-m = "(" 1*(simple-body-with-m ";") ")" [":" action-list]
    simple-body-with-s = state-list   "->" state [":" action-list]
    simple-body-with-m = message-list "->" state [":" action-list]

(This grammar needs to have comments and white space added to bring it into
line with the base grammar.)

If people like this, I'm happy to write wording to fit it into the
document.

-- 
Clive D.W. Feather  | Work:  <clive@demon.net>   | Tel:    +44 20 8495 6138
Internet Expert     | Home:  <clive@davros.org>  | Fax:    +44 870 051 9937
Demon Internet      | WWW: http://www.davros.org | Mobile: +44 7973 377646
THUS plc            |                            |

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