It is MscGen, but not as we know it
SDL has a feature called inline expressions. They allow you to group a bunch of arcs and label them somehow. To define a loop, for instance. Or a conditional statement.
UML2 sequence diagrams have a similar concept called "combined fragments" (see paragraph 14.3.3 of the linked document for details).
Behaviour specification is not a particularly strong suit of interaction diagrams (sequence charts/ diagrams/ collaboration diagrams), as Martin Fowler correctly notes in UML distilled. At this time (November 2015) MscGen does not support them.
Nonetheless inline expressions are damn handy. Hence Xù - a superset of MscGen that includes them.
This is an example of a Xù script describing an interaction that loops over a list of changes and sorts the old ones to a deletion queue, and the rest to a birth queue:
msc {
width=700, hscale=0.8;
a, b [label="change store"], c, d [label="necro queue"],
e [label="natalis queue"];
a =>> b [label="get change list()"];
a alt e [label="changes found"] {
b >> a [label="list of changes"];
a =>> c [label="cull old stuff (list of changes)"];
b loop e [label="for each change"] {
c =>> b [label="get change()"];
b >> c [label="change"];
c alt e [label="change too old"] {
c =>> d [label="queue(change)"];
--- [label="change newer than latest run"];
c =>> e [label="queue(change)"];
--- [label="all other cases"];
||| [label="leave well alone"];
};
};
c >> a [label="done processing"];
--- [label="nothing found"];
b >> a [label="nothing"];
a note a [label="silent exit"];
};
}
Rendered (e.g. with the online interpreter ) this looks like so:
As you can see, the syntax of the inline expressions is very similar to that of regular arcs, the only difference being that inline expressions have a section of arcs, enclosed by curly brackets.
spanarc =
_ from:identifier _ kind:spanarctoken _ to:identifier _ al:("[" al:attributelist "]" {return al})? _ "{" _ arclist:arclist _ "}" _ ";"To compare, this is how a regular arc looks:
regulararc =
_ from:identifier _ kind:arctoken _ to:identifier _ ("[" attributelist "]")? _ ";"Some more examples
break {
a => b [label="Can you do this?"];
b >> a [label="Fatal error"];
};
Arguments go into the label as free text.
loop [label="for each grain of sand on the beach"] {
a => beach [label="get grain"];
a => progeny [label="add"];
};
msc {
john, shed, bike;
john alt bike [label="wheather is nice"] {
john =>> shed [label="get(bike)"];
shed >> john [label="bike"];
john =>> bike [label="use"];
--- [label="else"];
||| [label="john stays at home"];
};
}
To separate sections to execute in parallel you can use a comment line, like so:
par {
a => b;
b >> a;
---;
a => c;
c => d;
}
If you're interested in the complete grammar: the parsing expression grammar we use to generate the parser is included in the source.
MscGen uses msc as a start token. As Xù was designed as an extension
to MscGen the same goes for Xù. If you want to be expressly clear your
script is a Xù script you can also use that:
xu {
arcgradient=20;
a,b,c;
a par c [label="Saying hello"] {
a =>> * [label="hi"];
---;
b =>> * [label="hi"];
---;
c =>> * [label="hi"];
};
a aboxc [label="Now we know each other a bit"];
}
Just like msgenny, Xù supports a "watermark" option: watermark="xù rocks!"; that puts a watermark diagonally on the rendered chart.
MsGenny also has support for inline expressions, the if-then-else construct above would look something like this:
john alt bike: weather is nice {
john =>> shed : get(bike);
shed >> john : bike;
john =>> bike : use;
--- : else;
||| : john stays at home;
};
- The command line interface handles by translating inline expressions to horizontal lines ("---")
# translate Xù back to MscGen
mscgenjs funkyInlineThings.xu --output-to funkyFlattenedInlineThings.mscgen- The on line interpreter, in debug mode, has an Export this chart to Vanilla MscGen option behind the
...button. - e.g. for the Johnny, bike, shed example
To start the interpreter in debug mode, pass debug=true to the url:
https://mscgen.js.org/index.html?debug=true
| feature | SDL-RT 2.3 inline expression | UML 2 combined fragment | xù |
|---|---|---|---|
| Alternatives - if with an optional else | alt | alt, else | alt |
| Alternatives - if with noelse | opt | opt | opt |
| Break - an exception occurred. After this nothing happens. | exc | break | excbreak
|
| Parralel - do several things at once | par | par | par |
| Weak sequencing - stuff within here can happen in any order across the instances | seq | seq | seq |
| Strict sequencing - stuff within here can happen in only this order across the instances | _not available_ | strict | strict |
| Negative - this is not happening | _not available_ | neg | neg |
| Critical region - this is important, execute at once | _not available_ | critical | critical |
| Ignore/ consider | _not available_ | ignore, consider | considerignore |
| Assertion - this stuff must be true | _not available_ | assert | assert |
| Loop - repeat this | loop | loop | loop |
| Reference - it's not here, but there's a diagram somewhere else detailing this | not available as an "inline expression" but as 'MSC reference' | _not available_ | ref |