Sequence Diagram: Incoming and Outgoing messages without participants on one end

[jayaprabhakar]

Proposal

Mermaid sequence diagram has been extremely easy to use. I have one small feature request. I need a way to represent Incoming and Outgoing messages. That is, messages where one end of the arrow has an unspecified participant/actor.

The common convention in most text-to-sequence diagram tools is to use '[' or ']'.

[->>Alice: Incoming Message from the Left
[<<-Alice: Outgoing Message to the Left

Alice->>]: Outgoing Message to the right
Alice<<-]: Incoming Message from the right

References:
https://plantuml.com/sequence-diagram#05984b1743e67542
https://sequencediagram.org/instructions.html#:~:text=Server%3Alater%20message-,Incoming%20and%20Outgoing%20Messages,-Incoming%20and%20Outgoing

Example

No response

Screenshots

PlantUML

sequencediagram.org

https://www.websequencediagrams.com/

[jayaprabhakar]

Another reason it is important is because, at present, in mermaid, a participant can be activated only if there is a message.

[jayaprabhakar]

What is the process to get this done?

• Is this change okay?
  • Technically, this will be considered backwards incompatible, because anyone who defined a participant as [ or ] will see a different behavior, however I don't see a reason why someone who do that.
  • We could document it if required or provide a way to disable this behavior if needed.
  • The grammar on mermaid has, the uni directional arrows are always specified left to write in the text --> -->> even if the diagram will show it right to left depending on the participants. So, To support incoming/outgoing messages we might have either add specifying arrows right to left or change the convention. '[' always mean left of participant. So A -->> [ will have an arrow pointing to the left of A. Similarly, ']' should mean right of the participant.
• Can someone who know the codebase point out where to make the changes?

I am happy to take this on, if I could get some guidance on how to implement.