Question

How to set labels on arrow with new Activity syntax ?

Answer 1

Right now, you cannot.

The most difficult part is to find a nice syntax.

Suggestion A:

@startuml
start
:first instruction;
-> this is an arrow
:second instruction;
end
@enduml
Drawbacks: it won't be possible to define arrow text on several lines

Suggestion B:

Add an ending semi-colon (;) at the end of the arrow definition
@startuml
start
:first instruction;
-> this is an arrow;
:second instruction;
-> This is an arrow
   that has a long text
   and is on several line;
end
@enduml

Any other suggestions (suggestion C, D...) welcome!

Comments

Suggestion B seems to be more flexible.
By the same time, it could be interesting to also add the possibility to force an arrow on the right or left.
In the same idea of proposed syntax for text on arrow, we could have:
-r-> to force the arrow on the right
-l-> to force the arrow on the left

Answer 2

It's possible on newer versions of PlantUML.