Softly safely spoken: Role playing for Session Types

Session types have made much progress at permitting programs be statically verified concordant with a specified protocol. However, it is difficult to build abstractions of, or encapsulate Session types, thus limiting their flexibility. Global session types add further constraints to communication, by permitting the order of exchanges amongst many participants to be specified. The cost is that the number of participants is statically fixed. We introduce Roles in which, similarly to global session types, the number of roles and the conversations involving roles are statically known, but participants can dynamically join and leave roles and the number of participants within a role is not statically known. Statically defined roles which conform to a specified conversation can be dynamically instantiated, participants can be members of multiple roles simultaneously and can participate in multiple conversations concurrently.