# Other Concepts

When looking at the code in our tests, you might see some new concepts:

  • Vats: All user code runs in what we call a vat. Within a vat, code is run synchronously. Communication with another vat happens asynchronously. The SwingSet infrastructure creates the vats and makes communication between vats possible.

  • E() and tildot (~.): Instead of obj.foo(), we can write E(obj).foo() or the syntactic sugar, obj~.foo() and get a promise for the result. The syntax means "deliver the message foo() to the actual object asynchronously, in its own turn, wherever and whenever it is, even if it is local." Using E or ~., you can talk asynchronously to local and remote objects in exactly the same way.

  • Presences: Presences are our name for the local object that stands in for the remote object. If obj is a presence of a remote object, we can send messages to the remote object by using "~." on obj, as in the above example.