Capabilities

Pony is a language based on data-race-free typing and garbage collection, and uses the actor model as well as something called reference capabilities. Reference capabilities compel the programmer to label pieces of data as mutable, immutable or isolated. If data is mutable, the compiler doesn’t allow the programmer to exchange the data between actors — when two actors access mutable data at the same time, they may make contradictory updates, or the data could get corrupted. Reference capabilities keep data safe and eliminate the need for locks to prevent concurrent data updates. With no locks, concurrent programs run faster. Down-sides to Pony are low API stability, few high-quality third-party libraries and limited native tooling.