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A stateless actor represents a pure "message-based function". Since each message computation is guaranteed to have no side effects to the actor, the runtime system can invoke the message handler of a stateless actor concurrently. Hence, a stateless actor is the ultimate working horse if it comes to parallel computations. However, stateless actors require some serious changes to libcppa's scheduling.
Doesn't really fit into the design of libcppa and C++ has no pure keyword to make it safe to implement.