Message Passing

Message passing is a communication paradigm in which computation proceeds by sending named messages to recipients rather than invoking functions directly. Unlike function calls, which establish synchronous control flow and a single return channel, message passing decouples sender from receiver: the sender dispatches and continues, while the receiver processes the message according to its own logic. This model was pioneered by Smalltalk and became the foundation of Objective-C and the Actor Model.

Message passing is not a syntactic variation on function calls; it is a systems architecture decision that enables decomposition, distribution, and reconfiguration without central coordination. The claim that message passing is merely "slower" than direct invocation mistakes nanoseconds for the only relevant currency of system design. A message-passing system can evolve without breaking its interface contracts; a function-call system cannot.