Editing Process calculus (section)

=== Communication ===
Interaction can be (but isn't always) a ''directed'' flow of information.  That is, input and output can be distinguished as dual interaction primitives. Process calculi that make such distinctions typically define an input operator (''e.g.'' <math>x(v)</math>) and an output operator (''e.g.'' <math>x\langle y\rangle</math>), both of which name an interaction point (here <math>\mathit{x}</math>) that is used to synchronise with a dual interaction primitive.

Should information be exchanged, it will flow from the outputting to the inputting process. The output primitive will specify the data to be sent. In <math>x\langle y\rangle</math>, this data is <math>y</math>.  Similarly, if an input expects to receive data, one or more [[bound variables]] will act as place-holders to be substituted by data, when it arrives. In <math>x(v)</math>, <math>v</math> plays that role. The choice of the kind of data that can be exchanged in an interaction is one of the key features that distinguishes different process calculi.