Monoidal categories provide an algebra of processes that compose sequentially and in parallel (MacLane). This algebra is abstract enough that it can deal with multiple paradigms of computation in a unified fashion: we have a monoidal category PAR of partial functions, a monoidal category STOCH of finite-support stochastic processes, or a monoidal category REL of relational queries. Moreover, monoidal categories have a practical graphical calculus in terms of string diagrams.
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