Monoidal computer III: A coalgebraic view of computability and complexity

Monoidal computer is a categorical model of intensional computation, where many different programs correspond to the same input-output behavior. The upshot of yet another model of computation is that a categorical formalism should provide a high-level language for theory of computation, flexible enough to allow abstracting away the low level implementation details when they are irrelevant, or taking them into account when they are genuinely needed. A salient feature of the approach through monoidal categories is the formal graphical language of string diagrams, which supports geometric reasoning about programs and computations. In the present paper, we provide a coalgebraic characterization of monoidal computer. It turns out that the availability of interpreters and specializers, that make a monoidal category into a monoidal computer, is equivalent with the existence of a universal state space, that carries a weakly final state machine for all types of input and output. Being able to program state machines in monoidal computers allows us to represent Turing machines, and capture the time and space needed for their executions. The coalgebraic view of monoidal computer thus provides a convenient diagrammatic language for studying not only computability, but also complexity.