Decorated Corelations

Abstract: Let $\mathcal C$ be a category with finite colimits, and let $(\mathcal E,\mathcal M)$ be a factorisation system on $\mathcal C$ with $\mathcal M$ stable under pushouts. Writing $\mathcal C;\mathcal M^{\mathrm{op}}$ for the symmetric monoidal category with morphisms cospans of the form $\stackrel{c}\to \stackrel{m}\leftarrow$, where $c \in \mathcal C$ and $m \in \mathcal M$, we give method for constructing a category from a symmetric lax monoidal functor $F\colon (\mathcal C; \mathcal M^{\mathrm{op}},+) \to (\mathrm{Set},\times)$. A morphism in this category, termed a \emph{decorated corelation}, comprises (i) a cospan $X \to N \leftarrow Y$ in $\mathcal C$ such that the canonical copairing $X+Y \to N$ lies in $\mathcal E$, together with (ii) an element of $FN$. Functors between decorated corelation categories can be constructed from natural transformations between the decorating functors $F$. This provides a general method for constructing hypergraph categories---symmetric monoidal categories in which each object is a special commutative Frobenius monoid in a coherent way---and their functors. Such categories are useful for modelling network languages, for example circuit diagrams, and such functors their semantics.