Optics for Premonoidal Categories
Abstract. We further the theory of optics or “circuits-with-holes” to encompass premonoidal categories: monoidal categories without the interchange law. Every premonoidal category gives rise to an effectful category (i.e. a generalised Freyd-category) given by the embedding of the monoidal subcategory of central morphisms. We introduce “pro-effectful” categories and show that optics for premonoidal categories exhibit this structure. Pro-effectful categories are the non-representable versions of effectful categories, akin to the generalisation of monoidal to promonoidal categories. We extend a classical result of Day to this setting, showing an equivalence between pro-effectful structures on a category and effectful structures on its free conical cocompletion. We also demonstrate that pro-effectful categories are equivalent to prostrong promonads.