# Limits of bimorphic lenses

arXiv: https://arxiv.org/abs/1808.05545 · HTML: https://mlsanity.com/paper/1808.05545/

## Key facts

- Cited by 4 papers in the map (data through 14 Oct 2026).
- First posted on arXiv 16 Aug 2018.

## Built on by (most cited first, 4 of 4)

- [Categorical Foundations of Gradient-Based Learning](https://mlsanity.com/paper/2103.01931/) (2021)
- [Category Theory in Machine Learning](https://mlsanity.com/paper/2106.07032/) (2021)
- [Deep Learning with Parametric Lenses](https://mlsanity.com/paper/2404.00408/) (2024)
- [Space-time tradeoffs of lenses and optics via higher category theory](https://mlsanity.com/paper/2209.09351/) (2022)

## Abstract (arXiv)

Bimorphic lenses are a simplification of polymorphic lenses that (like polymorphic lenses) have a type defined by 4 parameters, but which are defined in a monomorphic type system (i.e. an ordinary category with finite products). We show that the category of bimorphic lenses is complete when the base category is complete, cocomplete and cartesian closed, and so symmetric bimorphic lenses can be defined as spans of ordinary bimorphic lenses. This is in contrast to monomorphic lenses, which do not have pullbacks, and for which the category of spans can be defined in an ad-hoc way only when the lenses satisfy a certain axiom (the put-get law). This is a step towards a theory of symmetric polymorphic lenses. Bimorphic lenses additionally play an essential role in compositional game theory, and spans of bimorphic lenses are a step towards a compact closed category of open games.