# Profunctor

In category theory, a branch of mathematics, **profunctors** are a generalization of relations and also of bimodules. They are related to the notion of correspondences.

## Definition

A **profunctor** (also named **distributor** by the French school and **module** by the Sydney school) from a category to a category , written

is defined to be a functor

where denotes the opposite category of and denotes the category of sets. Given morphisms respectively in and an element , we write to denote the actions.

Using the cartesian closure of , the category of small categories, the profunctor can be seen as a functor

where denotes the category of presheaves over .

A correspondence from to is a profunctor .

### Composition of profunctors

The composite of two profunctors

is given by

where is the left Kan extension of the functor along the Yoneda functor of (which to every object of associates the functor ).

It can be shown that

where is the least equivalence relation such that whenever there exists a morphism in such that

### The bicategory of profunctors

Composition of profunctors is associative only up to isomorphism (because the product is not strictly associative in **Set**). The best one can hope is therefore to build a bicategory **Prof** whose

- 0-cells are small categories,
- 1-cells between two small categories are the profunctors between those categories,
- 2-cells between two profunctors are the natural transformations between those profunctors.

## Properties

### Lifting functors to profunctors

A functor can be seen as a profunctor by postcomposing with the Yoneda functor:

It can be shown that such a profunctor has a right adjoint. Moreover, this is a characterization: a profunctor has a right adjoint if and only if factors through the Cauchy completion of , i.e. there exists a functor such that .

## References

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