# nLab Kleisli object

### Context

#### 2-Category theory

2-category theory

## Structures on 2-categories

#### Higher algebra

higher algebra

universal algebra

# Contents

## Definition

Given a monad $t \colon a \to a$ in a 2-category $K$, the Kleisli object $a_t$ of $t$ is, if it exists, the universal right $t$-module or $t$-opalgebra. Equivalently, $a_t$ represents the functor $RMod(-,t)$ that takes an object $x$ of $K$ to the category of right $t$-modules $a \to x$.

This means that there is a morphism $f_t \colon a \to a_t$ and a 2-cell $\lambda \colon f_t t \Rightarrow f_t$ that induce an isomorphism $K(a_t,x) \cong RMod(x,t)$: given a right $t$-module $r \colon a \to x, \alpha \colon r t \to r$, there is a unique morphism $a_t \to x$ whose composite with $f_t$ (repsectively $\lambda$) is equal to $r$ (resp. $\alpha$).

## Examples

• The motivating example is that of Kleisli categories for monads in Cat.

• In a (locally ordered) bicategory of relations, the Kleisli object of a monad $t$ is part of a splitting of $t$ as an idempotent.

• For a monad $T \colon A ⇸ A$ in the bicategory Prof of profunctors, its Kleisli object consists of a category $A_T$ equipped with a bijective-on-objects functor $A\to A_T$. The category $A_T$ has the same objects as $A$, with hom-sets $A_T(a,b) = T(a,b)$. Identities and composition are given by the unit and multiplication of $T$.

Every functor $B \to A$ yields a monad $A(f,f)$ in $Prof$, whose Kleisli object is part of the (bijective on objects, fully-faithful) factorization $B \to A_{A(f,f)} \to A$ of $f$.

Because of this, we can identify a monad on $A$ in $Prof$ with a bijective-on-objects functor $A \to B$.

## Remarks

• A Kleisli object in a 2-category $K$ is the same as an Eilenberg-Moore object in $K^{op}$; see opposite 2-category. Kleisli objects for monads in $K^{co}$ can be identified with Kleisli objects for comonads in $K$.

• A Kleisli object for a monad $t$ in $K$ can equivalently be defined as a particular sort of weighted 2-colimit, namely the lax colimit of the lax functor $\ast \to K$ corresponding to $t$.

## References

An original article is

• R. Street, The formal theory of monads, J. Pure Appl. Alg. 2, 149–168 (1972)

Generalization from monads to more general categories enriched in a bicategory (where “Kleisli objects” are then called “collages”) is in

Revised on December 8, 2013 02:56:58 by Urs Schreiber (89.204.135.58)