nLab relative monad

Idea

A relative monad $T : \mathbf J \to \mathbf C$ is much like a monad except that it is not an endofunctor on one category, but more generally a functor between two different categories. To even formulate such a notion, (for instance the definition of the unit), the two categories have to be related somehow, typically via a specified comparison functor $J \colon \mathbf J \to \mathbf C$, in which case we say that $T$ is a monad relative to $J$. Ordinary monads are then the special case where $J$ is the identity functor.

Definitions

Let $\mathbf J, \mathbf C$ be categories and $J \colon \mathbf J \to \mathbf C$ be a functor.

As skew Kleisi triples

Definition

A relative monad $T$ on $J$ is a functor $T \colon \mathbf J \to \mathbf C$ equipped with

1. a unit $\eta_X \colon J X \to T X$ natural in $X \colon \mathbf J$,

2. a Kleisli extension $(-)^* \colon \mathbf C(J X, T Y) \to \mathbf C(T X, T Y)$ in both $X,Y \colon \mathbf J$

such that for every $X,Y,Z \colon \mathbf J$ and $k \colon J X \to T Y$ the following equations hold:

1. (left unitality) $k = k^* \circ \eta_X$,

2. (right unitality) $\eta_X^* = 1_{T X}$,

3. (associativity) $(\ell^* \circ k)^* = \ell^* \circ k^*$ for every $\ell : J Y \to T Z$.

Notice that for any $f \colon X \to Y$ in $\mathbf J$, one has

$$T f \;=\; \big( \eta_{J Y} \circ J f \big)^* \,.$$

Example

(ordinary monads as Kleisli triples/extension systems)
In the special case that $\mathbf{J} = \mathbf{C}$ and $J = id$ the identity functor, Def. reduces to the definition of ordinary monads, but in the guise known as “Kleisli triples” or “extension systems” which is nominally different from (though equivalent to) the way monads are traditionally presented in category theory/categorical algebra (namely as monoid objects internal to a category of endofunctors), but is exactly the form commonly used for monads in computer science.

As monoids in a skew-monoidal category

The notion of a skew-monoidal category is like that of a monoidal category except that the unitors and associators are not necessarily invertible. Monoids? may be defined in a skew-monoidal category analogously as to in a monoidal category.

In the general case that $\mathbf{J}$ is distinct from $\mathbf{C}$, the functor category $Func(\mathbf J, \mathbf C)$ lacks a natural monoidal category structure (as opposed to the case of endofunctors $Func(\mathbf{C}, \mathbf{C})$) so that the usual definition of monads as monoids cannot apply — but a suitable “skew” variant works:

The (skew-)composition of functors $F,G \colon \mathbf J \to \mathbf C$ may be defined by first extending $F$ along $J$ and then composing with $G$. The resulting composition product on $[\mathbf J, \mathbf C]$ is coherent but only laxly so, hence the need to appeal to skew-monoidal categories:

Theorem

(ACU14, Thm. 3.4)
Suppose $J \colon \mathbf J \to \mathbf C$ is such that $\mathrm{Lan}_J \colon [\mathbf J, \mathbf C] \to [\mathbf C, \mathbf C]$ exists (e.g. if $\mathbf J$ is small and $\mathbf C$ cocomplete). Then $[\mathbf J, \mathbf C]$ is skew-monoidal, with unit $J$ and product $F \circ^J G = (\mathrm{Lan}_J F) \circ G$, and a relative monad is a monoid in $([\mathbf J, \mathbf C], J, \circ^J)$.

When $J:\mathbf J \to \mathbf C$ is a free completion of $\mathbf{J}$ under colimits from some set $\mathcal{F}$ of indexing types, then this skew-monoidal structure on $[\mathbf J, \mathbf C]$ is properly monoidal, since it is equivalent to the $\mathcal{F}$-colimit preserving functors $\mathbf C\to\mathbf C$, and the monoidal structure is just functor composition.

Relative to a Profunctor

The above definition makes sense even more generally when $J$ is a profunctor $\mathbf J^{op} \times \mathbf {C} \to Set$, i.e., we require

1. a unit $\eta_X \colon J(X, T X)$, a natural in $X \colon \mathbf J$, that is an element of the end, $\eta \in \int_{X: \mathbf J}J(X, T X)$

2. a Kleisli extension $(-)^* \colon J(X, T Y) \to \mathbf C(T X, T Y)$ natural in both $X,Y \colon \mathbf J$

with essentially the same equations. This generalizes the previous definition by defining the profunctor to be $\mathbf C(J-,=)$.

Examples

Generic examples

Example

(relative monads induced from actual monads) Any actual monad $T$ on $\mathbf{C}$ induces a relative monad $T J$ on $J$, for any $J \colon \mathbf{J}\to \mathbf{C}$.

A concrete instance of this case is spelled out in Exp. below.

Example

A relative monad on the embedding $J \colon \mathbf{FinSet} \to \mathbf {Set}$ is the same thing as an abstract clone. These are equivalent to finitary monads and single-sorted algebraic theories.

Example

Fixing a category $\mathbb{V}$ with finite products, to give a Freyd category is to give a strong relative monad on the Yoneda embedding $\mathbb{V}\to [\mathbb{V}^{\mathrm{op}},\mathbf{Set}]$.

Example

The presheaf category-construction $(\mathbb{C}\mapsto [{\mathbb{C}}^{\mathrm{op}},\mathbf{Set}])$ may be regarded as a relative pseudomonad on the inclusion $\mathbf{Cat}\to \mathbf{CAT}$. (See also Yoneda structures.)

Example

A monad on $\mathbf C$ with arities in $\mathbf{J}\subseteq \mathbf C$ is the same thing as a relative monad for the embedding $\mathbf{J}\to \mathbf C$. (Here $\mathbf{J}\subseteq \mathbf{C}$ is required to be a dense subcategory, so that to give a functor $\mathbf{J}\subseteq \mathbf{C}$ is to give a functor $\mathbf{C}\to\mathbf{C}$ preserving $J$-absolute colimits.)

Specific examples

Example

(linear span)
We spell out the simple but maybe instructive example of the construction which sends a set $B$ to the vector space which it spans, i.e. to the $B$-indexed direct sum of some ground field $\mathbb{K}$, regarded in $\mathbb{K}$-vector spaces.

In detail, for $\mathbb{K}$ any ground field, consider:

1. $\mathbf{J} \coloneqq$ Set;

2. $\mathbf{C} \coloneqq \int_{B \colon Set} Vect_B$ (or “VectBund”, for short, see there for more) the category of indexed sets of vector spaces – hence of vector bundles over sets (i.e. over discrete topological spaces)

   $$\array{ \mathscr{H} \\ \big\downarrow \\ B }$$

   with possibly base-changing vector bundle maps between them:

   $$\array{ \mathscr{H} &\longrightarrow& \mathscr{H}' \\ \big\downarrow && \big\downarrow \\ B &\underset{f}{\longrightarrow}& B' }$$

3. the relativization functor given by sending a set to the trivial tensor unit-bundle over it:

   (1)$$\array{ J &\colon& Set &\longrightarrow& \int_{B \colon Set} Vect_B \\ && B &\mapsto& B \times \mathbb{K} }$$

Notice that for each map $f \;\colon\; B \to B'$ of base sets, there is a base change adjoint triple of functors

(2)$$f_! \dashv f^\ast \dashv f_\ast \;\;\colon\;\; Vect_{B} \leftrightarrow Vect_{B'} \,.$$

In particular, for $S' = \ast$ the terminal singleton set, the left base change along the unique $p_X \colon S \to \ast$ is the operation which forms the direct sum of the (fiber-)vector spaces in the bundle, and regards the resulting vector space as a bundle over the point:

$$(p_B)_! \;\colon\; Vect_B \longrightarrow Vect_\ast \,=\, Vect \,.$$

In view of this, we claim that the functor

$$\array{ Q &\colon& Set &\longrightarrow& \int_{B \colon Set} Vect_{B} \\ && S &\mapsto& (p_B)_!\big( B \times \mathbb{K} \big) \mathrlap{ \;\simeq\; \underset{b \colon B}{\bigoplus} \mathbb{K} } }$$

which may be understood as sending a set to its $\mathbb{K}$-linear span,

carries the structure of a monad relative to the functor $J$ from (1) with

1. unit given by

   $$\array{ \eta_{B} &\colon& B \times \mathbb{K} &\longrightarrow& \underset{b \colon B}{\bigoplus} \mathbb{K} \\ && (b,k) &\mapsto& (k)_b }$$

2. Kleisli extension given by

   $$\Big( B \times \mathbb{K} \xrightarrow{f} \underset{ \mathclap{b' \colon B'} }{\oplus} \mathbb{K} \Big) \;\;\;\;\mapsto\;\;\;\; \Big( \underset{b \colon B}{\oplus} \mathbb{K} \xrightarrow{ \big( f(b,-) \big)_{b \colon B} } \underset{b' \colon B}{\oplus} \mathbb{K} \Big)$$

This specific example may be understood as a special case of the general situation of relative monads induced from an actual monad (Exmp. ): Here the actual monad in question is:

$$\array{ \triangle &\colon& \int_{B \colon Set} Vect_B &\longrightarrow& \int_{B \colon Set} Vect_B \\ && \left[ \array{ \mathscr{V} \\ \big\downarrow\mathrlap{{}^p} \\ B } \;\;\; \right] &\mapsto& p_! \mathscr{V} }$$

This monad is in fact the reflective localization of the reflective subcategory-embedding of plain VectorSpaces into bundled/parameterized vector spaces:

$$Vect \;\simeq\; Vect_\ast \xhookrightarrow{\phantom{--}} \int_{B \colon Set} Vect_B \,.$$

Related pages

• relative adjunction

• relative comonad

• extension system, Kleisli triple, monad in computer science

References

The concept was introduced, in the context of monads in computer science, in:

• Thorsten Altenkirch, James Chapman, Tarmo Uustalu, Monads need not be endofunctors, Logical Methods in Computer Science 11 1:3 (2015) 1–40 [arXiv:1412.7148, pdf, doi:10.2168/LMCS-11(1:3)2015]