strong monad

In category theory, a strong monad is a monad on a monoidal category with an additional natural transformation, called the strength, which governs how the monad interacts with the monoidal product.

Strong monads play an important role in theoretical computer science where they are used to model computation with side effects.{{cite journal|last1=Moggi|first1=Eugenio|title=Notions of computation and monads|journal=Information and Computation|date=July 1991|volume=93|issue=1|pages=55–92|doi=10.1016/0890-5401(91)90052-4|url = http://www.disi.unige.it/person/MoggiE/ftp/ic91.pdf|doi-access=free}}

Definition

A (left) strong monad is a monad (T, η, μ) over a monoidal category (C, ⊗, I) together with a natural transformation t_A,B : A ⊗ TB → T(A ⊗ B), called (tensorial) left strength, such that the diagrams

:Image:Strong monad left unit.svg, Image:Strong monad associative.svg,

:Image:Strong monad unit.svg, and Image:Strong monad multiplication.svg

commute for every object A, B and C.

Commutative strong monads

For every strong monad T on a symmetric monoidal category, a right strength natural transformation can be defined by

$t'_{A,B}=T(\gamma_{B,A})\circ t_{B,A}\circ\gamma_{TA,B} : TA\otimes B\to T(A\otimes B).$

A strong monad T is said to be commutative when the diagram

:Image:Strong monad commutation.svg

commutes for all objects $A$ and $B$ .

Properties

The Kleisli category of a commutative monad is symmetric monoidal in a canonical way, see corollary 7 in Guitart{{Cite journal |last=Guitart |first=René |date=1980 |title=Tenseurs et machines |url=http://www.numdam.org/item/?id=CTGDC_1980__21_1_5_0 |journal=Cahiers de topologie et géométrie différentielle |language=en |volume=21 |issue=1 |pages=5–62 |issn=2681-2398}} and corollary 4.3 in Power & Robison.{{Cite journal |last1=Power |first1=John |last2=Robinson |first2=Edmund |date=October 1997 |title=Premonoidal categories and notions of computation |url=https://www.cambridge.org/core/product/identifier/S0960129597002375/type/journal_article |journal=Mathematical Structures in Computer Science |language=en |volume=7 |issue=5 |pages=453–468 |doi=10.1017/S0960129597002375 |issn=0960-1295}} When a monad is strong but not necessarily commutative, its Kleisli category is a premonoidal category.

One interesting fact about commutative strong monads is that they are "the same as" symmetric monoidal monads.{{Cite journal |last=Kock |first=Anders |date=1972-12-01 |title=Strong functors and monoidal monads |url=https://link.springer.com/article/10.1007/BF01304852 |journal=Archiv der Mathematik |language=en |volume=23 |issue=1 |pages=113–120 |doi=10.1007/BF01304852 |issn=1420-8938}} More explicitly,

a commutative strong monad $(T,\eta,\mu,t)$ defines a symmetric monoidal monad $(T,\eta,\mu,m)$ by $m_{A,B}=\mu_{A\otimes B}\circ Tt'_{A,B}\circ t_{TA,B}:TA\otimes TB\to T(A\otimes B)$
and conversely a symmetric monoidal monad $(T,\eta,\mu,m)$ defines a commutative strong monad $(T,\eta,\mu,t)$ by $t_{A,B}=m_{A,B}\circ(\eta_A\otimes 1_{TB}):A\otimes TB\to T(A\otimes B)$

and the conversion between one and the other presentation is bijective.

References

External links

[https://ncatlab.org/nlab/show/strong+monad Strong monad] at the nLab

Category:Adjoint functors

Category:Monoidal categories