Distributive law between monads

In category theory, an abstract branch of mathematics, distributive laws between monads are a way to express abstractly that two algebraic structures distribute one over the other. Suppose that ( S , μ S , η S ) {\displaystyle (S,\mu ^{S},\eta ^{S})} and ( T , μ T , η T ) {\displaystyle (T,\mu ^{T},\eta ^{T})} are two monads on a category C. In general, there is no natural monad structure on the composite functor ST. However, there is a natural monad structure on the functor ST if there is a distributive law of the monad S over the monad T. Formally, a distributive law of the monad S over the monad T is a natural transformation l : T S → S T {\displaystyle l:TS\to ST} such that the diagrams commute.

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