The theorem of factorisation forests shows the existence of nested factorisations — a la Ramsey — for finite words. This theorem has important applications in semigroup theory, and beyond. The purpose of this paper is to illustrate the importance of this approach in the context of automata over infinite words and trees. We extend the theorem of factorisation forest in two directions: we show th...

I introduce the notion of probability sheaf, which is a mathematical structure capturing the relationship between probabilistic concepts (such as random variable) and sample spaces. Various probability-theoretic notions can be (re)formulated in terms of category-theoretic structure on the category of probability sheaves. As a main example, I consider the Giry monad, which, in its original formu...

By the nonstandard analysis theory of turbulence, the governing equations of compressible turbulence are given. The equations can hold at non-uniform points, in fact, are new kind of equations. There are three choices. In the choice one, the second-order infinitesimal quantities are neglected, the closed equations about the point(monad)-average quantities are obtained. In other two choices, the...

The Hoare state monad provides a powerful means of structuring the verification of higher-order, stateful programs. This paper defines a new variant of the Hoare state monad, which, rather than being a triple of a pre-condition, a return type, and a post-condition, is a pair of a return type and a predicate transformer. We dub this monad the Dijkstra state monad. Using the Dijkstra state monad,...

Describing systems in terms of choices and their resulting costs rewards offers the promise freeing algorithm designers programmers from specifying how those should be made; implementations, can realized by optimization techniques and, increasingly, machine-learning methods. We study this approach a programming-language perspective. define two small languages that support decision-making abstra...

We describe a simplified categorical approach to Galois descent theory. It is well known that Galois descent is a special case of Grothendieck descent, and that under mild additional conditions the category of Grothendieck descent data coincides with the Eilenberg-Moore category of algebras over a suitable monad. This also suggests using monads directly, and our monadic approach to Galois desce...

By the nonstandard analysis theory of turbulence, the governing equations of compressible turbulence are given. The equations can hold at non-uniform points, in fact, are new kind of equations. There are three choices. In the choice one, the second-order infinitesimal quantities are neglected, the closed equations about the point(monad)-average quantities are obtained. In other two choices, the...

We describe a simplified categorical approach to Galois descent theory. It is well known that Galois descent is a special case of Grothendieck descent, and that under mild additional conditions the category of Grothendieck descent data coincides with the Eilenberg-Moore category of algebras over a suitable monad. This also suggests using monads directly, and our monadic approach to Galois desce...