Drawing a ball from an urn filled with balls of different colours is one of the basic models in probability theory. The probability of drawing a ball of a particular colour is determined by the proportion / fraction of balls of that colour. This paper introduces a new stochastic model for such urns: draw a ball, recolour (repaint) it, and put it back into the urn. One can distinguish four modes of drawing-and-recolouring, namely whether done proportionally or uniformly (both for drawing and recolouring). These modes can be reformulated in financial terms as redistribution of wealth or in biological terms as evolutionary drift. The resulting four operations form a coalgebra for the distribution monad, on the set of multisets of a fixed size. In fact they form a Markov chain and even a hidden Markov model, in combination with the frequentist learning map as emission channel. This paper identifies fixed points, capturing stable situations, for these four modes of drawing-and-recolouring.
We introduce effectful Mealy machines - a general notion of Mealy machine with global effects - and give them semantics in terms of both bisimilarity and traces. Bisimilarity of effectful Mealy machines is characterized syntactically, via free uniform feedback. Their traces are given a novel semantic coinductive universe in terms of effectful streams. We prove that this framework generalizes standard causal processes and captures existing flavours of Mealy machine, bisimilarity, and trace. This is an extended abstract for the manuscript Effectful Mealy Machines: Bisimulation and Trace that will appear in the proceedings of LiCS 2025 [3]; an extended version with proofs is also available (arxiv.org/abs/2410.10627) [4]. We additionally characterise causal processes as lax-natural transformations.
We investigate a representation of computable functions as total functions over 2(infinity), the set of finite and infinite sequences over {0, 1}. In this model, infinite sequences are interpreted as non-terminating computations whilst finite sequences represent the sum of their digits. We introduce a new definition principle, function space corecursion, that simultaneously generalises minimisation and primitive recursion. This defines the class of computable corecursive functions that is closed under composition and function space corecursion. We prove computable corecursive functions represent all partial recursive functions, and show that all computable corecursive functions are indeed computable by translation into the untyped lambda-calculus.
