Hello
PolyMath contains packages that implement different optimization and statistical analysis algorithms that are used in machine learning (like linear regression, least squares, hill climbing and so on). It also has an implementation of a genetic algorithm.
And there are several implementations of neural networks (this one is cool:
http://smalltalkhub.com/#!/~abergel/NeuralNetworks). I am working on my own implementation of neural nets (
http://smalltalkhub.com/#!/~Oleks/NeuralNetwork), you can read this, if you are interested:
https://medium.com/towards-data-science/single-layer-perceptron-in-pharo-5b13246a041d.
I didn't hear of any other ML algorithms implemented in Pharo, but maybe you will find something here:
http://smalltalkhub.com/list