Machines are demand-driven coroutines. They are similar to Pipes, Conduits, or Iteratees, but can support inputs of arbitrary complexity and are entirely pure. Instead of having monadic effects of its own, a Machine can be driven by a Driver which can have effects when feeding the Machine input or when reading its output.
You design a Machine by writing a Plan. You then compile the machine if it is to run once to completion, or designate it to run repeatedly. Plan provides a monadic API for building machines, but Machines themselves do not form a monad.
Simple machines that read from one input source are called a Process and processes form a Category. More generally you can attach a Process to the output of any type of Machine, yielding a new Machine.
More complicated machines provide other ways of connecting to them.