You could keep the state using the state monad, or by letting every reducer keep a transparent state in a linked list that whoever is pushing values through it

You could keep the state using the state monad, or by letting every reducer keep a transparent state in a linked list that whoever is pushing values through it has to handle. The reference implementation keeps it hidden using closures. This is mostly an API thing. In clojure you can pass a transducer when you create a channel. That way you can make a channel do just about anything. Send data in chunks of N. Filter Odd numbers. Or just do arbitrary transformations. It is a protocol for composable transformations of data being passed in one direction. It is not fancy. Not really hard to understand. A generalization of map, filter, and friends.