A Producer is an Unfold with an extract function added to extract the state. It is more powerful but less general than an Unfold.

A Producer represents steps of a loop generating a sequence of elements. While unfolds are closed representation of imperative loops with some opaque internal state, producers are open loops with the state being accessible to the user.

Unlike an unfold, which runs a loop till completion, a producer can be stopped in the middle, its state can be extracted, examined, changed, and then it can be resumed later from the stopped state.

A producer can be used in places where a CPS stream would otherwise be needed, because the state of the loop can be passed around. However, it can be much more efficient than CPS because it allows stream fusion and unecessary function calls can be avoided.