Regarding state: you can make thread safe transducers. The current SRFI 171 reference implementation is NOT thread safe. You can create a transducer and use it

Regarding state: you can make thread safe transducers. The current SRFI 171 reference implementation is NOT thread safe. You can create a transducer and use it across different threads no problem. But you cannot start a transducer and use the returned reducer in different threads. It uses hidden mutable state.