OK. It's a better map that can keep some state. (I scanned not read your doc and that was my takeaway; I have not thought about what I can do with state yet, particularly since I had never associated transducers with being able to keep state)

Can you give me an example of a classic problem (since we're talking about transformations, raytracers and compilers come to mind) where if you involve a transducer vs a map you get an interesting difference?

Edit: 'How state is kept is not specified': I assume that there are limitations to state keeping, particularly with the composability and performance pillars of transducers, but I'm just having a really hard time synthesizing everything.