> they seem to me more implementable in C++ than in Haskell

If you don't mind the overhead, it's ContT wrapping some base monad. Or you could make it integral. That is more specific than transducers, I grant. But there's not much they can do that ContT over like Array does not.

I think it's too bad they were invented in Clojure, because they're brutally hard to use right and write in clojure.

I have some interesting work I could share in private on using them to provide stream fusion properties to pipelines of channels. Hit me up on keybase if you'd like to see. I don't want to open source it because then people will expect me to explain them and maintain them and I don't feel like it :)