[dgb23]

That's returning a transducer which is hairy implementation wise regardless of compilation.

[ltultraweight]

Transducers are awesome! They are so elegant. If one is interested in transducers I liked Ben Sless's intro: https://bsless.github.io/transducers-intro/ or in video "workshop form" https://www.youtube.com/watch?v=1sC71eb9Ox0