I wrote this, which I find more usable than racket's loops: https://rikspucko.koketteriet.se/bjoli/goof-loop

It does not use mutation, and should produce just as efficient code as the racket loops. It is also extensible (and easily portable to racket!). It is written almost exclusively in syntax-rules.

Guile fibers can be found here: https://codeberg.org/fibers/fibers