This is a blast from the past! And indeed, I use Eio these days to do direct-style IO; you can see an example of a HTTP/1.1 parser done in that style, and there's nary a monad in sight! https://tangled.org/anil.recoil.org/ocaml-requests/blob/main...

OCaml's effects are similar to thethe delimcc hack used in the article, except that it's nicely integrated into OCaml 5 and very high performance; basically a stack switch: https://anil.recoil.org/papers/2021-pldi-retroeff.pdf