[lower]

The OCaml implementation used to be hard-coded in the type system, but has now been rewritten (in 4.02, I think) using GADTs.

https://ocaml.org/meetings/ocaml/2013/proposals/formats-as-g...

https://caml.inria.fr/mantis/view.php?id=6017

The compiler only does syntactic desugaring of format strings now.