I haven't ever seen that in OCaml. Probably an F# extension.
If you're going to complain about OCaml's syntax, I would start with having to use different arithmetic ops for int vs. floating point (+ vs. +.) and different indexing operators for strings and arrays ( s.(0) vs. s.[0] ).
You get used to it pretty quickly, and OCaml more than makes up for it in other ways, but it is a bit annoying.
He mentioned that he did a bunch of work object-system work on Caml that was published, but the INRIA guys didn't put it into the main version when they developed OCaml. It's possible it got transmitted into F# that way. Next time I see him I'll ask about it.
If you're going to complain about OCaml's syntax, I would start with having to use different arithmetic ops for int vs. floating point (+ vs. +.) and different indexing operators for strings and arrays ( s.(0) vs. s.[0] ).
You get used to it pretty quickly, and OCaml more than makes up for it in other ways, but it is a bit annoying.