OCaml variant augmentation boilerplates

Question

Is there a way to efficiently "augment" or attach some more information without much boilerplate?

That is, given types

type orig =
 | A
 | B of orig
 | C of orig * orig
and type dat = int
and type aug =
 | AugA of dat
 | AugB of dat * aug
 | AugC of dat * aug * aug

and functions

let _cnt = ref 0
let gen_dat () =
 let _ = _cnt := !_cnt + 1 in
 !_cnt
let rec augment (o: orig) : ao =
 match o with
 | A -> AugA (gen_dat ())
 | B o -> AugB (gen_dat (), augment o)
 | C (o1, o2) -> AugC (gen_dat (), augment o1, augment o2)

I think this is a trivial structural recursion on its components, and I have to do repeat this type of identical manipulations on several types with 10+ patterns (Multiple augs each with its own augment function associated).

This leads to a long, tedious boilerplate code. Is there a way to abstract away this manipulation in OCaml, e.g., with help of ppx?

• Chris: It looks like maybe type aug might be defined as type aug = Aug of int * orig. While there likely is a way to do this with ppx (which I have not mastered nor explored in depth) this might also verge into XY problem territory depending on what you're ultimately trying to achieve.
• Jay Lee: @Chris I'm sorry for the confusion; There was a typo while translating my code in to a MWE: aug should be a recursive data type using itself, not orig. In such case, isn't type aug = Aug of int * orig not viable?
• Chris: Suggest rewriting gen_dat as let gen_dat () = _cnt := !_cnt + 1; !_cnt. The local let binding is unnecessary.
• Maëlan: I’m sure someone will come up with relevant PPX tools. In the meantime, if you have the possibility to modify the orig type, perhaps you could make it a type constructor that takes the dat type as a parameter. Then you’d just have to write a recursive function map_dat : ('a -> 'b) -> 'a aug -> 'b aug (or have it generated by PPX magic), which might be a useful factorization?
• Maëlan: I really meant 'a aug -> 'b aug. You’d have only one inductive type, and you’d define orig as unit aug.