In Ocaml, this bit of ugliness is not needed. By not exposing the implementation type in the interface, one can have a type that is an int but is not usable as an int outside of the model. There is no added indirection or cost because the types disappear at compile-time, after they have been checked.

foo.mli

type t
val of_int : int -> t
val to_int : t -> int
val add : t -> t -> t

foo.ml

type t = int
let of_int i = i
let to_int t = t
let add = (+)

Will compile:

Foo.to_int (Foo.add (Foo.of_int 1) (Foo.of_int 2))

Won’t compile:

Foo.add 1 2