Why do we have type 'f field in a bunch of places?

[mimoo]

I finally understood why we have type 'f field = field in a bunch of places : D

Basically if you have a type field in your interface, and use this field everywhere in the interface (e.g. val thing : t -> field) then you can't apply that interface to a module that's generic over a field.

So for example, I'm writing my interface for Cvars with an abstract type field:

module type Cvar_intf = sig
  type field

  type t [@@deriving sexp]

  val to_constant_and_terms : t -> field option * (field * int) list

  (* and so on... *)

but if I want to apply it to my module T:

module T = struct
  type 'f t =
    | Constant of 'f
    | Var of int
    | Add of 'f t * 'f t
    | Scale of 'f * 'f t
  [@@deriving sexp]

  type 'f cvar = 'f t [@@deriving sexp]

I can't. I want to be able to write this:

module T : Cvar_intf with type field = 'f = (* ... *)

which I can't. Using a type type 'f field is the only solution for an interface that can be apply to both a generic implementation (over a field in this case) and an instantiated implementation (where the field is known to be Backend.Field.t for example).