> Thanks for your explanations! To get closer to the problem that
> caused me to investigate the error: so there is no way to make f1
> into an argument in the following function iter_uniques (other than
> as a method or record field)?
>
> let f1 add empty = List.fold_right add ["foo"; "bar"; "bar"] empty ;;
> let f2 = print_endline ;;
>
> let iter_uniques comparison_fun =
>   let module StringSet =
>   Set.Make(struct
>     type t = string
>     let compare = comparison_fun
>   end) in StringSet.iter f2 (f1 StringSet.add StringSet.empty)
> ;;

Hmm, you could use a ref (which yes, is a record type).

let dyn_comparer = ref compare
let dyn_compare a b = !dyn_comparer a b

module SS = Set.Make(struct type t = string let compare = dyn_compare end);;

let iter_uniques f1 f2 = SS.iter f2 (f1 SS.add SS.empty);;