config_via_effects.ml

(* Example due to @emillon. Mistakes my own. *)

type conf = { precision : int }

effect Get_conf : conf

let with_conf (conf: conf) ~f =
  try f () with effect Get_conf k -> continue k conf
  
let get_precision () = (perform Get_conf).precision
  
type term =
  | Float of float
  | Plus of term * term
  
(* A pretty-printer for [term]s that supports dynamically-specified float precision. *)
let rec pp_term ppf = function
  | Float f -> Format.fprintf ppf "%.*f" (get_precision ()) f
  | Plus (a, b) -> Format.fprintf ppf "(%a) + (%a)" pp_term a pp_term b
  
let () =
  let term = Plus (Float 1., Plus (Float 1., Float 1.)) in
  with_conf { precision = 2 } ~f:(fun () ->
    Format.printf "%a\n" pp_term term
  )