Hlist for format printing

hlist_and_format.ml

module L = struct
  type 'args t =
    | [] : unit t
    | (::): 'a * 'b t -> ('a -> 'b) t
end

let rec print_list: type args result.
  Format.formatter -> args -> args L.t -> unit  =
  fun ppf print -> function
    | [] -> ()
    | arg :: rest -> print_list ppf (print arg) rest

let fprintf ppf fmt args =
  print_list ppf (Format.fprintf ppf fmt) args

let fatal_error fmt args =
  let ppf = Format.err_formatter in
  fprintf ppf fmt args;
  fprintf ppf "@." [];
  failwith "Unrecoverable error"

let test = if false then fatal_error "%s" ["foo";"bar"]