let ( let* ) o f = Result.fold ~ok:f ~error:Result.error o

type extended_status =
  [ Dream.status
  | `See_Other_Redirect of (string * string) list
  ]

let ( let** ) o f =
  match o with
  | Error (kind, msg) -> begin
    match kind with
    | `See_Other_Redirect headers ->
      Dream.respond ~status:`See_Other ~headers msg
    | #Dream.status as status -> Template.err (status, msg)
  end
  | Ok v -> f v