type error = errortype t = tval exists : t -> Fpath.t -> (bool, error) Stdlib.result Lwt.tval delete : t -> Fpath.t -> (unit, error) Stdlib.result Lwt.tval write : Cstruct.t -> ?off:int -> ?len:int -> [> `Write ] fd -> (int, error) Stdlib.result Lwt.tval read : Cstruct.t -> ?off:int -> ?len:int -> [> `Read ] fd -> (int, error) Stdlib.result Lwt.tval close : 'a fd -> (unit, error) Stdlib.result Lwt.t