linear type File
    { handle : i32 }

    --a private type, "evidence" for file state
    adt Opened<File f>{ c_opened :: Opened<f> }
    adt Closed<File f>{ c_closed :: Closed<f> }

    new :: () -> (f::File, Closed f)

    open :: (this::File) -> Closed this -> Path -> Either (f::File, Closed f, Error) (f::File, Opened f)

    read :: (this::File) -> Opened this -> int -> Either (f::File , Closed f, Error) (f::File, Opened f, Bytes)
    write :: (this::File) -> Opened this -> Bytes -> Either (f::File, Closed f, Error) (f::File, Opened f)

    close :: (this::File) -> Opened this -> (f::File, Closed f)

File.new () =
    let
        f = file { InvalidHandle }
    in
        (f, c_closed f)

File.open this ev_closed path =
    let h = fopen(path, "rw")
    in if h == 0
    then let f = File{h} in (Left f, c_closed f, "can't open the file")
    else (Right f, c_opened f)