Why is the output type bool in this OCaml fun?

I'm sure I'm missing something, but where is the bool?

let iter t ~f =   let rec loop !t  = function    |Some element -> ~f element.value; loop element.next    |None -> () 

Expecting unit, have bool in type of iter:

('a element option -> unit) ref -> f:('a -> 'b) -> bool

Also compared to this version

let iter t ~f =   let rec loop   = function    |Some element -> f element.value; loop element.next    |None -> () in loop !t

Why 'a element option ref vs. ('a element option -> unit)?