let kerning = ref [] in
    match ctx.ttf.ttf_kern with
    |None -> ();
    |Some(kern) ->
        List.iter (fun t ->
            match t.ks_def with
            | Kern0 (fmt) -> kerning := List.map(fun p ->
                {
                    font_char_code1 = p.kern_left;
                    font_char_code2 = p.kern_right;
                    font_adjust = round((float_of_int p.kern_value) *. scale *. 20.)
                })fmt.k0_pairs;
            | Kern2 (fmt) -> ();
        )kern.kern_subtables;

Error: This expression has type Swf.font_layout_data but is here used with type unit.