let length str =
    let len = String.length str in
    
    let rec length_helper str curr = 
    if str = "" then 0 else 1 + length_helper (String.sub str 1 (len - curr - 1)) (curr + 1)
    
    in length_helper str 0;;

We use this code internally at Google. JFC.

By Mark, 2018-04-21 21:51:57