Finding the number of different letters in a string, ignoring spaces

letter_cover.ml

let string_fold s f ini =  (* this is a left fold *)
  let len = String.length s in
  let rec loop i res = 
    if i >= len then res else loop (i+1) (f s.[i] res) 
  in
  loop 0 ini
  
let explode s =   (* a right string fold wouldn't need the reverse *)
  List.rev @@ string_fold s (fun c l -> c :: l) []
  
let unique l = 
  List.fold_right (fun i l -> if List.mem i l then l else i :: l) l []
  
let letter_cover s = 
  explode s 
  |> List.filter (fun c -> c <> ' ') 
  |> unique 
  |> List.length