Excel Formula Leetcode problem in OCaml (https://leetcode.com/problems/design-excel-sum-formula/)

excel.ml

let rec range start stop = if start > stop then [] else start :: range (succ start) stop
let sum xs = List.fold_left (+) 0 xs
let (>>) f g a = a |> f |> g

module type Excel = sig
  type t
  val create: int -> char -> unit 
  val set: int -> char -> int -> unit
  val get: int -> char -> int
  val sum: int -> char -> string list -> int
end

module NaiveExcel () : Excel = struct 
  type cell = int * char
  type range = cell * cell
  type content = Number of int | Sum of range list 
  type t = content array array

  let ci c = int_of_char c + 1 - int_of_char 'A' (* ci, as in, column index *)

  let get_range ((sr, sc),(lr, lc)) = 
    let sc,lc = ci sc, ci lc in 
    range sr lr
    |> List.concat_map (fun row -> 
       range sc lc
       |> List.map (fun col -> row, col)) 

  (* A range can be a single cell (e.g A1) 
     or two cells seperated by a ':' (e.g A1:C3) *)
  let parse_range ur = 
    let parse_cell single = 
      match Str.split (Str.regexp "") single with 
      | [col;row] ->  int_of_string row, col.[0] 
      | _ -> failwith "Range parse error"
    in 
    match String.split_on_char ':' ur with
    | [start;finish] -> parse_cell start, parse_cell finish 
    | [single] -> let c = parse_cell single in c, c
    | _ -> failwith "Range parse error"
  
  let state : t ref = ref [||]
  let update st = state := st
  let updateIndex (y, x) value = !state.(y-1).(x-1) <- value 
  let index  (x, y) = !state.(y-1).(x-1)

  let create height width =
    let width = ci width in
    let t = Array.init height (fun _ -> Array.init width (fun _ -> Number 0)) in 
    update t

  let set row column value = updateIndex (row, ci column) (Number value) 
  
  let rec evaluate (expr : content) : int = 
    match expr with
    | Number n -> n
    | Sum ranges -> List.fold_right (get_range >> List.map (index >> evaluate) >> sum >> (+)) ranges 0

  let get row column = evaluate (index (row, ci column))

  let sum row column urs = 
    let ranges = List.map parse_range urs in 
    updateIndex (row, ci column) (Sum ranges) ;
    get row column
end

let () = 
  let module E = NaiveExcel () in 
  E.create 3 'C' ;
  E.set 1 'A' 2 ;
  let  _ = E.sum 3 'C' ["A1"; "A1:B2"] in 
  E.set 2 'B' 2 ;
  let result = E.get 3 'C' in 
  Printf.printf "C3 = %d" result