module rec Expr : sig
  type expr =
    | Num of int
    | Add of expr * expr
    | Mul of expr * expr
    | Seq of Stmt.stmt * expr
  val eval : expr -> int
end = struct
  type expr =
    | Num of int
    | Add of expr * expr
    | Mul of expr * expr
    | Seq of Stmt.stmt * expr
  let rec eval = function
    | Num k -> k
    | Add (e, f) -> eval e + eval f
    | Mul (e, f) -> eval e * eval f
    | Seq (s, e) ->
        Stmt.run s ;
        eval e
end

and Stmt : sig
  type stmt =
    | Print of string
    | Compute of Expr.expr
    | Exit
  val run : stmt -> unit
end = struct
  type stmt =
    | Print of string
    | Compute of Expr.expr
    | Exit
  let run = function
    | Print msg -> print_endline msg
    | Compute e -> Printf.printf "%d\n" (Expr.eval e)
    | Exit -> exit 0
end