let rec eval expr lst =
    match expr with
    | ConstInt i                  -> (pushNum (IntVal i)) :: lst
    | PrimOp ("+", lOper, rOper)  -> (evalNumericExpr (+) lOper rOper) :: lst
    | _                 -> failwith "Syntax error"

and evalNumericExpr op lOper rOper =
    let left = eval lOper
    let right = eval rOper
    match (op) with
    | (+) -> "ADD"
    | _   -> failwith "Syntax error"

let compile expr =
    eval expr []