如何使用megaparsec获取AST节点的源码范围？

import Text.Megaparsec
import Text.Megaparsec.Char
import qualified Text.Megaparsec.Char.Lexer as L

-- 定义AST数据类型
data Expr = Lit Int | Add Expr Expr

-- 定义解析器
expr :: Parsec () String Expr
expr = do
  x <- integer
  space
  symbol "+"
  space
  y <- integer
  return (Add (Lit x) (Lit y))

-- 定义整数解析器
integer :: Parsec () String Int
integer = L.decimal

-- 定义符号解析器
symbol :: String -> Parsec () String String
symbol = L.symbol space

-- 解析文本并获取AST节点的源码范围
parseExpr :: String -> Either (ParseErrorBundle String ()) Expr
parseExpr = parse expr ""

-- 获取AST节点的源码范围
getRange :: Expr -> (SourcePos, SourcePos)
getRange ast = (start, end)
  where
    start = getSourcePos $ L.sourcePosPretty $ L.statePos $ state ast
    end = getSourcePos $ L.sourcePosPretty $ L.statePos $ stateEnd ast

-- 示例用法
main :: IO ()
main = do
  let input = "1 + 2"
  case parseExpr input of
    Left err -> putStrLn $ errorBundlePretty err
    Right ast -> do
      let (start, end) = getRange ast
      putStrLn $ "AST节点的源码范围："
      putStrLn $ "文件名：" ++ sourceName start
      putStrLn $ "起始行号：" ++ show (sourceLine start)
      putStrLn $ "起始列号：" ++ show (sourceColumn start)
      putStrLn $ "结束行号：" ++ show (sourceLine end)
      putStrLn $ "结束列号：" ++ show (sourceColumn end)