parseExpr :: String -> Expr
parseExpr input =
  case parse (contents expr) "<stdin>" input of
    Left err -> error (show err)
    Right ast -> ast
parseExpr :: String -> Expr
parseExpr input =
  case parse (contents expr) "<stdin>" input of
    Left err -> error (show err)
    Right ast -> ast
parseExpr input =
  case parse (contents expr) "<stdin>" input of
    Left err -> error (show err)
    Right ast -> ast