使用`map`遍历递归数据结构时如何通过总计检查

Question

例如，我有以下代码：

data Tree = Branches (List Tree) | Node Int

sumTree : Tree -> Int
sumTree (Branches xs) = sum (map sumTree xs)
sumTree (Node x) = x

如果我希望它通过总体检查，则必须手动展开地图：

data Tree = Branches (List Tree) | Node Int

sumTree : Tree -> Int
sumTree (Branches xs) = sum' xs where
  sum' [] = 0
  sum' (x::xs) = sumTree x + sum' xs
sumTree (Node x) = x

如果我想避免在所有地方都使用assert_total，那么函数式编程就可以发挥很多功能。有什么方法可以解决此问题？