相关疑难解决方法(0)

例如,给出以下树数据类型:

data Tree a = Node [Tree a] | Leaf a deriving Show
type Sexp = Tree String

如何使用高阶组合器表达"漂亮"功能,该组合打印出适当缩进的树？例如:

sexp = 
    Node [
        Leaf "aaa", 
        Leaf "bbb",
        Node [
            Leaf "ccc",
            Leaf "ddd",
            Node [
                Leaf "eee",
                Leaf "fff"],
            Leaf "ggg",
            Leaf "hhh"],
        Leaf "jjj",
        Leaf "kkk"]
pretty = ????
main = print $ pretty sexp

我希望该程序的结果是:

(aaa 
   bbb 
   (ccc 
       ddd 
       (eee 
           fff) 
       ggg 
       hhh) 
   jjj 
   kkk) 

这是一个不完整的解决方案,使用"fold"作为组合器,不实现缩进:

fold f g (Node children) = f (map (fold f g) children) …