小编Mat*_*att的帖子

我对Haskell很新,我正在努力研究如何遍历一棵n-ary树.作为输出,我希望获得Leaf值列表(因为分支没有值),因此对于testtree,这将是:4,5

到目前为止我的定义是:

data Tree a = Leaf a | Branch [Tree a] deriving (Show)

travTree                    :: Tree a -> [a]
travTree (Leaf x)           = [x]
travTree (Branch (x:xs))    = travTree x : travTree xs

testtree = Branch [(Leaf "4"), (Leaf "5")]

但它给出了错误:

Couldn't match expected type `Tree a'
  against inferred type `[Tree a]'
In the first argument of `travTree', namely `xs'
In the second argument of `(:)', namely `travTree xs'
In the expression: travTree x : travTree xs

我假设这是因为xs是一个树列表,它期待一棵奇异的树.有没有办法做到这一点？我一直在尝试地图功能,顺序如下:

travTree …