不知道如何正确键入我的 N 叉树

Question

I made the following N-ary tree data type:

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

with the following method:

listLeaf::Tree a -> [a]
listLeaf (Leaf a) = [a]
listLeaf (Node a (l:list)) = concat $ listLeaf l

I'm trying to make listLeaf so that it gives me a list that contains each element of a leaf inside my tree. This means that if I have the following tree: Node 3 [Leaf 1, Leaf 2, Node 1 [Leaf 3]] , my objective would be to get [1,2,3].

As you probably noticed the type of my function doesn't add up because it expects a list of a, but I'm giving it a list of lists. Can't wrap my head around how I can have the base case (leaf) return me a type [a] and at the same time have the Node case give me the same kind of type. I feel like I need to use a higher order function aside from concat, but I got no clue. Any hints?

Answer 1

You are here only calling listLeaf on l , the first element of the list, so you will only obtain the leftmost leaf. Furthermor concat then does not make much sense, ince you have a list of values, not a list of lists of values.

You can use concatMap:: (a -> [b]) -> [a] -> [b] to perform a mapping on all elements and concatenate the lists of the results, so:

listLeaf :: Tree a -> [a]
listLeaf (Leaf a) = [a]
listLeaf (Node _ ) =  listLeaf cs