反转Haskell中的自定义嵌套列表

Question

I am working on trying to reverse a nested list in Haskell. I am aware that nested lists are not a thing in Haskell so I defined one:

data NestedList a = Elem a | SubList [NestedList a]

I also have a flatten function:

flatten :: NestedList a -> [a]
flatten (Elem x) = [x]
flatten (SubList x) = concatMap flatten x

Now I wish to write my reverse function. The function is defined as:

myreverse :: NestedList a -> NestedList a

Which I think makes sense because I'm just rearranging the elements within the list.

I understand how to write a basic reverse function, and I also know that for Haskell's standard lists reverse is already defined.

My question is: how do I deal with the case where the head of the list is also a list? What I know needs to happen is that I reverse the head of the list and put it back onto the reverse of the tail. But how to achieve this?

Answer 1

why not this way

rev :: NestedList a -> NestedList a
rev (Elem a) = Elem a
rev (SubList xs) = SubList $ map rev $ reverse xs

if you add deriving (Show) to your data definition,

Prelude> rev $ SubList [Elem 1, SubList [Elem 2, Elem 3]]
SubList [SubList [Elem 3,Elem 2],Elem 1]

Prelude> rev $ SubList [Elem 1, SubList []]
SubList [SubList [],Elem 1]

Answer 2

Your nested list is actually a tree with elements at leavess:

                 SubList
               /         \
        SubList           Elem 4
    /      |      \
Elem 1   Elem 2   Elem 3

So your myreverse would be a horizontal flip, ie recursive reverse of each list in SubList , as other answer points out.

The lesson here: Visualising data-structures helps to understand and implement actions on them.