[Haskell-beginners] Defined list data type compared to Haskell List

[David McBride]

In NestedList, the List constructor takes a regular list of NestedLists.
Therefore when pattern matching on it you can get access to those nested
lists.  In your code, x is the first NestedList, and xs is the rest of the
NestedLists.

On Tue, Jan 26, 2021 at 4:32 PM Lawrence Bottorff <borgauf at gmail.com> wrote:

> I'm following this <https://wiki.haskell.org/99_questions/Solutions/7>
> and yet I see this solution
>
> data NestedList a = Elem a | List [NestedList a] deriving (Show)
>
> flatten1 :: NestedList a -> [a]
> flatten1 (Elem a   )   = [a]
> flatten1 (List (x:xs)) = flatten1 x ++ flatten1 (List xs)
> flatten1 (List [])     = []
>
> What I find puzzling is this line
>
> flatten1 (List (x:xs)) = flatten1 x ++ flatten1 (List xs)
>
> where I see
>
> (List (x:xs)) as an argument. How is the NestedList type also able to be
> expressed as a normal consed list with x:xs argument? How is (:)
> interacting with NestedList?
>
> LB
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