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