Personally, I think this problem shows why sane type systems are useful. In Haskell data NestedList a = Single a | Nested [NestedList a] -- Or equival

Personally, I think this problem shows why sane type systems are useful. In Haskell data NestedList a = Single a | Nested [NestedList a] -- Or equivalently, type NestedList = Free [] flatten :: NestedList a -> [a] -- This is pretty much the only reasonable definition of flatten the compiler will accept given this type flatten (Single a) = [a] flatten (Nested xs) = concatMap flatten xs -- or given the Free variation of NestedList -- flatten = retract