[Haskell-cafe] to make a statically known sized non-empty list

Hi haskell-cafe, so for fun and profit I was trying to make a non-empty list data type, where the size of the non-empty prefix would be known for the type-system with the following code, but it fails with an error "No instance for (Functor (NonEmpty * a)) ..." for the last line. To me it seems that all should be known, but something must be missing.. data Nat = Zero | Succ Nat data family NonEmpty (n :: Nat) a data instance NonEmpty Zero a = Tail [a] data instance NonEmpty (Succ n) a = Head a (NonEmpty n a) instance Functor (NonEmpty Zero) where fmap f (Tail xs) = Tail (fmap f xs) instance Functor (NonEmpty (Succ a)) where fmap f (Head x xs) = Head (f x) (fmap f xs) -- Markus Läll