Hi Markus, Rather than doing this... On 24/10/13 21:35, Markus Läll wrote:
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)
...you are probably better off using a GADT like this: data NonEmpty2 (n :: Nat) a where Tail2 :: [a] -> NonEmpty2 Zero a Head2 :: a -> NonEmpty2 n a -> NonEmpty2 (Succ n) a Now you can pattern-match on something of type `NonEmpty2 n a` in order to learn more about the value of `n`, whereas with a data family, you need to know the value of `n` in advance. This makes it harder to use. For example, it is easy to convert in one direction: convert :: NonEmpty2 n a -> NonEmpty n a convert (Tail2 xs) = Tail xs convert (Head2 x xs) = Head x (convert xs) but going in the other direction is more complicated, and requires some kind of dependent pi-type or singleton construction. Hope this helps, Adam -- Adam Gundry, Haskell Consultant Well-Typed LLP, http://www.well-typed.com/