## Also Functor inside \(\Hask\)

\(\mathtt{[a]}∈\ob{\Hask}\) but is also a category. Idem for `Int`.

`length` is a Functor from the category `[a]` to the cateogry `Int`:

- \(\ob{\mathtt{[a]}}=\{∙\}\)
- \(\hom{\mathtt{[a]}}=\mathtt{[a]}\)
- \(∘=\mathtt{(++)}\)

⇒

- \(\ob{\mathtt{Int}}=\{∙\}\)
- \(\hom{\mathtt{Int}}=\mathtt{Int}\)
- \(∘=\mathtt{(+)}\)

- id: `length [] = 0`
- comp: `length (l ++ l') = (length l) + (length l')`