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')