Example: List

• [] :: * -> * an Endofunctor
• \(⊙:M×M→M\) a nat. trans. (join :: M (M a) -> M a)
• \(η:I→M\) a nat. trans.

-- In Haskell ⊙ is "join" in "Control.Monad"
join :: [[a]] -> [a]
join = concat

-- In Haskell the "return" function (unfortunate name)
η :: a -> [a]
η x = [x]