list via codensity

[juan_gandhi]

List a = Codensity Endo a = forall r. (a -> r -> r) -> r -> r

    nil :: List a
    nil = \f z -> z

    cons :: a -> List a -> List a
    cons x xs = \f z -> f x (xs f z)

    append :: List a -> List a -> List a
    append xs ys = \f z -> xs f (ys f z)

    foldr :: (a -> r -> r) -> r -> List a -> r
    foldr f z xs = xs f z

Basically, it's like lambda.

Src: https://golem.ph.utexas.edu/category/2012/09/where_do_monads_come_from.html#c042100