The Derivative of a Regular Type is its Type of One-Hole Contexts by Conor McBride was mentioned on LtU several times.

If you enjoyed it, try a new paper by the same author:

Clowns to the left of me, jokers to the right (Dissecting Data Structures).

More generic programming, more parallels between data types and calculus, more fun.

As usual for Conor's paper, it's short and full of (sometimes obscure) humor. Beware of typos, though.