Lambda the Ultimate

Functional Pearl: Species and Functors and Types, Oh My! Brent Yorgey, draft.

We've discussed species many times before, and discussed at least one introduction for functional programmers. Here's another, this time considerably less technical. I would say this paper succeeds fairly well in giving a conceptual overview and some useful intuition. If you found the Carette and Uszkay paper sketchy in places, you may find this a gentler introduction.

On the other hand, I don't really see how it counts as a Functional Pearl...