Last week we took a deep dive into the map function. First we saw that map on arrays is unique amongst all the functions that transform arrays and satisfy a simple property. That was kind of amazing, because it shows that map wasn’t invented because it was convenient, rather it was only sitting there waiting for us to discover it.

In today’s episode we want to explore the idea of “contravariance”. It’s a kind of composition that hides right in plain sight in the programming world, but for whatever reason we never really come face-to-face with it. So we want to dedicate some time to really study it and build intuitions for it, because it will allow us to uncover new ways of composing things that are otherwise difficult to see.