This post made the concept of “indexing,” used across mathematics, click for me.

Profunctors are bifunctors that are contravariant in their first type argument and covariant in their second one.

We've seen that contramap is a powerful operation, but the name isn't fantastic. We propose a much more intuitive name for this operation, and in doing so make our code much easier to read.

it is often a surprise for people to learn that function composition is the `fmap` of functions