Here's a place to discuss Chapter 4.

Chapter 4 is largely about profunctors. I just ran into a nice explanation of how profunctors show up in Haskell:

* Dan Piponi, [Profunctors in Haskell](http://blog.sigfpe.com/2011/07/profunctors-in-haskell.html?m=1).

We've also got some threads going where you can discuss exercises in Chapter 4:

* [Exercise 4 - Chapter 4](https://forum.azimuthproject.org/discussion/2228)
* [Exercise 7 - Chapter 4](https://forum.azimuthproject.org/discussion/2229)
* [Exercise 9 - Chapter 4](https://forum.azimuthproject.org/discussion/2231)
* [Exercise 10 - Chapter 4](https://forum.azimuthproject.org/discussion/2232)
* [Exercise 12 - Chapter 4](https://forum.azimuthproject.org/discussion/2233)
* [Exercise 15 - Chapter 4](https://forum.azimuthproject.org/discussion/2234)
* [Exercise 17 - Chapter 4](https://forum.azimuthproject.org/discussion/2235)
* [Exercise 21 - Chapter 4](https://forum.azimuthproject.org/discussion/2237)
* [Exercise 25 - Chapter 4](https://forum.azimuthproject.org/discussion/2238)