google_ad_section_start(name=default) Friday, May 23, 2014 Cofree meets Free > {-# LANGUAGE RankNTypes, MultiParamTypeClasses, TypeOperators #-} Introduction After I spoke at BayHac 2014 about free monads I was asked about cofree comonads. So this is intended as a sequel to that talk. Not only am I going to try to explain what cofree comonads are. I'm also going to point out a very close relationship between cofree comonads and free monads. At the beginning of the talk the Google Hangout software seems to have switched to the laptop camera so you can't see the slides in the video . However the slides are here . Cothings as machines I often think of coalgebraic things as machines. They have some internal state and you can press buttons to change that internal state. For example here is a type class for a machine with two buttons that's related to a magma...