In re-frame, we'd like to use pure event handlers because they provide some important advantages, (mentioned in a previous post about coeffects in re-frame): local reasoning, easier testing, and events replay-ability. However, as we said, to build a program that does anything useful, it's inevitable to have some side-effects and/or side-causes. So, there will be many cases in which event handlers won't be pure functions. We also saw how using coeffects in re-frame allows to have pure event ha...