I like your post, and think we need more people like you doing this. One critique: I think you need to do a much better job explaining your first part on recursion.

You need to walk through exactly what you are doing in your sum function, because it is not at all clear to the uninitiated where in the hell the "acc" variable comes from. Instead of explaining any of this part, which is perhaps the most difficult part for a non-FP person to grasp, you jumped straight to "by the way, use loop when there's a lot of numbers."

My coworker's comments when I sent him the link:

"Ok, I get it...... what's going on with the sum?..... whaaat?"

Thanks for doing what you're doing. I look forward to reading more of your work.