I am happy to be able to announce that starting today and continuing over the next few weeks, we are going to make the materials that our “Introduction to Haskell” course is based on available for free.
“Sum types” is perfectly fine and perhaps even the better term. I’ve chosen Union Types here because I think it conveys more intuition to someone who doesn’t already know what it is. I think it’s still accurate, even though strictly speaking one should probably insist on calling it a tagged union type.