I certainly wouldn't say "go learn Haskell first" :-). As a rule of thumb, I always type unknown things in to Wikipedia. In this case the rather cheesy c&p job "Hindley-Milner-style algebraic types" has as top hit the article on the Hindley–Milner type system. Unfortunately that seems to have been written by mathematicians, who as usual have somehow managed to make an incredibly abstruse Wikipedia article about something relatively straightforward. Um. I think I am making your point for you :-).

So, I have just made that phrase a link to https://en.wikipedia.org/wiki/Algebraic_data_type which is quite a good article. I was resisting making every technical term a link (especially to general references sources) but you're not the first to trip over this. And actually maybe I should stop thinking of Wikipedia as a general reference source and instead realise that it's actually a universal specialist reference source - which is why the maths articles are inscrutable...