This is probably an unpopular opinion - Haskell, I used to think it must be cool (and useful) since people go on about it so much. I spent quite a bit of time learning it, and imho the usefulness to practicing programmers is marginal at best. It does present some useful techniques that are making their way into languages (e.g. swift optionals), but in general it didn't live up to the hype for me. I feel a lot of the things they go on about are overly complified to impress.