Things in Haskell which don't exist elsewhere at all
The best thing that exists in Haskell and nowhere else I've found isn't a library at all:It's the experience of grubbing around in unfamiliar libraries (in my case a database lib and a visualization lib), not knowing much Haskell and struggling to get the damn thing to compile. The very first time I got the types to line up, the program ran and I had my timeseries graph from the database.This is a magical, life changing experience.Any other language I've used would have been many iterations of, "this is null", "you passed a string where it was supposed to be a list of strings" (looking at you python)! Now that I know it's possible, I much prefer to do my thinking up front, and know that there's a compiler out there that's got my back :-)