I often think of Haskell as one of the friendliest communities. Granted, not on SO, but on IRC.
I agree —- I find the Haskell IRC very friendly, even to the point of helping people do research and figure things out instead of just dropping answers. And there’s a beginners IRC for all those stupid questions I have
The Haskell channel(s) on the FP Slack (https://fpchat-invite.herokuapp.com/) are also exceptionally welcoming and I've seen people go above and beyond helping people again and again.
This is not the first time I have seen this claim, but it's so much in contrast with my experience that I don't know what to think.
There seems to be some cliques of Haskell developers on mostly non-communicating communities. I would imagine they self-select but I have actually only encountered any hostility on Stack Overflow, and the Haskell part of it, besides being business as usual for the site, also is a mostly useless knowledge base (to the point that I remove the site from searches to decrease the noise).