I don't have a great read on how it happened at Mercury originally; I assume that they started using it near the beginning since lots of Haskellers like Nix and

I don't have a great read on how it happened at Mercury originally; I assume that they started using it near the beginning since lots of Haskellers like Nix and it solves some pain points with packaging, and they've been using it ever since. The key insight is that it's been able to scale and still work pretty well even as Mercury is hitting a few hundred developers. There's a bit of friction here and there, but the overall dev experience is pretty good; in a number of ways, it's distinctly better than what I've seen in places that heavily use Docker.