While this is nice, of course, I'm not sure that is outcome is unique to Haskell/Stack. It seems like you could accomplish a similar level of reproducibility by building a Docker image or bundling dependencies in some other way.
It is not clear to me how Docker solves the issue of pinning dependencies; I would rather have a file that states the exact version of every package to install, than an opaque blessed container image that has some versions installed, and I do want to have the versions used under source control. Generating the image would not be reproducible (in the sense of having the same Python files inside it) without pinning versions somewhere anyway, right? Or am I missing something obvious?
This isn't a huge issue, but still it's nice in declarative systems like Stack and NixOS not to have to worry about that kind of thing.