This looks great! My only complaint is that sometimes I do want to work on a quick script outside of a stack environment, and this seems like an all-or-nothing proposition. (I use haskell for pandoc filters quite a bit, for example, and setting up stack for that seems like overkill.) I'm sure a bit of elisp could make it fall back to haskell mode if it can't find a local .stack dir.
If there's no stack.yaml it uses the global ~/.stack/global/stack.yaml.
So you should be able to open any random file and it'll work out. E.g. if I open ~/X.hs and import Lucid, I can just run stack build lucid in my ~/ and now M-x intero-restart and it has access to that package.
