stack works quite well for cabal projects, but it's not that convenient for quick experimentation, where I just want to write some code to a file and run it or play with it in ghci, without creating a cabal file.

Here are some things that could be improved:

1. stack init currently refuses to create a stack.yaml when there's no cabal file
2. Even if there are no local packages, stack build could still build extra-deps.
3. It would be nice if stack ghci file.hs worked. stack exec ghci file.hs works for me, but that's probably not obvious to a Haskell newcomer (and Haskell newcomers really need a simple way to run ghci)
4. Related to the previous two items, we also need a way to force building a snapshot package so that it is available to non-cabalized stack ghci. I remember there was stack dep once, but it is no longer there.