[dllthomas]

> The millions of syntax extensions in ghci, some of which are incompatible with each other. This means ProjectA and ProjectB can be written in completely different, possible incompatible, 'dialects' of Haskell.

To my knowledge, there are no GHC extensions that are incompatible on a project level (that is, you could have one enabled in one file, another enabled in another, and it would break things). I'm much less confident that there are no extensions that are incompatible if you try to use them within the same file, but I am not able to think of any off the top of my head.