Hi Geoffrey,
I don't know much about subtrees, but that might be another possibility?
the main point about subtrees is, that you've just one repository and you're merging a directory of this repository with 'git subtree' with some other git repository. subtrees and submodules both try to handle the use case if you want to incorporate a third party repository into your own repository and would like to merge the changes in both directions. I think that subtrees are easier for the developer working on the repository, because there's only one repository, but it's a bit more hassle merging the third party repository. submodules are harder for the developer, because there're multiple repositories, but merging the third party repository might be a bit easier. GHC devs might have other reasons for using submodules, because they want to separate things or they're afraid that the resulting one repository might get too big, but I think that there should be good reasons for using submodules, because a lot of workflows (like branching) are such a hassle with submodules. Greetings, Daniel