Question: How does Mercurial deal with garbage collection? After all, the desire for garbage collection is by far not unique to Git -- any version control system that has the equivalent of `commit --amend` and rebase should provide it.
As for not needing the extension, it seems to me that having "dangling" commits would be very difficult to use without some decent visualization of the dangling commits, such as what the blog post shows with `hg show`.
> It's probably also worth noting that we have an implementation detail leaking into user space here.
I find this comment absolutely fascinating, because truly, this is not an implementation detail leak at all.
The point of the purely functional data structures isn't to achieve atomicity (although potentially being a bit more robust to power loss etc. is certainly a nice side effect), it's a way of thinking about version control. I've never heard functional programmers use atomicity as the main argument for immutable data structures, either...
The whole point of Git's design is that it chose a robust and crystal clear way of thinking about distributed versioning as its underlying model of what version control is, and then simply provided tools for manipulating that DAG. Some of those are a bit ugly because of how the system grew over time, but still, this is how software should be designed: have a clear model of what the data is, then provide tools for manipulating it.
For what it's worth, the underlying data store of Git could quite easily support "unnamed" tip commits as well. What you'd need is a change in the garbage collection policy, and "porcelain". Which brings me back to the question of how Mercurial does it.