immutable Universe

[dcz-self]

Using Queries is currently awkward because creating them requires a &mut Universe. In my case, this means either a RefCell on the database which wraps Universe.

This pushes errors from compile-time to runtime, which is not so great.

I looked into creating the Query, and the only thing the &mut is for is to put additional symbols in the symbols store. This gave me the idea: why not bundle symbols with the Query and call it the UniverseQuery?

struct UniverseQuery {
  symbols: Symbols,
  query: Query,
}

This makes the original Universe obsolete for this query, but the problems with &mut are gone.

In this patch, I decided that the original Universe doesn't have to be copied with every query, so I sketched the type for a lighter SymbolsOverlay:

struct SymbolsOverlay {
  symbols: &SymbolsStore, // lots of symbols
  overlay: SymbolsStore, // only symbols from this query
}

and I stopped there so far.

Does this look like a good direction?

[fatho]

Definitely makes sense to not pollute the universe with random symbols users might've used in all their queries.

The main thing that's important is that the underlying RuleSet in the universe doesn't get further modified before the query is then actually run, because then you might get conflicts. But your design neatly prevents that, so I think it should be good to go this way.

I commented on a few things I noticed, but I don't see any major blockers.

[dcz-self]

I removed the WIP, as all feedback has been addressed.

[fatho]

Thanks a lot! I still have a few smaller comments, but the overall design looks good to me!

BTW, since I noticed some formatting changes (one fixing something cargo fmt would not agree with, but another one that cargo fmt would do differently), I added cargo fmt to the CI checks, so if you could rebase once more?

[fatho]

Awesome!

[dcz-self]

Let me know when a new release is out, I want to incorporate those APIs in my own release.

[fatho]

Let me know when a new release is out, I want to incorporate those APIs in my own release.

This PR and the others are now available as v0.4.0 on crates.io.