cells - a language server for CEL

This weekend, I built a language server for CEL called cells. It currently supports semantic highlighting, diagnostics, documentation on hover, signature help, references, document highlighting (for variable references), completion, and formatting. CEL is a pretty simple language, so a language server for it doesn’t really need to support things like go to definition or some of the other niceties that language servers typically support. It only works on a single .cel file at a time, which is a filetype currently supported only in neovim nightly but will be supported in neovim 0.12.

Building this out, I brought over some of the ideas that I had integrated when I was building out some of the Buf LSP server at $WORK, like that tests should have separate testdata files so that they could be independently verified by opening up an LSP-powered editor and taking a look at the files themselves. On top of this, making the tests mostly table-based made it really simple for additional test cases to be added.

Of course, building this in a weekend took working with AI (using pi and amp). One thing I had in mind was trying out the other LSP “frameworks” in the go ecosystem; unfortunately, most seem to be pretty unmaintained these days. go.lsp.dev progress has largely stalled, and tliron/glsp was the only other major contender, which seemed to be in a similar state.

At the end of the day, the agent suggested that we ought to just borrow what charmbracelet had down with powernap, and vendor in the gopls protocol code while maintaining the license, and then “write” our own JSONRPC server, based on the sourcegraph implementation, but without the legacy cruft. I may end up upstreaming some of this approach to buf lsp.

I’m not sure how many other LSP features cells needs to be considered “complete”, although I’ll continue to track CEL language features (as it’s still in a pre-1.0 state as a language), and I’d like to add most of the LSP features as functionality in the CLI, similar to gopls’ CLI interface.

cells turned out to be a pretty perfect weekend project for working with agents: LSP is fairly well-specified, with existing patterns, a tight feedback loop, and a great reference library in cel-go.

Fun times to be building things. 👷‍♂️