Language server/completion engine

[YangchenYe323]

Feature request checklist

• There are no issues that match the desired change
• The change is large enough it can't be addressed with a simple Pull Request
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Change
Summary of the proposed change and some details about what problem
it helps solve.

We use CEL extensively as our user-facing query language, and we implemented custom harnesses that creates env based on db schema and turn the AST into sql queries. A big challenge is to document and guide users to a brand new project/environment, with different fields and different extensions.

Since everything the user needs is already in the environment, I think it'd be great if we can take the environment and allow the user to navigate by tab completion. Would it be considered in scope of this project to accept work on building the compiler/type checker infrastructure to support such use cases? I'm happy to work on it, and would also appreciate some guidance on where to start, potential challenges, or it is fundamental impossible.

Example
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Alternatives considered
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[TristonianJones]

Hi @YangchenYe323, are you suggesting to create a Language Server Protocol (LSP) implementation for CEL, so it would work with VS Code or similar IDE? If so, I think that's definitely doable. It might need some changes to the CEL parser to accomodate incomplete expressions better, but if you're interested I think such a contribution on github.com/cel-expr domain would be possible. We'd have to seed the repo with some source to kick start things, but it would be pretty neat.

[YangchenYe323]

Hi @TristonianJones , yes an LSP implementation is what I had in mind, and I'm interested in contributing. Would you give some suggestions as to where to start, e.g., what's the recommended approach and where in the parser should I poke and understand.

[TristonianJones]

Hi @YangchenYe323, you could try to vibe-code some of the basics. The main difficulty is communicating the CEL configuration consistently to the LSP. It's doable, but the simplest approach is to hard-code the type system and function definitions into the LSP to support code completions.