nREPL is a really interesting protocol for developer tools. It's extensible, but one of the basic operations is eval().
If your nREPL server doesn't support a given operation, you can just send an eval request to achieve the same result!
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Defining extensible macros, and a helpful approach of classifying syntax into distinct categories:
I'm really excited to learn about Luakit, a browser that's extensible in Lua! https://luakit.github.io/
I've followed uzbl and Conkeror with interest but both are inactive these days.
GitHub is exploring some really interesting editor designs with Xray: https://github.com/atom/xray/blob/master/README.md
It uses CRDT to represent text (rather than rope data structures) and mixes web tech (primarily JS) with Rust to produce a design that's fast but still extensible.