Modern Tcl may be big, but it is not too big to embed. I know, because I embedded it into a Go system at a previous employer. Unfortunately, it is proprietary, so I may not point you to the repo, but it required remarkably little glue code.

I also evaluated Python and Lua. As others have noted, Python appears to be a right royal pain to embed. I actually had more experience with Lua prior to that project, but that experience leads me to believe that Lua is, generally, the wrong language.

Looking back on the project, I would not hesitate to do the same thing again, ideally as open source so that other Go projects can embed a scripting language. It got the job done, and did it well. Non-programmers on our team were able to write both configuration and logic successfully & productively, and apparently enjoyed the experience. Tcl itself ended up being a pleasure to use. While I personally would have enjoyed something like Embeddable Common Lisp, I think that would have been to much to ask of the rest of the team.

Interestingly, the four languages Lua, Python, Tcl and Lisp each can be said to take one idea and run with it. Lua is everything-is-a-hash-table (well, almost everything); Python is everything-is-an-object; Tcl is everything-is-a-string; and Lisp is everything-is-a-list (well, in theory: in practice it is really everything-is-an-object). I don’t know if this says anything deep about scripting languages, but it is at least interesting — right?