How to track down the code during debugging?

Question

The problem I'm trying to troubleshoot now is specific to recent neovim 0.5.0 (exact version I'm using is NVIM v0.5.0-ef3230f35 but this issue reproduces in the same way on all my machines each running slightly different recent nvim).

The problem does not occur in vim.

It shows like this:

Error detected while processing function 9[30]..<SNR>30_callback:
line   21:
Vim(let):E730: using List as a String

Now I know that <SNR>30 means that I can look up the script the failure is occurring in :scriptnames, however I really kind of still need a line number. The line 21 is not right because the file corresponding to 30 (which is definitely the right script) has a comment at the top of it that is far longer than 21 lines...

So I'm curious if I can extract any meaning out of 9[30] or _callback. And perhaps line 21 is relative to something. Possibly it's an offset into some callback function?

Answer 1

I did find that the 21st line inside of a function named s:callback in the file referenced by <SNR>30 (in my case fzf.vim) is where the error is being thrown (the code is echoerr v:exception).