fix: allow lambdas and Lua functions as doc handlers

[gwuen]

When g:vimtex_doc_handlers holds a lambda expression or bare Lua function, the iterator itself will become a Funcref. To avoid E704, this Funcref variable name must start with a capital letter.

In practice, this means one can directly pass in a Lua function:

vim.g.vimtex_doc_handlers = { MyVimtexDocHandler }

Instead of having to create a Vim wrapper function, which can be call()ed directly by its name:

function! MyVimtexDocHandler(context)
  return v:lua.MyVimtexDocHandler(a:context)
endfunction

let g:vimtex_doc_handlers = ['MyVimtexDocHandler']

[gwuen]

By the way, I'd be really happy if someone can explain to me why :h lua-call clearly says:

v:lua without a call is not allowed in a Vimscript expression: Funcrefs cannot represent Lua functions.

While the error message suggests that the Lua function I passed is now held as a Funcref:

E704: Funcref variable name must start with a capital: l:handler

Is this because the bare Lua function is treated the same as a lambda expression when handled inside Vim script? This is at least the behavior I observed, but I couldn't find any conclusive information in the Neovim docs or online.

So if my reasoning in the commit text is wrong, feel free to correct me :)

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And a note for anyone considering using a Lua function: From inside Lua, there's absolutely no way¹ to modify the context dictionary passed to the handler outside its scope, see :h lua-vimscript:

Objects passed over this [Lua-Vimscript] bridge are COPIED (marshalled): there are no "references".

So one will want to implement all the doc-handling logic inside the function, not relying on handlers down the list to open the selected candidate. Alternatively, use a Vimscript wrapper to modify the dictionary.

Footnotes

1. Please let me know if you think there is! ↩

[lervag]

Sorry for the slow response!

When g:vimtex_doc_handlers holds a lambda expression or bare Lua function, the iterator itself will become a Funcref. To avoid E704, this Funcref variable name must start with a capital letter.

Yes, that's right, and I think your change is both safe and useful! Thanks!

By the way, I'd be really happy if someone can explain to me why :h lua-call clearly says: …

I'm sorry, I can't explain that.

And a note for anyone considering using a Lua function: From inside Lua, there's absolutely no way to modify the context dictionary passed to the handler outside its scope, see :h lua-vimscript: …

Thanks, that's useful to know. And yes, I think you are right that there is no way to change the vimscript reference of the context dictionary here if you are moving over to Lua.

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In any case, thanks for the contribution!

[gwuen]

Sorry for the slow response!

No worries at all! I appreciate your feedback. I'm genuinely impressed that you've been actively maintaining this plugin for so many years – huge thanks for your dedication!

[lervag]

No worries at all! I appreciate your feedback. I'm genuinely impressed that you've been actively maintaining this plugin for so many years – huge thanks for your dedication!

Great, and thanks for the kind words! It's been fun to keep working on this. 😊