[vim/vim] vim.list({}) index starts with 0 while neovim and lua index starts with 1 (#6342)

[Prabir Shrestha]

I opened an issue in neovim to have compatibility with vim and neovim on the apis related to neovim. neovim/neovim#12537.

Does any one know why the vim.list in lua starts with 0 instead of 1? Arrays in lua starts with 1 and neovim seems to default to 1.
This has been one of the source of bugs in lua plugins in vim and neovim. Example of bugs:

1. natebosch/vim-lsc@97fee80. vim-lsp also uses the same diff algorithm so updating should be simple. prabirshrestha/vim-lsp#821

Add support for neovim
There is a difference between vim.eval and vim.api.nvim_eval when
passed a list. With vim.eval the collection is zero indexed, with
vim.api.nvim_eval it is one indexed. Pass an offset of 1 to correct
for this by evaluating has("nvim") as a third argument.

2. prabirshrestha/asyncomplete.vim#101

Problem
First candidate is truncated in vim when smart completion is enabled.
Cause
According to document of vim, the index of list starts from 0. However, according to the document of neovim, the index of table starts from 1.
Solution
Pass the start and end indexes from vim script to lua script.

Lua:

array = [1,2,3] 
print(array[1])

Neovim:

let s:array = [1,2,3] 
lua print(vim.api.nvim_eval('s:array')[1])

Vim:

let s:array = [1,2,3] 
lua print(vim.eval('s:array')[1])

This is the output and vim seems to be an outlier here.

Lua	Neovim	Vim
1	1	2

My thought is that we make the list index at 1 and make it a breaking change given that there are almost no one using lua plugins currently and only few plugins need to update their code. If the index is starting at 0 instead of 1 it won't play well with the entire lua ecosystem since everyone will assume it starts with 1 except vim.

One could do something like this if they really wanted to support old version of vim.

let s:array = [1,2,3] 
lua <<EOF
let offset = vim.fn.has('patch-8.2.x') == 1 then -1  else 0 end
print(vim.eval('s:array')[1 + offset])
EOF

As for me I would personally disallow old version of vim without this patch to run.

if !has('patch-8.2.x') | finish | endif

Thoughts?

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[Christian Brabandt]

I would think it starts at 0 as this is consistent with vimscript api.

[Prabir Shrestha]

The problem with starting with 0 is that non of the existing lua libraries or functions that works in lua ecosystem will work in vim. There is an entire luarocks ecosystem I would like to take advantage of https://luarocks.org/.

Here is a simple example. This print 1 2 3 in neovim and pure lua if local array = {1,2,3} but throws error in vim.

let s:array = [1,2,3]

lua

 <<EOF
local array = vim.eval('s:array')
--local array = {1, 2, 3}

local function concat(arr)
     local text = ''
     for i=1, #arr do
         text = text .. ' ' .. arr[i]
     end
     return text
end

print(concat(array))
EOF

So the proposal is that if a vim list is accessed in vimscript index starts at 0 if it is accessed in lua it starts with 1. This only has impact on devs writing in lua.

[Bram Moolenaar]

Most language use index zero for the first element of a list or array. If Lua does something different, better make sure that only happens when actually writing Lua. I suppose that means when converting a Vim list to Lua you just use the right Lua type for it and it should just work.

[Prabir Shrestha]

Sent a PR for this. #6347.

[vim-dev ML]

Hi,

On Fri, Jun 26, 2020 at 12:49 PM Prabir Shrestha <vim-dev...@256bit.org>
wrote:

> Sent a PR for this. #6347 <https://github.com/vim/vim/pull/6347>.
>
>
>
As you are working on enhancing the Vim Lua interface, it will be useful to
improve the
test coverage for if_lua.c by adding some more tests:

https://codecov.io/gh/vim/vim/src/5d3c9f8c2a0fc29ba4ac8e0f052378b64d9e3dd3/src/if_lua.c


Thanks,
Yegappan

[Yegappan Lakshmanan]

Hi,

On Fri, Jun 26, 2020 at 12:49 PM Prabir Shrestha <vim-dev...@256bit.org> wrote:

Sent a PR for this. #6347.

[Prabir Shrestha]

Sounds good. Currently focusing more on compatibility with lua and neovim (neovim/neovim#12537) and making sure we have the right apis exposed to make lua a viable alternative for plugin authors. But as I add new features I'm trying to add much tests as possible.

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[Prabir Shrestha]

Closing since it has the PR has been merged.

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[Prabir Shrestha]

Closed #6342.

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