TITLE: First setup for nvim-lsp with nvim-cmp
DATE: 2021-11-30
AUTHOR: John L. Godlee
====================================================================


I used to use ncm2 for autocompletion in neovim, then I switched to 
coc.nvim for a short time, but found it awkward to maintain. 
Recently I set up nvim-cmp, which uses neovim's built-in language 
server client to source external language servers for different 
languages.

  [ncm2]: https://github.com/ncm2/ncm2
  [neovim]: https://neovim.io/
  [coc.nvim]: https://github.com/neoclide/coc.nvim
  [nvim-cmp]: https://github.com/hrsh7th/nvim-cmp

I use auto-completion fairly sparingly, but I do find it incredibly 
useful for two main things: completing file paths, and suggesting 
snippets from Ultisnips. I also sometimes use language 
auto-completion when writing R. Possibly in the future I'd also 
like to use auto-completion for writing LaTeX, Javascript, Julia 
and CSS, but for now I've just set up the language server for R.

  [Ultisnips]: https://github.com/SirVer/ultisnips

The R language server is installed by running 
install.packages("languageserver") in R. I use the Nvim-R vim 
plugin to turn neovim into something more like a full IDE. Nvim-R 
provides syntax highlighting for R code, allows interaction with an 
open R repl either in a tmux split or a nvim terminal buffer, among 
many other features. I use radian to provide a smarter R repl.

  [Nvim-R]: https://github.com/jalvesaq/Nvim-R
  [radian]: https://github.com/randy3k/radian

To set up nvim-cmp I largely followed the example configuration 
provided in the nvim-cmp README.md, but I've added an extra bit at 
the end to disable buffer word completion in file types which 
normally contain prose. I've pasted the config below for Nvim-R, 
Ultisnips, and nvim-cmp.

I wish I understood a bit better what the different bits of lua 
code actually do (e.g. the calls to cmp.setup.cmdline), but I found 
the online documentation to be quite lacking.

  ![Screenshot demonstrating Nvim-R and 
nvim-cmp](https://johngodlee.xyz/img_full/nvim_lsp/scrot.png)

    " Plugins
    call plug#begin('~/.vim/plugged')

    Plug 'SirVer/ultisnips' " Snippets
    Plug 'jalvesaq/Nvim-R'
    Plug 'neovim/nvim-lspconfig'
    Plug 'hrsh7th/cmp-nvim-lsp'
    Plug 'hrsh7th/cmp-path'
    Plug 'hrsh7th/cmp-buffer'
    Plug 'hrsh7th/cmp-cmdline'
    Plug 'hrsh7th/nvim-cmp'
    Plug 'quangnguyen30192/cmp-nvim-ultisnips'

    call plug#end() 

    " nvim-cmp
    set completeopt=menuone,noselect

    lua <<EOF
      local cmp = require'cmp'

      -- Setup nvim-cmp
      cmp.setup({
        -- Specify snippet engine
        snippet = {
          expand = function(args)
            vim.fn["UltiSnips#Anon"](args.body) 
          end,
        },
        -- Keybindings
        mapping = {
          ['<C-n>'] = cmp.mapping.select_next_item({ behavior = 
cmp.SelectBehavior.Insert }),
          ['<C-p>'] = cmp.mapping.select_prev_item({ behavior = 
cmp.SelectBehavior.Insert }),
          ['<Down>'] = cmp.mapping.select_next_item({ behavior = 
cmp.SelectBehavior.Select }),
          ['<Up>'] = cmp.mapping.select_prev_item({ behavior = 
cmp.SelectBehavior.Select }),
          ['<CR>'] = cmp.mapping.confirm({ select = true }),
        },
        -- Define completion sources
        sources = cmp.config.sources({
          { name = 'nvim_lsp' },
          { name = 'ultisnips' }, 
          { name = 'buffer' },
          { name = 'path' },
        })
      })

      cmp.setup.cmdline('/', {
        sources = {
          { name = 'buffer' }
        }
      })

      cmp.setup.cmdline(':', {
        sources = cmp.config.sources({
          { name = 'path' }
        }, {
          { name = 'cmdline' }
        })
      })

      -- Setup lspconfig
      local capabilities = 
require('cmp_nvim_lsp').update_capabilities(vim.lsp.protocol.make_cl
ient_capabilities())

      -- R language server options
      require('lspconfig')['r_language_server'].setup {
        cmd = { "R", "--slave", "-e", "languageserver::run()" },
        filetypes = { "r", "rmd" }
      }

    EOF

    " Only enable minimal completion in files containing prose
    autocmd FileType markdown,tex,txt lua require'cmp'.setup.buffer 
{
    \   sources = {
    \     { name = 'path' },
    \     { name = 'ultisnips' }, 
    \   },
    \ }

    " Nvim-R
    let R_external_term = 0
    let R_source = '~/.vim/tmux_split.vim' 
    let R_assign = 0
    let R_objbr_place = 'BOTTOM'
    let R_objbr_h = 30
    let R_min_editor_width = 80
    let R_objbr_opendf = 0 
    let r_indent_comment_column = 0
    let r_indent_align_args = 0
    let r_indent_ess_comments = 0
    let r_indent_ess_compatible = 0
    let R_non_r_compl = 0
    let R_rmdchunk = 0
    let R_rnowebchunk = 0
    let R_app = "radian"
    let R_cmd = "R"
    let R_hl_term = 0
    let R_args = []
    let R_bracketed_paste = 1

    nmap <LocalLeader><Enter> <Plug>RDSendLine
    vmap <LocalLeader><Enter> <Plug>REDSendSelection

    " Ultisnips
    let g:UltiSnipsExpandTrigger="<tab>"
    let g:UltiSnipsJumpForwardTrigger="<c-l>"
    let g:UltiSnipsJumpBackwardTrigger="<c-h>"
    let g:UltiSnipsEditSplit="vertical"
    let g:UltiSnipsSnippetDirectories=[$HOME.'/.vim/Ultisnips']

Update 2021-12-05

I spent about 3 days trying to work with nvim-cmp, but I found that 
if I had more than two R scripts open in separate nvim instances, 
the fans on my MacBook Pro would start going crazy and htop showed 
the CPU maxed out. For me that’s not good enough, and I can get 
by with the built-in completion provided by Ctrl-x Ctrl-f / ctrl-o 
etc. So that’s a bit disappointing, and I’m not sure whether it 
will ever get better. I see that nvim v0.60 has just been released, 
but reading through the release notes I see no improvements to LSP 
support. As an uninformed user it's difficult to know if the 
bottleneck is nvim-cmp, the R language server, or nvim.