Command works in ex mode but not during init (call)

Question

When I run this command in ex mode it works without a problem:

:call deoplete#custom#source('ultisnips', 'rank', 1000)

However, when I put it in my vimrc I get an error:

Error detected while processing /Users/me/config/nvim/plugins/deoplete.vim:
line   14:
E117: Unknown function: deoplete#custom#source

I have the deoplete plugin installed via a git submodule and deoplete works in vim. I just can't use call with any of its commands. Am using Neovim 0.3.3. I feel perhaps my init is being sourced before the plugin has finished loading but I am not sure how to go about sorting this out.

Answer 1

According to the OP, the plugin directory is located inside the 'packpath', which means the plugin is not available until after the vimrc is processed:

When Vim starts up, after processing your .vimrc, it scans all directories in
'packpath' for plugins under the "pack/*/start" directory.  First all those
directories are added to 'runtimepath'.  Then all the plugins are loaded.
See |packload-two-steps| for how these two steps can be useful.

[...]

Note that the files under "pack/foo/opt" are not loaded automatically, only the
ones under "pack/foo/start".  See |pack-add| below for how the "opt" directory
is used.
Loading packages automatically will not happen if loading plugins is disabled,
see |load-plugins|.
To load packages earlier, so that 'runtimepath' gets updated: >
    :packloadall
This also works when loading plugins is disabled.  The automatic loading will
only happen once.

(:help packages)

While the help system mentions using :packloadall to load packages earlier, I offer an alternative suggestion: the after directory. By the time after files load, packages have been loaded. In my own Dotfiles, I keep a directory ~/.vim/after/plugin/config/¹ with short files to customize my plugins.

As an example, my config for ale is in ~/.vim/after/plugin/config/ale.vim:

if ! exists(':ALEInfo')
  finish
endif

let g:ale_set_signs = 0

let g:ale_linters = {
      \ 'python': 'all',
      \ 'java': [],
      \ }

The trick is generally to find a reliable way of testing for the presence of the plugin: when they define commands or "skip" variables (e.g. g:loaded_{name}), we can test those. Autoload'ed functions (file#name) are harder because they don't exist until they are used.

In your case, I'd use g:loaded_deoplete in a file like ~/.vim/after/plugin/config/deoplete.vim (use the equivalent for neovim):

if ! exists('g:loaded_deoplete')
  finish
endif

" your custom code here
call deoplete#custom#source('ultisnips', 'rank', 1000)

---

1. The name plugin does not mean plugin as a set of code from GitHub or elsewhere, but rather a plugin script (cf. :help plugin). The name config is not significant except that it helps me organize my after/plugin directory: all the configuration lies there. Note that all subdirectories of a plugin directory are searched for scripts.

Answer 2

You could try using the VimEnter autocmd:

augroup DeopleteOptions
   au!
   autocmd VimEnter * 
   \ if exists('*deoplete#custom#source') | 
   \     call deoplete#custom#source('ultisnips', 'rank', 1000) |
   \ endif
augroup END

The if avoids errors if you are running somewhere you don't have all your plugins installed.