set rtp in a function does not work

Question

I have the following configuration for OCaml development which works fine:

" --------------- OCaml -----------------------------
" Merlin
let g:opamshare = substitute(system('opam config var share'),'\n$','','''')
execute 'set rtp+=' . g:opamshare . '/merlin/vim'
" ocp-indent
execute 'set rtp^=' . g:opamshare . '/ocp-indent/vim'
" enable deoplete
let g:deoplete#enable_at_startup = 1

Given that I use differents languages, I want to just use this configuration with OCaml files so I tried to create a function and to call it if the FileType is ocaml:

augroup OCaml_Environment
  autocmd!
  autocmd FileType ocaml :call OCamlEnvSetup()
augroup END

function! OCamlEnvSetup()
  let g:opamshare = substitute(system('opam config var share'),'\n$','','''')
  execute 'set rtp+=' . g:opamshare . '/merlin/vim'
  " ocp-indent
  execute 'set rtp^=' . g:opamshare . '/ocp-indent/vim'
  " enable deoplete
  let g:deoplete#enable_at_startup = 1
endfunction

But now I can not access to commands defined in merlin/vim for example. What am I doing wrong?

Edit

Based on the answers I modified my function:

augroup OCaml_Environment
    autocmd!
    autocmd FileType ocaml :call OCamlEnvSetup()
augroup END


function! OCamlEnvSetup()    
   let g:opamshare = substitute(system('opam config var share'),'\n$','','''')
   execute 'set rtp+=' . g:opamshare . '/merlin/vim'
   execute 'runtime ' . g:opamshare . '/merlin/vim'
   " ocp-indent
   execute 'set rtp^=' . g:opamshare . '/ocp-indent/vim'
   execute 'runtime ' . g:opamshare . '/ocp-indent/vim'
endfunction

let g:deoplete#enable_at_startup = 1

With this I have the following error:

[deoplete] Merlin not found, make sure ocamlmerlin is in your path and merlin's vim plugin is installed.

After that I tried to use a filetype-plugin too, creating the file ~/.config/nvim/ftplugin/ocaml.vim with the following content:

let g:opamshare = substitute(system('opam config var share'),'\n$','','''')
execute 'set rtp+=' . g:opamshare . '/merlin/vim'
execute 'runtime ' . g:opamshare . '/merlin/vim/plugin/**/*.vim'
" ocp-indent
execute 'set rtp^=' . g:opamshare . '/ocp-indent/vim'
execute 'runtime ' . g:opamshare . '/ocp-indent/vim/plugin/**/*.vim'

But now it looks like the configuration is not loaded, because for example, I can not use the :MerlinTypeOf that is available with merlin.vim.

Answer 1

Have a look at :help startup. If you set up 'runtimepath' in your ~/.vimrc, Vim will later load the plugins found there (:help load-plugins).

Now if you do the 'runtimepath' setup later, Vim is ready to source any autoload functions from there, but the plugin configuration itself (that would make use of the autoload functions), has never been loaded! Therefore, you need to explicitly :runtime any plugins and filetype detections yourself. In the comments, you've indicated that you've tried that via runtime g:opamshare . '/merlin/vim':

• Vim's evaluation rules are different than most programming languages. You need to use :execute in order to evaluate a variable (or expression); otherwise, it's taken literally; i.e. Vim uses the variable name itself as the argument.
• :runtime needs a list of files / file glob, and you want to get all plugin files (i.e. from the plugin subdirectory).

:execute 'runtime' g:opamshare . '/merlin/vim/plugin/**/*.vim'

Wait, there are also filetype plugins

Please note that your approach is only recommended for general plugins that you however only want to use in the context of a certain filetype. For plugins that truly are specific to a filetype, there's :help filetype-plugins built into Vim, and that is far more elegant that the approach described here.

Answer 2

As you apparently struggle with setting this up yourself, an alternative may be delegating the implementation to a plugin:

• vim-plug is a fully-featured plugin manager (that offers much more than just runtimepath manipulation); it explicitly supports On-demand loading for faster startup time
• enabler.vim is a more dedicated plugin to load plugins/bundles as needed