In my vimrc how to test if a plugin is installed?

Question

My use case is the following, but I think a general solution, working for as many plugins as possible would be preferable:

I use the submodes plugin to create a submode handling windows. In my .vimrc I have several lines containing calls to functions defined by this plugin (like that:)

" The submode is entered whith <Leader>k and exited with <Leader>
call submode#enter_with('WindowsMode', 'n', '', '<Leader>k', ':echo "windows mode"<C
call submode#leave_with('WindowsMode', 'n', '', '<Leader>')
" Change of windows with hjkl
call submode#map('WindowsMode', 'n', '', 'j', '<C-w>j')

When I am on a machine where I use my .vimrc but I haven't installed the plugin yet, I get errors on Vim starting: E117: Unknown function :submode#enter_with.

This is normal that this happen since the plugin isn't installed, Vim can't know the function. But the message on my shell necessitating a key press to disappear is pretty annoying.

---

To avoid that I've tried several things

• Some (good) plugins declare a variable g:loaded_pluginname which is accessible at the time of the .vimrc sourcing and can be used to test if the plugin exists. Unfortunately submode (and other plugins) doesn't provide such a variable (nor any variable available at .vimrc sourcing time).
• Of course, I could use :silent! in front of all the lines causing the message to mute them. The problem is that it doesn't feels really clean to proceed this way: I'd like keeping the potential error messages when the plugin is installed.

• The last solution I've found is to check if the file defining the plugin exists and put the call to the function in this if:

  if (filereadable($HOME . "/.vim/plugged/vim-submode/autoload/submode.vim"))   
  

  The fact is that it doesn't seem efficient nor reliable to me.

Thus I'm looking for the most generic way to programmatically test from my .vimrc if a plugin is already installed.

Answer 1

Normally exists('*submode#enter_with') can be used, however it was returning false in my case.

So @VanLaser suggested a pretty simple solution using the try and catch vimscript instruction to execute the lines which echoed errors when the plugin wasn't installed.

For now I use this solution, it doesn't allow to determine if a plugin is installed or not but at least my instructions are run when it's possible and I got rid of the error messages when the instructions can't be executed:

try
    " Create a submode to handle windows
    " The submode is entered whith <Leader>k and exited with <Leader>
    call submode#enter_with('WindowsMode', 'n', '', '<Leader>k', ':echo "windows mode"<CR>')
    call submode#leave_with('WindowsMode', 'n', '', '<Leader>')
    " Change of windows with hjkl
    call submode#map('WindowsMode', 'n', '', 'j', '<C-w>j')
catch
    "echo "The functions can't be executed"
endtry

I think I could even put the following in the catch block to automatically try to install the missing plugin.

catch
    echo "vim-submode is not installed, trying to install it"
    PlugInstall vim-submode
endtry

Answer 2

For autoload plugins i proceed in two steps.

First i test whether the function exists. If not i execute :runtime autoload/path/pluginname.vim and i test again whether the function exists.