`set` option with variables

Question

I am trying to rewrite adding a string to option with variable. The original string is

set rtp+=~/.vim/bundle/Vundle.vim

The following is the working let way:

let MYVIMDIR='/home/loom/foo'
let &rtp .= ','.expand(MYVIMDIR).'/bundle/Vundle.vim'

I am interested how rewrite it with set. The following code (and some other variations) doesn't work

let MYVIMDIR='/home/loom/foo'
set rtp+=expand(MYVIMDIR).'/bundle/Vundle.vim'

What is the right way to use variables in expression with set?

Answer 1

You could use :exe, but this is extremely cumbersome to use and you'll need to escape a few things.

I used to use my own path fixing function. It looked like:

exe 'set rtp+='.lh#path#fix(somevariable)
" with standard tools, may be it'd be (untested)
exe 'set rtp+='.escape(somevariable, ' \|,')

let &rtp = expression is really our friend.

But indeed, it doesn't play well with +=. That's why I've came up with lh#path#munge() that relies on another function lh#list#add_if_new(). Strictly speaking set += is emulated by lh#list#add_if_new(). munge() keeps pathnames that do exist, or actually globbing patterns that match things.

" Function: lh#path#munge(pathlist, path) {{{3
function! lh#path#munge(pathlist, path) abort
  if type(a:pathlist) == type('str')
    let pathlist = split(a:pathlist, ',')
    return join(lh#path#munge(pathlist, a:path), ',')
  else
    " if filereadable(a:path) || isdirectory(a:path)
    if ! empty(glob(a:path)) " support '/**' for &tags
      call lh#list#push_if_new(a:pathlist, a:path)
    endif
    return a:pathlist
  endif
endfunction

" Function: lh#list#push_if_new(list, value) {{{3
function! lh#list#push_if_new(list, value) abort
  if index(a:list, a:value) < 0
    call add (a:list, a:value)
  endif
  return a:list
endfunction

They are defined and maintained in lh-vim-lib.

Answer 2

You could use an environment variable defined within your current shell session.

let $myvimdir = '/home/abc/vimfiles'

That way you can use other variables to compose the path as well:

let $myvimdir = $HOME.'/vimfiles'

And set the runtime path:

set rtp+=$myvimdir/bundle/Vundle.vim

More information on environment variables within vim :help expr-env