How can I set a variable for a single Vim Ex command only?

Question

I often set or modify a shell variable in Bash for the scope of a single command only:

$ FOO=BAR <command>

Is it possible to set or modify a variable in Vim for the scope of a single Ex command only?

Suppose I want to modify the path variable for a single :find command. I don't want to remember the old value and reset it afterwards using :set path=.

Answer 1

Unfortunately there is nothing like that.

In my library plugin, I've implemented a generic feature that takes care of restoring options, variables (where restoring may means :unlet), mappings, or trigger some other actions at the end of a sequence of operations. But it's not meant to be used on the fly. It's too verbose for that.

  let cleanup = lh#on#exit()
        \.restore('&l:statusline')
        \.restore('&isk')
        \.restore('b:varname')
        \.register('au! SomeAuGroup')
        \.restore_cursor()
        \.restore_buffer_mapping('<c-y>', 'i')
        .....
  try
     actions...
  finally
     call cleanup.finalize()
  endtry

---

Back to your concern, I guess you'd like something simple to use. We could imagine something like :MyExec -protect:path=tmp\ value :the old ex-command. The problem I see is parsing the temporary value sent to path. I've tried several times to have commands able to reconstruct strings and take other parameters, it's a plain headache. At best we need some unique separator. Something like -- maybe ?

This could give

command! -nargs=1 ProtectExec call s:ProtectExec(<f-args>)

function! s:ProtectExec(cmdline) abort
  " Only accepts a single -override parameter
  let parts = split(a:cmdline, '\s\+--\s\+')
  if len(parts) < 2
    throw "ProtectExec: Don't forget to separate the ex-command from the overriden variable/option with '--'"
  endif
  let cleanup = lh#on#exit()
  try
    if parts[0] =~ '\v-over%[ride]:'
      let [all, var, val; dummy] = matchlist(parts[0], '\v-over%[ride]:([^=]+)\=(.*)')
      call cleanup.restore(var)
      if empty(var)
        throw "ProtectExec: Cannot extract variable name and value from `".parts[0]."`"
      endif
      " If only options and only one option would be supported,
      " then varname could be used in the the finally clause instead 
      " of the generic on#exit() object.
      let varname = lh#option#is_set_locally(var) ? '&l:'.var[1:] : var
      exe 'let '.varname.' = '.val
    endif
    exe parts[1]
  finally
    call cleanup.finalize()
  endtry
endfunction

That is used with:

:ProtectExec -override:&path='toto' -- echo "tmp:".&path

Note that lh#on#exit().restore() relies on another function: lh#option#is_set_locally() that I just had to fix in order to correctly recognize global-local options that haven't been locally overridden. This part has been a little tricky (we need to know how the option shall be restored: with :set or :setlocal ).

let s:k_option_fullname = {
      \ 'ai': 'autoindent',
      \ 'bs': 'backspace',
      \ 'efm': 'errorformat',
      \ 'et': 'expandtab',
      \ 'ft': 'filetype',
      \ 'isk': 'iskeyword',
      \ 'rtp': 'runtimepath',
      \ 'sw': 'shiftwidth',
      \ 'ts': 'tabstop',
      \ 'tw': 'textwidth'
      \ }
function! lh#option#is_set_locally(option_name, ...) abort
  let bufid = get(a:, 1, '%')
  if a:option_name =~ '^&\(\([lg]:\)\@!.\)*$'
    " options with no explicit scope
    let options = getbufvar(bufid, '&')
    " Before 7.4.434, getbufvar() returns an empty string instead of an
    " empty dict when nothing is found
    " Also, older version of vim don't return local options with
    " getbufvar(bid, '&')
    " In global-local options case, an empty local option is an option that
    " hasn't been overriden
    let option_name = get(s:k_option_fullname, a:option_name[1:], a:option_name[1:])
    if !empty(options) && has_key(options, option_name)
      if type(options[option_name]) == type('')
        return !empty(getbufvar(bufid, '&l:'.option_name))
      else
        return getbufvar(bufid, '&l:'.a:option_name[1:]) != getbufvar(bufid, '&g:'.option_name)
      endif
    endif
  endif
  return 0
endfunction

IMO, unless you have many cases of commands and options you'd like to override, you'd better off with a dedicated command for searching with a temporary path -- but don't forget to check whether 'path' is currently a local or a global option.