How to prevent whitespace associated with items in the statusline from taking up space, when the item is not shown?

Question

Consider the following statusline that makes use of %r to display [RO], when readonly has been set.

The problem is that when readonly is not set, two spaces occur between %f (file path) and %y (file type). The problem is even worse, if I have several items, that may sometimes not be shown, separated by spaces.

Is there a way to completely remove an item from the statusline, when it's not shown, so that associated whitespace isn't shown?

set laststatus=2
set showcmd
set showmode

set statusline=%f
set statusline+=\ %r
set statusline+=\ %y

Answer 1

Yes, it's parentheses (aka "item group"). Sort of

let &statusline = '%f%( %r%)%( %y%)'

Some time ago I also wrote a plugin (actually a function) to assist in building status line expression from simple "blocks".

Answer 2

An alternative is to wrap more things in square-brackets, obviating the need for spaces (a modified version of what I start with):

let &statusline = '%([%f]%)%(%m%r%w%h%)%(%y%)'

Not all of the grouping is necessary, but I actually break my statusline out over multiple lines in my vimrc, so I just group everything. And there's this from :help statusline:

    When all items in a group becomes an empty string (i.e. flags that are
    not set) and a minwid is not set for the group, the whole group will
    become empty.  This will make a group like the following disappear
    completely from the statusline when none of the flags are set. >
        :set statusline=...%(\ [%M%R%H]%)...

Answer 3

Another option is to use a conditional:

set statusline+=%{&readonly?'\ [RO]':''}

This allows you to also change what is shown. For example, I have:

set statusline+=%{&readonly?'\ !!':''}