March 01, 2011

End-of-Line Whitespace in Vim

Whitespace characters at the ends of lines are sloppy and (almost completely) useless. Not to mention when you notice and remove them, they clutter up your version control history. That can be mitigated by using git diff --ignore-space-at-eol, but it's better to never let it become a problem. So I have two settings in my vimrc that help me avoid committing EOL whitespace to any of my hobby or work projects.

The first one highlights EOL whitespace so you can tell that it's even there, but in a way that isn't obnoxious. Vim does offer a builtin option to do this. But it sucks.

set list
set listchars=trail:.

These two settings make it so when whitespace occurs at the end of a line, each character is displayed as a blue period. This is good in theory, but actually it means that whenever you're typing new content, these blue periods show up every single time you're done typing a word and continue on to the next one. This is what I mean by obnoxious. Of course there's going to be lots of fleeting EOL whitespace if I'm typing a sentence or a line of code! So a few years ago Jesse Luehrs and I banged our heads against our own separate walls until we came up with something that worked for us. I haven't seen this used anywhere else but it works perfectly for us so I wanted to share it.

autocmd InsertEnter * syn clear EOLWS | syn match EOLWS excludenl /\s\+\%#\@!$/
autocmd InsertLeave * syn clear EOLWS | syn match EOLWS excludenl /\s\+$/
highlight EOLWS ctermbg=red guibg=red

What this does is highlight EOL whitespace with a red background except on the line you're editing. Which means it's not obnoxiously telling you about every time you hit the space bar. The highlighting only occurs when you leave insert mode or if you move the cursor to a different line, such as by hitting enter. Which we've found to be exactly the kind of highlighting we want.

The syn clear EOLS is needed so when you switch modes, the appropriate syntax highlighting rule runs instead of both of them. The first line has two components of abominable Vim-specific regex: \%# matches cursor position, and \@! is like a negative lookahead from Perl for the previous atom. So in effect this matches end of line whitespace, except when the cursor is inside that whitespace.

The other tool I use to combat EOL whitespace is to unceremoniously execute it.

nmap <leader>w :%s/\s\+$//<CR>:let @/=''<CR>

This sets up a new normal-mode command <leader>w. The <leader> is like a user-specific namespace for custom commands; for most people it's going to be , but I have it set to \. The :let @/='' bit empties the / register so that the /\s\+$/ regular expression is not used for the n command or hlsearch highlighting. Unfortunately you lose whatever was in @/ before you ran \w but that kind of problem hasn't affected me in practice.

Some people run a whitespace stripper like this in a BufWritePre autocommand. But I don't like that solution because sometimes whitespace at the end of a line is important — such as in Markdown. Instead, the configuration I've described gives you tools for dealing with EOL whitespace sanely. Got any more?

Several people on reddit have offered alternate highlighting solutions.

Update - I'm stealing reddit user Amablue's improvements to the whitespace stripper because it no longer stomps on @/ and it reinstates the cursor position. I also like his <leader><space> over <leader>w.

function! <SID>StripTrailingWhitespace()
    " Preparation: save last search, and cursor position.
    let _s=@/
    let l = line(".")
    let c = col(".")
    " Do the business:
    " Clean up: restore previous search history, and cursor position
    let @/=_s
    call cursor(l, c)
nmap <silent> <Leader><space> :call <SID>StripTrailingWhitespace()<CR>