Vim: toggle quickfix window

.vimrc

" quickfix
function! s:redir(cmd)
  redir => res
  execute a:cmd
  redir END

  return res
endfunction

function! s:toggle_qf_list()
  let bufs = s:redir('buffers')
  let l = matchstr(split(bufs, '\n'), '[\t ]*\d\+[\t ]\+.\+[\t ]\+"\[Quickfix\ List\]"')

  let winnr = -1
  if !empty(l)
    let bufnbr = matchstr(l, '[\t ]*\zs\d\+\ze[\t ]\+')
    let winnr = bufwinnr(str2nr(bufnbr, 10))
  endif

  if !empty(getqflist())
    if winnr == -1
      copen
    else
      cclose
    endif
  endif
endfunction
nnoremap <silent> qo :<C-u>silent call <SID>toggle_qf_list()<Cr>