function Defer(command, callback)
  let l:buffer = []
  call job_start(a:command, {
  \ "out_io": "pipe",
  \ "out_cb": { pipe, text -> add(l:buffer, text) },
  \ "close_cb": { pipe -> a:callback(l:buffer) }
  \ })
endfun

comm -nargs=* Defer call Defer(<q-args>, { buffer -> 0 })