jobstart callback wont assign variable outside of function

Question

I have a job I want to run in the background of vim, and update my status line with the periodic results of that job.

I'm able to setup the timer, call the external job and get my response back. I'm able to echom the response and see it in :messages.

I am unable to get my global variable to take hold of the new value though.

Here is the vimscript:

let g:please_vim_why = 'an echo example'
let g:why_vim_why = -1
function! TimerRun(timer)
  call jobstart('date', {'on_stdout': 'TimerCallback'})
endfunction
function! TimerCallback(j, d, e)
  echom a:d[0]
  let g:why_vim_why = a:d[0]
endfunction
let timer = timer_start(2000, 'TimerRun', {'repeat': -1})

Run via nvim -u godwhyvimwhydoyouhateme.vim to load it as your vimrc.

Now if you go :echo g:please_vim_why, you'll see the response, 'an echo example'.

If you run :messages, you'll see a repeated output of the date.

If you run :echo g:why_vim_why you will see no output.

I thought maybe it might be a:d is scoped and gets garbage collected after the function, but let g:why_vim_why = 'not a good joke' . a:d[0], sets the variable to 'not a good joke' but doesn't concatenate anything.

Why VIM? Why?

Answer 1

The on_stdout callback will be called twice for each execution of date in this example.

First it will be called with the result from date as an array with two elements ['the-date', ''] where '' is an artifact of the newline output by date. (If you run a job such as echo -n "no new line", you'd only receive ['no new line'].

Second it will be called with [''] to signal EOF on stdout.

The second call is clobbering the first, which makes it appear as if the result is going missing somehow.

You can either check the length of the argument (i.e. if len(a:d[0])...) or pass the option 'stdout_buffered': 1 to jobstart to only receive output when all output from the command is collected.

All this is explained in :h on_stdout.