Re: tabulated-list-revert runs hook before re-printing the table

[Sebastian Wiesner <swiesner@lunaryorn.com>]

Am 15.09.2014 um 00:43 schrieb Stefan Monnier <monnier@iro.umontreal.ca>:

>> You mean that a mode derived from `tabulated-list-mode' would add a function
>> to this hook in order to re-compute `tabulated-list-entries'?  
> 
> That's what it looks like, yes.  Try a "grep tabulated-list-revert-hook
> **/*.el" in Emacs's sources and see for yourself.

Indeed, I now remember that I saw this pattern when I studied other tabulated list modes to find out how it is supposed to be used.  I also remember, though, that I also didn't understand its motivation back then.

>> Why would a mode do that, rather than just using a function for
>> `tabulated-list-entries' that directly returns the list of entries?
> 
> I don't know, but my guess is that it's perceived that
> tabulated-list-entries would be called too often and would hence be too
> taxing on CPU resources?

Is that really the case?  I looked at tabulated-list.el, and I don't think so.

`tabulated-list-print' calls tabulated-list-entries once if its a function, and `tabulated-list-print' itself is only called at two places:  In the revert function, which `tabulated-list-revert-hook' is called as well, and in the function that handles explicit sorting, i.e. the user pressing S or clicking on the header line.

As far as I can see, using the hook instead of a `tabulated-list-entries' would only make explicit sorting a little more efficient.

Imho, it seems that the real use case for this pattern is to abuse `tabulated-list-entries' as data structure.  At least, that's the case in package.el, which uses `package-list-entries' to compute package updates, rather than `package-alist', which would be the right data structure for this purpose, imho.

Anyhow, I was just curious, so never mind.  I've solved my original problem by simply adding my own hook to my derived mode.

>> Unrelated to that question, could we have a hook run *after*
>> `tabulated-list-print' by `tabulated-list-revert'?
> 
> It would make sense for it to run after-revert-hook, I guess.
> 

Indeed.  It should probably run `before-revert-hook' as well, before `tabulated-list-revert-hook'.  I wonder why there's even a separate hook for tabulated-list-mode. 

Thanks for your help, and your feedback
Sebastian Wiesner