Sorry, don't know what went wrong. Is it okay that I send you this link to my original message (which does include the code)?

On Mon, 26 Oct 2020 at 11:45, Lars Ingebrigtsen <larsi@gnus.org> wrote:
dalanicolai@gmail.com writes:

> The code above should work correctly. However when you try to use the
> variable defined in the first two lines for the width within the
> definition of the tabulated-list-format then no tabular list is printed
> but instead Emacs shows an error that the width is not a number or
> marker (number-or-marker-p).
>
> I guess this is not the desired behavior?

Could you show the code that is failing?

--
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   bloggy blog: http://lars.ingebrigtsen.no