[texhax] question about \vfil

[Robin Fairbairns]

> Could somebody please explain me why this:
> 
> 	\vfil abc\vfil\bye
> 
> doesn't have the same effect as this:
> 
> 	\vbox to \vsize {
> 	        \vfil
>         	abc
> 	        \vfil
> 	}
> 	\bye

for two reasons.

1. \vfil is discardable at the top of a page (just like space is
   discardable at the start of a line), so your first example doesn't
   even see the first \vfil
2. \bye contains \vfill, so even if you manage to get the first \vfil
   noticed, it's overwhelmed by the "greater infinity" of \vfill

> The former one is simpler and thus nicer. But there \vfil-s seem to be
> ignored. I do not understand why because as far as my humble knowledge,
> TeX starts to process the input in vertical mode. So the initial \vfil
> shouldn't be ignored.
> 
> I would really appreciate some explanation or some link to a relevant
> information. I do not understand the horizontal/vertical mode stuff very
> well.

it's all described in the texbook, and no doubt also in tex by topic
(google for it -- it's a good "free"[*] book).

correct the two errors i mentioned, and you have

\vbox{}\vfill abc\vfill\bye

but now "abc" is 33% of the way down the page: tex has added up the
\vfill's and apportioned space appropriately.  the second \vfill above
isn't needed:

\vbox{}\vfill abc\bye

(your \vbox to\vsize works because it's not a page, even though it's
the same size as one.)

[*] as in, if you can, you're invited to send a donation to the author