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From: barr@TRIPLES.MATH.MCGILL.CA (Michael Barr)
Newsgroups: comp.text.tex
Subject: \the \baselineskip
Message-ID: <9102172148.AA04807@triples.math.mcgill.ca>
Date: 17 Feb 91 21:48:31 GMT
Sender: daemon@ucbvax.BERKELEY.EDU
Lines: 57

Indeed if you run the code:
 
\the\baselineskip
\baselineskip=2.0\baselineskip
\the\baselineskip
\bye
 
through tex, the output is
12.0pt12.0pt
 
If you run the code
 
\the\baselineskip
\baselineskip=2.0\baselineskip{}
\the\baselineskip
\bye
 
through tex, the output is
12.0pt 24.0pt
 
If you run
 
\the\baselineskip
\baselineskip=2.0\baselineskip\relax
\the\baselineskip
\bye
 
the output
12.0pt24.0pt
 
What is going on here is one of the most pernicious gotcha's in tex.
The fact is that when tex reads a numerical quantity (and dimensions are
also stored as numbers, in terms of sp's), tex has to read far enough to
ascertain that the number has actually finished.  If it discovers that
the number actually finished a while back, it does not go back and
reread what has already been read.  In the first case, it read the
\baselineskip=2.0\baselineskip and then continued past the carriage
return (which was swallowed by the control sequence) to read the second
\the, which apparently caused it to read the \baselineskip and then
executed the code that it had absorbed.  At that point, the value of
\baselineskip was indeed 24.0pt, but what printed was the value when it
was read.  For some reason, it doesn't do this if instead of \the you
execute \showthe\baselineskip.  I don't understand why the two behave
differently; I can only assume one takes place in the stomach and the
other in the esophagous, or something like that.  TeX's digestive tract
is something of a black hole, at least to me.
 
The remaining two contain a pair of braces or a \relax, both of which
signal TeX that the dimension it is examining has ended and so it
doesn't continue to read on.  LaTeX has \relax inserted at various dates
in strategic places, almost surely to forestall unexpected occurrences
like this when a macro appears in a place not anticipated at the time it
was originally coded.  If you insert the pair of braces, the carriage
return isn't swallowed and you get a space in there; with relax, you
don't get the space.
 
Michael Barr