TeX - LaTeX Stack Exchange is a question and answer site for users of TeX, LaTeX, ConTeXt, and related typesetting systems. It only takes a minute to sign up.
Sign up to join this community
In TeX, code like I\kern37ptlike\hskip100ptcake works perfectly well. That makes me wonder how exactly the syntax of primitive commands like \kern, \hskip etc. work? How does TeX determine where their arguments end?
Does TeX just utilise the fact that (as far as I know) all length units consist of two alphabetic characters?
(I cannot see the need for a MWE.)
The argument for \kern is a ⟨dimen⟩, so TeX first looks for a ⟨decimal number⟩ (which can be an integer in any admissible representation, or a fractional number in decimal notation, with period or comma for separating the fractional part) and then for a ⟨unit of measure⟩, that is, a pair of letters representing one of the accepted units (pt, cm, mm, in, dd, pc, cc, bp, sp, em, ex). Any number of spaces between the numbers and the units are ignored, an optional space can follow the unit (it is looked for with expansion). Any other pair of letters will trigger an error message
! Illegal unit of measure (pt inserted).
Thus \kern37pclike has a correct value for \kern, but \kern 37 pc like would have given exactly the same result.
Be careful with \kern, because it's a command that does different things if called in vertical or horizontal (or math) mode. Using \enspace before having started a paragraph will make you scratch your head.
The argument to \hskip should instead be a ⟨skip⟩; the same as before applies, but after the ⟨dimen⟩ stating the natural width, TeX looks for the keyword plus or minus (with expansion). If it finds it, it looks again for a ⟨dimen⟩ with the same rules as before, but the ⟨decimal number⟩ can also be followed by fil, fill or filll (not the real truth, but a good approximation) again with a trailing optional space that's looked for and ignored. Notwithstanding if plus is found or not, TeX looks for a minus keyword, same rules.
So \hskip 2pt minuscule chances of errors will trigger a puzzling error message.
The same as before is valid whenever TeX is looking for a ⟨dimen⟩ or a ⟨skip⟩. In case of doubts, particularly with a ⟨skip⟩, add \relax, which is what \setlength, \hspace and \vspace do in LaTeX.
fil(|l|ll)?
:-D. (Seriously, I could see it was his without looking at the name.)
fil looks for a following l which looks for a following l. So \hskip 0pt plus 1 fil L l minus 2 fiLL is a legal specification. And \hskip 0pt plus 1fil Let's go will make you scratch your head. ;-)
\kern , pt will give you a kern of 0.
l(ell) in the first case, which is not a length, then decides that the kern is ended, inserts the kern, and keeps going with the ell. The\hskipdoes the same, but withc. In case of an space, it's gobbled. Better use\kern37pt likeand\hskip100pt cake. (Disclaimer: I'm a not specially proficient in TeX.)\kern{37pt}breaks. TeX is really the worst when it comes to consistent syntax.\kern37pt, the{..}is not the syntax. Of course it crashes.\kern{37pt}if\kernweren't already a TeX dimension command, which has its own syntax.\def\whatever{\afterassignment\dowhatever\dimen0=}, then\whatever37ptlikewould work and use\dowhatever, after storing37ptin\dimen0.