[texhax] Constructing a very thin space

[Paul Isambert]

Philip Taylor <P.Taylor at Rhul.Ac.Uk> a écrit:
> Karl Berry wrote:
> 
> > I do not have an example at hand, but without the space after `em',
> > TeX will still be reading and expanding tokens after the macro is
> > called (to look for the <space> that's supposed to be there).
> 
> To look for the <space> that's supposed to be /where/, Karl ?
> 
> Considering \thinspace as an example, and using DEK's
> definition from Plain.TeX :
> 
> \def\thinspace{\kern .16667em }
> 
> No space is expected after \thinspace, which is automatically
> terminated by any non-letter; no space is expected after \kern .16667em,
> since all 2-letter units are self-terminating and
> there are no optional plus/minus modifiers, so I cannot see
> to what "<space> that's supposed to be there" you are referring.

Units can be followed by an optional space, so they’re not
“self-terminating”; if \thinspace did not include a space in its
definition, expansion would continue until a space is found, see:

    \def\aspace{ }
    a\kern0pt\aspace b

with no space between the two letters. Similarly, “a\kern0ptb” and
“a\kern0pt b” are the same thing. So the space at the end of
\thinspace is quite important.

Best,
Paul