[luatex] scanning for undefined command names

[Hans Hagen]

On 10/26/2016 2:25 PM, David Carlisle wrote:
> Hans,
>
>> there is no token \UNDEFINED so it cannot be reported as such
>
> I think that is a terminology problem that is the source of the confusion.
>
> there is no entry in the internal table of control names that
> presumably you are looking up here but \UNDEFINED is a token in the
> usual TeX sense \UNDEFINED can take part in token lists
> \def\foo{\UNDEFINED}  if  \UNDEFINED isn't a "token" what is it?
>
> Actually while writing the above, I guessed that the following would
> work and in fact it does
>
>
> \def\z{\UNDEFINED}
> \directlua{texio.write_nl("[" .. token.scan_csname() .. "]")}\par
> \directlua{texio.write_nl("[" .. token.get_next().csname .. "]")}\par
> \directlua{texio.write_nl("[" .. token.scan_csname() .. "]")}\UNDEFINED
> \directlua{texio.write_nl("[" .. token.get_next().csname .. "]")}\UNDEFINED
>
> produces
>
> [par]
> [par]
> [UNDEFINED]
> [UNDEFINED]
>
>
> but I wonder if the manual could be clarified, not everyone would guess that:-)

indeed (i bet that there are a few more confusing cases, esp because tex 
itself when scanning puts stuff back while the token.* functions are 
just picking things up)

i think can add your example as explanation somewhere

Hans


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