[metapost] Outputtemplate influences %& format comment parsing in verbatimtex

[Ben Frank]

Hi,

On 24.12.23 19:33, luigi scarso wrote:
> Here I have
> 
> $> mpost test.mp <http://test.mp>
> This is MetaPost, version 2.02 (TeX Live 2023) (kpathsea version 6.3.5)
> (/<...>/tex-2023/texmf-dist/metapost/base/mpost.mp <http://mpost.mp>
> (/<...>/tex-2023/texmf-dist/metapost/base/plain.mp <http://plain.mp>
> Preloading the plain mem file, version 1.005) ) (./test.mpfatal: Command 
> failed: etex --parse-first-line --interaction=nonstopmode mpK6cPmm.tex; 
> see mpxerr.log
>  >> test.mp <http://test.mp>
>  >> test.mpx
> ! ! Unable to read mpx file.
> l.10    label.lft(btex
>                         \textbf{J} etex,origin);
> Transcript written on test.log.

That's the error I mentioned you would get because the wrong format is 
chosen due to the wrongly output format comment.
As I said in my original post, you get the wrong format when the 
`outputtemplate` line is parsed by Metapost but the right format is 
chosen if you comment it out.

To repeat my original question:

 > Why does it [the example] not compile when fed to mpost as is (because
 > of the space before %&latex carrying into the output preventing its
 > parsing as a format comment) but compile fine when the outputtemplate
 > line is commented (i.e., the space seems to be gobbled as expected)?

I.e., my question boils down to why is there a ` %&latex` in the TeX 
stream which prevents processing as a LaTeX document because the space 
prevents the parsing of the format comment. And my observation is that 
this space seems to be part of the interaction with `outputtemplate` 
because commenting out the `outputtemplate` line results in `%&latex` 
(without space) which is a working format comment.

As I said, you will not get the error with

```
% outputtemplate := "%j-%2c.mps";
% ^^ commenting this out will make it work

verbatimtex %&latex
   \documentclass{article}
   \begin{document}
etex

beginfig(1)
   label.lft(btex \textbf{J} etex,origin);
endfig;

end;
```

That is, the `outputtemplate` influences the space handling. And I would 
like to know why and whether that is a bug or if it is a known feature, 
what is its rationale (i.e. outputtemplate influencing how verbatimtex 
inserts spaces into the TeX stream).

I hope this repetition of my question helps understanding what I am 
asking. Otherwise, feel free to ask a more detailed question, although I 
am not sure I can provide anything you cannot reproduce on your 
installation.

Best,
Ben