Using a custom .fmt with Emacs/AUCTeX

Question

I am attempting to use a plain TeX format, say, foo.fmt (based on XeTeX), with Emacs/AUCTeX (actually Aquamacs with built-in AUCTeX). I'm unable to get AUCTeX to recognize the custom format. In TeXShop I would simply say:

% !TS-program=foo

I have tried various "Local Variables" such as

%%% Local Variables:
%%% TeX-engine: foo
%%% End:

followed by C-c C-n. But so far nothing seems to work. TeXing stops with an error for an undefined command that was defined in foo.fmt. The format file is findable with kpsewhich. What am I missing?

Answer 1

The problem is that AUCTeX will use this kind of call:

Running `TeX' on `testfmt' with ``xetex  -file-line-error  --synctex=1 -interaction=nonstopmode "\input" testfmt.tex''

Due to the \input a first line %&foo to use foo.fmt format file is ignored, for example with this file:

%&foo
\foo
\bye
% Local variables:
% TeX-engine: xetex;
% End:

where macro \foo was defined in the dumped foo.fmt.

You can see it originates in "%'" in TeX-expand-list-builtin and its usage in TeX-command-list

("TeX" "%(PDF)%(tex) %(file-line-error) %(extraopts) %`%S%(PDFout)%(mode)%' %t" TeX-run-TeX nil
  (plain-tex-mode ams-tex-mode texinfo-mode)
  :help "Run plain TeX")

I have customized the variable to contain this other entry

("texfmt" "%(PDF)%(tex) %(file-line-error) %(extraopts) %S%(PDFout)%(mode) %t" TeX-run-TeX nil
  (plain-tex-mode)
  :help "Fait (xe|lua|pdf)tex")

The only difference is I removed the %'. edit: I also removed the %` whose elisp definition makes it look like it is to be used paired with %' but this would need confirmation by AUCTeX people.

Then when I open a buffer with file above I see in the menu "Command" a new entry texfmt. I choose that and xetex is run and uses the foo.fmt.

Running `texfmt' on `testfmt' with ``xetex  -file-line-error   -interaction=nonstopmode testfmt.tex''
This is XeTeX, Version 3.14159265-2.6-0.99999 (TeX Live 2018) (preloaded format=foo)
 restricted \write18 enabled.
entering extended mode
(./testfmt.tex [1] )
Output written on testfmt.pdf (1 page).
Transcript written on testfmt.log.

TeX Output finished at Thu Apr 19 00:11:19

À propos formats with xelatex, not xetex, see this for a difficulty.