Minimal Testing Environment For LuaMetaTeX

Question

The new LuaMetaTeX typesetting engine praises itself as being "lean and mean", but the ConTeXt package that it comes shipped with is actually pretty obese (around 270 MiB when installed). To get some basic understanding of how it works, I would like to run it with the minimal subset of files required for a kind of "hello world" test, converting into PDF the famous "short story" example from the TeXbook ("Once upon a time, in a distant galaxy called Ööç, there lived a computer named R. J. Drofnats...").

Using pdfTeX, this is quite simple (see manual https://ftp.tu-chemnitz.de/pub/tex/systems/doc/pdftex/manual/pdftex-a.pdf). Create one single folder C:\TeX without any subdirectories and copy the following files into it:

1. Binaries: pdftex.exe, pdftex.dll, kpathsealib.dll (from http://mirrors.ctan.org/systems/win32/w32tex/pdftex-w32.tar.xz).

2. Font metrics and outlines: cm*.tfm and cm*.pfb (from https://ctan.org/tex-archive/fonts/cm/tfm and https://ctan.org/tex-archive/fonts/cm/ps-type1/bakoma/pfb).

3. TeX files: plain.tex, hyphen.tex, story.tex (from https://ctan.org/tex-archive/macros/plain/base).

4. Font map file pdftex.map (created manually):

    cmbx10 CMBX10 <cmbx10.pfb
    cmr10 CMR10 <cmr10.pfb
    cmsl10 CMSL10 <cmsl10.pfb
    [...]
   

5. Open a command prompt window, change directory and set environment variables to the current folder:

    cd C:\TeX
    set t1fonts=.
    set texfontmaps=.
    set texfonts=.
    set texformats=.
    set texinputs=.
    set texmf=.
    set texmfcnf=.
    set tfmfonts=.
   

6. Create memory dump format file plain.fmt:

    pdftex --ini ./plain.tex \dump
   

7. Typeset document story.pdf:

    pdftex --fmt=plain --output-format=pdf ./story.tex \end
   

Now do the very same thing with LuaTex (see manual http://www.pragma-ade.com/general/manuals/luatex.pdf):

8. Download and extract the archive http://lmtx.pragma-ade.nl/install-lmtx/context-win64.zip to another folder and run the included install.bat script. Copy luatex.exe and luametatex.exe to the C:\TeX test folder.

9. Create memory dump format file plain.fmt:

    luatex --ini ./plain.tex \dump
   

10. Typeset document story.pdf:

    luatex --fmt=plain --output-format=pdf ./story.tex \end
    

The problem comes with the new LuaMetaTeX engine (see manual http://www.pragma-ade.com/general/manuals/luametatex.pdf). The command

    luametatex --ini ./plain.tex \dump

gives error messages

    startup error : missing find_log_file callback

and

    startup error : missing open_data_file callback

unless a Lua initialization file lmtx-ini.lua defining callback functions is also loaded, but obviously these ones are not sufficient:

    callback.register ("find_log_file", function () return "plain.log" end)
    callback.register ("open_data_file", function () return true end)

because now, the command

    luametatex --lua=lmtx-ini.lua --ini ./plain.tex \dump

yields a really fundamental failure:

    This is LuaMetaTeX, Version 2.09.33  (INITEX)
    runtime error : input file './plain.tex' is not found, quitting

So how do I typeset my story.pdf using LuaMetaTeX in this minimal environment?

[Edit] As @Marcel points out, essential input-output functionality from TeX and a PDF backend must be rebuilt with Lua. To awaken the LuaMetaTeX engine to life somehow, my question modifies to: How to get his extracted LuaMetaLaTeX script collection (https://github.com/zauguin/luametalatex) running in the above standalone folder? This command fails:

    luametatex --lua=./luametaplain-init.lua --ini ./plain.tex \dump

Answer 1

LuaMetaTeX aggressively removed many parts which can be implemented in Lua, therefore making the engine itself much smaller and shifting responsibilities from the engine to the macro/Lua format. This allows significantly more cutomization without wasting no space with no longer used engine functionality. But this also means that a lot more components are required to actually use the engine. After all, many of the things which are no longer present in the engine itself still have to be provided by someone.

Some examples:

• You have to implement all the callbacks for finding and reading files correctly. E.g. find_log_file has to return a valid path to a log file, open_data_file has to return a table implement the file reader interface. (Of course, if you return true or another invalid value from a callback like open_data_file which is used to open input files then this gets treated as an error, in this case the engine things that the file is missing) This allows the engine itself to longer contain the distribution specific file finding routines like TeX Live's kpathsea.
• Instead of including PDF, DVI, or any other backends, any attempts to shipout the current page are ignored and the loaded macro package has to implement an output driver itself.
• The engine does not include code to read fonts from TFM files, if you want to load such fonts you have to write a TFM file parser.
• Some primitives, including rather important ones like the file i/o support (\openin/\openout/\read/\readline/\write/\ifeof/\closein/\closeout) are no longer included and have to be implemented by the format if needed.
• Hyphenation patterns are no longer part of the dumped format and the Lua and/or TeX code is responsible for loading them if necessary.
• ...

In general, if you want an engine which can be used without much supporting code, then LuaMetaTeX is not the right engine for you. If you are looking for a more or less LuaTeX compatible implementation of most of these independent form the ConTeXt version, you can look at my LuaMetaLaTeX repo (which also includes a luametaplain format which is using the same code for plain TeX instead of LaTeX).