Add traceback to help debug "Arguments are not sufficiently instantiated" error from functor/2 from bad markup

[kamahen]

No description provided.

[kamahen]

The ordering of this use_module seems to be important.

[kamahen]

These 2 set_prolog_flag calls didn't have the desired effect ... but I couldn't find another flag for setting the option for the write.

This is what I got when running from cmake:

ERROR: [2,630] tex:item_commands([_484|_486],[],[_490|_492]) at /home/peter/src/swipl-devel/build.debug/home/library/ext/ltx2htm/latex2html.pl:983
ERROR: [1,904] tex:items([\(classitem,...),'\n'|...],[[...|...]|_554]) at /home/peter/src/swipl-devel/build.debug/home/library/ext/ltx2htm/latex2html.pl:961
ERROR: [1,890] tex:env(description([],['\n'|...]),[html('<dl class="latex">'),_624|...]) at /home/peter/src/swipl-devel/build.debug/home/library/ext/ltx2htm/latex2html.pl:935

but when I ran it by hand (calling main0/1), and did the debugger u command, I got more useful information:

   Exception: (1,887) tex:translate_env(description([], ['\n', \(classitem, [{'PlTerm'}]), '\n', 'Generic', 'Prolog', term|...]), file, file, _353630) ? 

which helped me figure out where the bad markup was.

[JanWielemaker]

The catch_with_backtrace/3 is in any case not needed as that is already in main/0. I don't know whether it works properly. Adjusting the amount of information captured from the trace depends on the application and could be a good idea for this case. Although markup errors are better reported in some other way ...

If you want to change these, you cannot do that in the exception handler. That is too late. The stack is assembled after detecting we want a backtrace (because of catch_with_backtrace/3 or the exception is unhandled) and before the stack is unwound and the handler is called.

I think the way to go is that if there is a normal Prolog exception in the code, this needs to be debugged. In some cases temporary changing the stack printing is a way to find the issue.

[kamahen]

It appears that the use_module(library(prolog_stack)) needs to come before use_module(library(main)) to get a backtrace.
I agree that a backtrace isn't a very good tool; but without that, all I had was one of the less useful error messages in Prolog: "Arguments are not sufficiently instantiated" error from functor/2, which needs a backtrace to figure out the cause.

Because I've fixed the underlying problem, I can't reproduce this error now. Anyway, this particular problem is at

item_commands(List, [], List) :-
    List = [Cmd|_],
    functor(Cmd, \, _),
    arg(1, Cmd, TexCmd),
    tex_command_function(TexCmd, item),
    !.

so the call to functor/3 could have a catch/3 around it and could print out the first few items after the Cmd, I suppose. Without the backtrace, there's no context at all for figuring things out.

So, does it make sense to at least add use_module(library(prolog_stack))?

[JanWielemaker]

I see. It reproduces using this. Gets a nice backtrace with prolog_stack loaded and not without. I'm not sure where to change that though. I kind of dislike the Python approach to always print a backtrace. As a normal user of some app, it tends to be annoying.

:- use_module(library(prolog_stack)).
:- initialization(main, main).

main(_Argv) :-
    a(10).

a(N) :- N > 0, N1 is N-1, a(N1).
a(_) :-
    atom_concat(_,_,_).

You can also get the desired effect after deleting the use_module(library(prolog_stack)) and running

 swipl -g "use_module(library(prolog_stack))" t.pl

Possibly it does make sense for ltx2html to include it by default as it is not an end-user application anyway.

[kamahen]

https://www.swi-prolog.org/pldoc/man?predicate=catch_with_backtrace/3 says:

This is good practice for a catch-all wrapper around an application.

This use of catch_with_backtrace/3 is for situations where you think you've handled all exceptions, to allow debugging the code (the particular bug that I triggered, for example, which gave an uninstantiated variable to functor/3). Anything else can be handled by a regular catch/3.

Presumably, we should also set debugger_write_options and/or answer_write_options so that the backtrace has sufficient context for figuring out what's wrong in the .tex file? (I apologize for not preserving the original file so that we could figure out the real problem - a bug in latex2html.pl)