[WWdevel] Backward-compatible Parser-based answer checkers

[Davide P.C. <dp...@un...>]

Folks:

Over the long weekend, I have been working on a method of making the 
traditional answer checkers (like std_num_cmp() and fun_cmp()) use the 
new Parser, and I have a working version of PGanswermacros.pl that 
implements it.  It turns out not to have been as difficult as I thought 
it might be, and I think I was able to map all the various parameters 
onto corresponding Parser parameters successfully.  Of course, this is 
a pretty major switch, so there will be some problems, I'm sure.

I have created a new subdirectory of the Parser tree that contains the 
needed files.  It is pg/lib/Parser/Legacy, and there is a README file 
there that explains how to install and use it.  You should also update 
your copy of the Parser and Value packages, as there are some changes 
within them that I made while working on this.

Since PGanswermacros.pl is preloaded (unlike most .pl files), this 
means that the change has to be made for the whole server at once.  I 
have arranged, however, for the original answer checkers to remain 
available, and you can switch back on a site-wide, course-wide or 
problem-by-problem basis.  If you turn the new checkers OFF for the 
site, and ON for problem files that you want to test, you should be 
able to try this out even on an active system without causing trouble 
for existing courses.  Again, see the README for details.

All the changes to PGanswermacros.pl are in NUM_CMP and FUNCTION_CMP, 
which have been essentially replaced by functions that set up an 
appropriate Parser context, create a Parser object from the correct 
answer, and return its answer checker.  This effectively turns all the 
traditional answer checkers into ones that use the new Parser in a 
backward compatible way so that existing problem files do not need to 
be modified.  Any answer checkers that rely on the standard ones (like 
the ones in the union_problib) will also be taking advantage of the new 
Parser.  Note, however, that these replacement NUM_CMP() and 
FUNCTION_CMP() do not necessarily store the same values and fields in 
the AnswerHash, so code that takes advantage of private data from a 
checker's AnswerHash may no longer operate correctly.

I have not done a huge amount of testing in existing problem files, but 
I did run through some of the more complicated ones from the Union 
problem library that used nested answer checkers, and they seem to work 
OK.  Let me know if you try this out, and what results you get.  I'm 
sure it is not a finished product.

Davide