[WWdevel] Predefined constants like NONE and DNE

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

Folks:

When I wrote the Parser code, one of the things I was after was 
consistency in terms of error messages.  Since the same parser was 
being used for ALL types of answers, it would know about the other 
types of objects no matter what type of answer you were looking for.  
For example, if you are checking for a numeric answer, the Parser still 
will give appropriate error messages even when the student types an 
interval: it will say something to the effect that you typed an 
interval but that it expected a number.  Since you have the ability to 
change the Context, however, that somewhat undermines this, since you 
can remove its knowledge of some types of objects (e.g., the Numeric 
class does not know about points).

When I developed the Legacy module for the Parser, which makes num_cmp 
and fun_cmp use the Parser answer checkers, I implemented contexts that 
mirrored the traditional num_cmp and fun_cmp environments as closely as 
I could.  This is the right thing to do for backward compatibility, but 
I'm not sure it is the best idea for the future.  For example, all the 
default Parser contexts include definitions for NONE, INF, INFINITY, 
-INF, -INFINITY, and DNE, as common strings that might be used in other 
parts of the problem, so the Parser should recognize these and respond 
appropriately when the student enters them when some other type of 
answer is expected (usually they are marked incorrect silently).  I 
took these out of the num_cmp and fun_cmp contexts, however, so if a 
student types NONE in an answer blank for num_cmp, he or she will get 
an error message instead (but it will at least be "'NONE' has no 
meaning in this context" rather than a message about an undefined 
variable 'N').

I think that in the long run, NONE, DNE, and the infinities should all 
be defined all the time.  Do you think these should be added back into 
the Parser-based versions of num_cmp and fun_cmp?

Davide