Spirit: addition of new productions

From: joel de guzman (isis-tech_at_[hidden])
Date: 2001-06-11 20:57:34

Hi,

Some BNF variants allow: 

r ::= a
r ::= b

Which is equivalent to:

r ::= a | b

Douglas Gregor noted that allowing the addition of new 
productions would be extremely useful to be able to, for 
instance, turn on/off extensions to a language at the parser 
level. Example:

if (allow_typeof)
    typespec = TYPEOF >> '(' >> expr >> ')'
             | TYPEOF >> '(' >> type_id >> ')';

if (allow_template_typedefs)
    type_decl = template_header typedef;

Essentially, the declarative nature of (E)BNF productions 
when inlined with imperative C++ statements yield an 
uncanny mix. The C++ assignment operator substituting 
as BNF's ::= operator when applied to the addition of 
new productions might cause confusion. 

The |= operator, as suggested by George Heintzelman, 
may used for this purpose. Yet, this too might cause 
confusion as one might expect &=, -= and ^= to be 
equally applicable.

Also, after some deliberation, both cases impose runtime 
and code size penalty. Thus, I've come up with a very 
simple solution:

Undefined Rules:

An undefined rule matches nothing. This can be used to 
turn on/off extensions to a language at the parser level. 
For example:

r = feature_a | feature_b;
if (allow_feature_b)
    feature_b = /*...define feature_b...*/;

If the flag allow_feature_b is false, feature_b will be left 
undefined and will match nothing when parsing proceeds. 
So, the rule r will only have only one alternative.

Comments?
-Joel de Guzman