[antlr-interest] Re: Preprocessors - academic question

[John Allen Green]

Exactly.

Er, dunno what to add to that... :-)

John


--On 28/06/2002 6:38 AM -0700 mzukowski at yci.com wrote:

> What is the problem you are trying to solve?  Do you need to make changes
> that propagate back into unpreprocessed code?
> 
> Monty
> 
>> -----Original Message-----
>> From: John Allen Green [mailto:greenj at ix.netcom.com]
>> Sent: Thursday, June 27, 2002 3:15 PM
>> To: antlr-interest at yahoogroups.com
>> Subject: Re: [antlr-interest] Re: Preprocessors - academic question
>> 
>> 
>> 
>> --On 27/06/2002 10:43 AM +0200 John Allen Green wrote:
>> 
>> >>> Given a preprocessor which allows branching and unrestricted code
>> >>> substitutions, is it impossible to work with the semantics of the
>> >>> preprocessing itself?
>> >>> 
>> >>> It would seem to me that since you could have code substitution 
>> >> doing code
>> >>> substitution, it would be impossible to build a parse tree of the
>> >>> preprocessing.
>> 
>> 
>> I think I've finally figured out what's been bothering me. Here's some
>> valid Progress code:
>> 
>> &GLOBAL-DEFINE XX &GLOBAL-DEFINE HI "Hello world!"
>> {&XX}
>> DISPLAY {&HI}.
>> 
>> Of course, C doesn't let you get away with the above 
>> nonsense. What I think
>> the above shows is that we can't build a complete graph of the
>> preprocessing without actually evaluating the preprocessing 
>> expressions and
>> expansions.
>> 
>> Here's an expression that would have to be evaluated:
>> 
>> &IF OPSYS = "VMS" &THEN
>>   &GLOBAL-DEFINE OSINCLUDE include/vms.i
>> &ENDIF
>> {{&OSINCLUDE}}
>> 
>> We'd have to evaluate for both true and false on OPSYS = 
>> "VMS". If we never
>> evaluated for true, we'd never evaluate the text in the 
>> include file, and
>> our graph would be incomplete.
>> 
>> If we have to evaluate for every possible combination of 
>> conditions, then
>> for some compile units we might be talking about an 
>> astronomical number of
>> possible states, and graphing becomes impractical.
>> 
>> A friend suggested that this sort of graphing problem is 
>> similar to other
>> problems as well, and mentioned NP Complete. I've no 
>> education for this
>> stuff, so I'll have to hit the books for this one.
>> 
>> Cheers,
>> John
>> 
>> 
>>  
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