a nifty feature for c preprocessor

[David Brown]

On 29/12/11 22:05, R A wrote:
>
>> The gcc developers, and everyone else involved in the development
>> of C as a language, are perhaps not superhuman - but I suspect
>> their combined knowledge, experience and programming ability
>> outweighs yours.
>
> given. but do you have a consensus of the community that this feature
> is not worth including? i haven't even heard but from a few people
> saying that "it's not worth it because if it was, 'we're the ones to
> have thought about it'".
>
> computing science (may i call it a science??), is all about
> objectivity and examining the theories and evidence. it should be
> prepared re-examine everything when a new idea is introduced or
> challenged.
>
> so if it's a bad idea, explain to me exactly why; not go about human
> politics.
>

I don't speak for the gcc community - either for the users or the 
developers.  But occasionally I can help out on this mailing list, and 
let the real developers get on with their work.

There are a number of reasons why you wouldn't want an evaluation system 
as part of the C preprocessor (disregarding any thoughts about it being 
hard to do, impossible to standardise, and redundant given all the other 
ways to achieve the same ends).

There is no standard environment for C.  Should calculations be done as 
32-bit integer?  What about targets that use 16-bit integers (or even 
64-bit integers)?  You want to call the function "eval" - what about 
existing code that uses the word "eval", either as a pre-processor macro 
or as part of the C code?  You want to allow recursive macros - that 
opens a wide range of problems for getting consistency and clear 
definitions (and typically requires being able to define macros in at 
least two different ways - one recursive, one literal).  You want to 
allow the evaluation of symbols and expressions by the preprocessor - 
that means it has to understand the syntax and semantics of these 
expressions, rather than just doing simple text manipulation.  And it 
leads to more questions when macros and expressions are mixed - when do 
you do "evaluation", and when do you do simple text expansion?  How do 
you write macros that have the "eval" function in them?

I don't expect or want any answers here - I don't think anybody is 
looking for a discussion aiming to implement the eval idea.  I just want 
to point out that there are many, many questions involved here before 
anyone could even think about implementing it.  And I don't think they 
could be resolved in a way that is consistent, practical, and adds 
anything to C or the C pre-processor that is not better handled by an 
external code generator or an independent macro language.  No one wants 
to re-invent the wheel.