Re: Preprocessor functionality equivalent ideas needed

[Marin David Condic <nobody@noplace.com>]

While I agree with the general concept of separate bodies for separate 
implementation-specific modules, I have occasionally run into situations 
where it would have been nice if I had a compiler directive of some sort 
to go one way or another depending on some condition. This is especially 
true where I might need to keep code portable between two different 
compilers. Also its a big problem when what needs to be different is in 
the *specification* of something. (Yes, I know, "One more layer of 
indirection...." can go fix it - but why should it be so hard?)

Maintaining two sets of files has a problem - its two sets of files! :-) 
You change something in one file & you have to remember to make the 
update to the other one. You want to give the code off to someone else, 
you thus have to supply both sets of files and sufficient information to 
enable them to build it either way. Someone unaware of the parallel 
paths can really mess things up. Some sort of conditional compilation 
can simplify matters. (The existence of pragma Assert and pragma Debug 
in the Gnat compiler attest to at least *some* usefulness to conditional 
compilation.)

I have sometimes been able to simulate conditional compilation by 
putting code into an if statement with a constant flag. Some compilers 
will optimize away the unreachable code. But that doesn't help much in 
declarative parts or in situations where a set of statements is 
acceptable to one compiler, but not another. I'd hate to see Ada develop 
into the unholy mess that C is with all of its macros and other 
confusing garbage, but maybe a couple of pragmas that let alternate 
versions of the code exist within a single file might not be a bad thing?

MDC

Jeffrey Carter wrote:
> 
> 
> Generally, you define a package specification for the functionality you 
> require, then implement different bodies for it for the different 
> situations.
> 


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Marin David Condic
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