Re: Standard Ada Preprocessor (Was: why ada is so unpopular ?)

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

That might be an answer. It at least goes a long way towards alleviating 
most of the problems. I'd have some concerns about mentioning actual 
filenames in the pragma because that can be incredibly system dependent 
and all you have to do is move some directory tree and the whole thing 
collapses. I've seen that problem show up in C and would prefer not to 
have it show up in any Ada code.

If, OTOH, it let me use some kind of symbolic name in much the same way 
as I already do with a package name, then you can at least figure that 
each compiler or platform has some mechanism for finding a body already 
and now it just does so with a different name. (On Gnat, it looks for an 
appropriately named file. On Aonix, it looks in its compilation database 
for the right unit to associate. On ??? - it uses whatever means it has 
natively.)

Maybe its something like this:

with Some_Package ;
if (some_condition) then
     for Some_Package'Body use GNAT_Some_Package ;
else
     for Some_Package'Body use AONIX_Some_Package ;
end if ;

I'm not saying it would have to be this syntax or that a pragma wouldn't 
be better. Just saying that I want the compiler to apply its normal 
interpretation rules to find "GNAT_Some_Package" and pretend that its 
name is "Some_Package".

Something like that might work.

MDC

Jacob Sparre Andersen wrote:
> 
> Would a pragma in the package specification declaring which file the
> package body should be read from depending on some compilation
> parameters (compiler, operating system, etc.) do the job?
> 
> Something like this, where the body is grabbed from the file named by
> the first "Body_Source_File" pragma with a matching condition.  If no
> "Body_Source_File" pragma has a matching condition, the compiler just
> uses its default routine for finding the source code for the package
> body.
> 
> with System;
> with System.Compiler_Flags;
> 
> package Hiding_Compiler_Specific_Details is
>    pragma Body_Source_File
>      (Condition => System.Compiler_Name = "GNAT",
>       File      => "GNAT/HCSD.ada_body");
>    pragma Body_Source_File
>      (Condition => System.Compiler_Flags.Set ("--with-gnu-gettext"),
>       File      => "GNU-Gettext/HCSD.ada_body");
>    pragma Body_Source_File
>      (Condition => True,
>       File      => "default_implementations/HCSD.ada_body");
> 
>    ...
> end Hiding_Compiler_Specific_Details;
> 
> This would limit the language handling of conditional compilation to
> selecting package/unit bodies, but I think it would solve most of the
> problems mentioned in this discussion, without risking the whole
> preprocessing hell seen in too many C programs.
> 
> Greetings,
> 
> Jacob


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Marin David Condic
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