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

["Robert I. Eachus" <rieachus@comcast.net>]

Marin David Condic wrote:

> Ada ought to have some sort of answer for how to deal with maintaining a 
> single body of code that has to compile for more than one environment. 
> Even if you have the same target hardware and two different compilers 
> (even from the same manufacturer sometimes!) you can have statements 
> that will compile on one but not on the other. Isolation with separate 
> bodies is sometimes difficult to do and always complicates the build 
> process. Some form of conditional compilation would make the job easier.

Technically what we expected way back when was that users would write 
code that depended on the value of System.System_Name, which was made an 
Ada constant for just that reason.  (Compilers could evaluate "if 
System.System_Name = VAX then..." at compile time to eliminate any 
overhead.)

In practice, two things prevented that.  First, vendors didn't list all 
supported systems in package System, usually just the one you were 
compiling for. But more important was that the Ada culture quickly 
became to avoid dependencies on System for any reason whatsoever.

Now, we are where we are:

-----------------------------------------------------------
with Ada.Text_IO; use Ada.Text_IO;
with System; use System;
procedure System_Names is
   package Name_IO is new Enumeration_IO(System.Name);
begin
   Put(" The Available System Names are: ");
    for I in System.Name'Range loop
      Name_IO.Put(I);
      if I = System.Name'Last
      then
        Put_Line(".");
      else
        Put(", ");
        if Col > 60 then New_Line; end if;
      end if;
    end loop;
    New_Line;
    Put(" Current System.System_Name is: ");
    Name_IO.Put(System.System_Name);
    Put_Line(".");
end System_Names;
-----------------------------------------------------------

E:\Ada\Test>system_names
system_names
  The Available System Names are: SYSTEM_NAME_GNAT.

  Current System.System_Name is: SYSTEM_NAME_GNAT.

Is there any compiler that produces a useful output?  Of course, with 
GNAT at least I can modify system and recompile it.  But of course, the 
usual is to do the easier thing, and have some other system dependent 
package defined by the project will all the dependencies depending on 
that.  But that forces projects to do the version control that should 
come from the compiler switches.  (The code generator has to know enough 
about the target to patch up System.  But for that to work the type 
System.Name has to contain a useful set of names.)

Incidently this is the first program I have written in a while that had 
a bug not caught by the compiler.  For some reason Are was capitalized 
in one of the message strings. ;-)

-- 
                                           Robert I. Eachus

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