Re: Why .ads as well as .adb?

[Lucretia <laguest9000@googlemail.com>]

On Monday, 3 June 2019 14:37:46 UTC+1, John Perry  wrote:

> So I was surprised that some people made absolute negations of the possibility along these lines:
> 
>     "You can't generate specification from implementation."

I knew Wirth had done this already, but he missed the point.
 
> I especially wonder this since one of them had just referred to gnatchop, whose online documentation provides an example that does precisely that [1]. So, when I read statements like the ones above, I wonder (a) if people understand that I don't mean Ada-as-is, but a hypothetically modified Ada (I am not proposing such a modification Ada; again, it's just curiosity); and (b) if I misunderstand what people mean by "specification".

In Ada, I've written code which is organised like this:

src/blah.ads
src/blah.adb
src/linux/blah-separate.adb
src/windows/blah-separate.adb
src/meh.ads
src/linux/meh.adb
src/windows/meh.adb
etc.

blah.ads is the interface to that package, it is what every OS sees, one interface, that's it. blah.adb is the body where it's portable. separate.adb contains a "separate" which is a subprogram which is separated from the blah.adb because it's platform specific. In the gpr file, I enable the OS specific directories. The meh package is similar except it has a whole body which is platform specific and therefore separated into the OS specific directory.

You can, if you want, create constants for the OS as well somewhere, i.e.

Inside src/oses.ads:

package OSes
   type OS is (Linux, Windows, etc.);
end OSes;

Inside src/linux/platforms.ads:

-- Similar for Windows.

with OSes;

package Platforms
   OS : constant OSes.OS := Linux;
end Platforms;

Then inside actual code:

with Platforms;

...

case Platforms.OS is
   when Platforms.Linux =>
      -- Linux code.
   when others =>
      -- Should be stripped from the final binary.
end case;

This method gives you compile time code stripping due to a static type system and the constant OS, platform specific code in a directory and using stubs in platform specific directories. It's very powerful and a shitload less messy than C and C++ methods of makefiles and preprocessor.

Luke.