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Date: Mon, 3 Jun 2019 11:51:31 -0700 (PDT)
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Subject: Re: Why .ads as well as .adb?
From: Lucretia <laguest9000@googlemail.com>
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Xref: reader01.eternal-september.org comp.lang.ada:56444
Date: 2019-06-03T11:51:31-07:00
List-Id: <comp.lang.ada>

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 possib=
ility along these lines:
>=20
>     "You can't generate specification from implementation."

I knew Wirth had done this already, but he missed the point.
=20
> 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 u=
nderstand 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 in=
terface, 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 bla=
h.adb because it's platform specific. In the gpr file, I enable the OS spec=
ific directories. The meh package is similar except it has a whole body whi=
ch is platform specific and therefore separated into the OS specific direct=
ory.

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 :=3D Linux;
end Platforms;

Then inside actual code:

with Platforms;

...

case Platforms.OS is
   when Platforms.Linux =3D>
      -- Linux code.
   when others =3D>
      -- Should be stripped from the final binary.
end case;

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

Luke.