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From: "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>
Newsgroups: comp.lang.ada
Subject: Re: Why .ads as well as .adb?
Date: Sun, 2 Jun 2019 08:39:23 +0200
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Xref: reader01.eternal-september.org comp.lang.ada:56433
Date: 2019-06-02T08:39:23+02:00
List-Id: <comp.lang.ada>

On 2019-06-02 02:48, John Perry wrote:

> I understand that Ada, like Modula-2 and Modula-3, and arguably like C++, requires a definition file (.ads) as well as an implementation file (.adb). With Oberon, Wirth moved away from definition files, using a symbol to indicate which module identifiers should be exported. (Someone else may have done this before him; it's just that I'm most familiar with this history.) Most languages I'm familiar with these days do something similar, either via public/private or some other mechanism.

It is general design principle of separation specifications from 
implementations. Are you arguing against the principle or against 
keeping these things separately. The latter has evident advantages for 
code base maintenance, team development, testing, separate compilation 
etc. BTW, you can stuff bodies and specifications in the same file. It 
is purely compiler's business. See gnatchop for GNAT. So I consider it 
is the former you are against.

> As far as I can tell, though, Ada has stuck with the two separate files, rather than, say, generating an .ads from an .adb with export markup.

That is not possible. You cannot generate specification from 
implementation and conversely. In both cases there is additional 
information missing. It could be two different languages. Even in the 
languages that confuse these things, declarations have syntax different 
from definitions. Compare also Ada's declarative regions with C++'s 
ad-hoc declarations:

    declare
       X : Integer;
    begin
       ... -- Some code
       X := 1;
    end;

    {
       ... // Some code
       int X;
       X = 1;
    }

> Is there a reason Ada hasn't moved to this simpler structure?

See J-P's post. BTW when writing OS-dependent code it is very convenient 
to have one *.ads file and several *.adb files. It ensures that all 
client code is OS-independent, since it sees only the *.ads. And, in the 
tool-chain you just switch to another *.adb when changing the target. 
Compare that with C++'s #ifdef infestations.

-- 
Regards,
Dmitry A. Kazakov
http://www.dmitry-kazakov.de