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From: Jeffrey Carter <spam.jrcarter.not@spam.not.acm.org>
Newsgroups: comp.lang.ada
Subject: Re: Can .ads be compiled alone?
Date: Thu, 06 Nov 2014 23:09:03 -0700
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Date: 2014-11-06T23:09:03-07:00
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On 11/06/2014 10:21 PM, moixa wrote:
> 
> I have only two files, machinery_1_3.ads [1] and machinery_1_3.adb [2], in
> some directory, when I `gnat compile machinery_1_3.ads`, it told me compile
> error, (but no further information).
> 
> A stackoverflow post[3] said that it's a good practice to separate compile
> the spec and the body, so, why I cannot compile my .ads?

This is an idiosyncrasy of GNAT's source-based compilation model. When you
compile something that withs a package, GNAT also looks at the source file for
the pkg spec. So GNAT normally only compiles a pkg body; typically the message
when one tries to compile a spec that has a body is something like "cannot
produce object code for a package specification" (I haven't tried it recently).
You can check a pkg spec that has a body with -gnatc, and compile a spec that
doesn't have a body, or compile a spec along with its body by compiling the body.

Other compilers have other ways of dealing with this, and require that a pkg
spec be compiled before you can compile something that withs it.

-- 
Jeff Carter
"In the frozen land of Nador they were forced to
eat Robin's minstrels, and there was much rejoicing."
Monty Python & the Holy Grail
70