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From: Simon Wright <simon@pushface.org>
Newsgroups: comp.lang.ada
Subject: Re: Gnat Problem - Freezing too soon
Date: Sat, 02 Mar 2019 21:35:51 +0000
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Date: 2019-03-02T21:35:51+00:00
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russ lyttle <lyttlec@removegmail.com> writes:

> I meant why it solved this particular problem. As far as I can tell,
> the authors never used the "not null" in any of their books. Would
> other compilers accept the code as written, or should the code be
> considered erroneous?

'not null' is clearly better style.

With regard to

   task type Sporadic (S : Any_Sporadic_State;
		       A : Any_Sporadic_Thread_Interface)
   with Priority => S.pri
   is

GNAT CE 2018 fails, GNAT GPL 2017 & earlier & GCC 9 succeed. You can see
that making the type 'not null' would eliminate a check (check would be
done at the call site instead), so that the internal tree generated by
the compiler would be different, and the compiler may have been changed
in this region for other reasons.

With regard to

   protected type Sporadic_Agent (S: Any_Sporadic_State)
   with Priority => S.Ceiling_Prioriy
   is

all the compilers are seriously confused:

   sporadics.ada:30:22: no selector "Ceiling_Prioriy" for type "Sporadic_State'Class" defined at line 17
   sporadics.ada:30:23: possible misspelling of "Ceiling_Priority"
   sporadics.ada:30:23: possible misspelling of "Ceiling_Priority"
   sporadics.ada:30:23: possible misspelling of "Ceiling_Priority"

and you only have to look at the task equivalent above to see that it
should have been OK. This one looks like another compiler error to me.

> As Simon pointed out replacing "with Priority => S.pri" with "pragma
> Priority(S.pri)" also works. They never used that form either.
>
> Why would the authors prefer one form over the others?

The aspect is the newer form; the book was published in 2016, so they'd
have gone with that.