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From: Eric Hughes <eric.eh9@gmail.com>
Newsgroups: comp.lang.ada
Subject: Defect with formal package parameters
Date: Thu, 19 Jun 2008 08:14:15 -0700 (PDT)
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Date: 2008-06-19T08:14:15-07:00
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GNAT 2008 (just installed) fails to compile the following example,
which was cut down from existing code to isolate the problem.  This is
the error message.
> foo.ada-spec:16:34: actual for "Operation" in actual instance does not match formal

Changing the syntax to eliminate "others =>", which is what the
commented-out line does, allows the example to compile without error.
Unfortunately, that's not a viable workaround, because the real code
requires named parameters.

At present I am assuming this is a defect with GNAT 2008 rather than
with the Ada 2005 language definition.  I had the identical problem
with GNAT 2007.  I had stopped development back then, a few months
ago, in the apparently-misplaced hope that a new compiler version
would fix this.

I should point out, for those without significant experience in
generic programming, that this defect completely blocks a huge class
of generic layering techniques that are primarily of interest to
library authors.  Code that does not admit significant reconfiguration
does not require this technique.

Eric
=======================================================
package Foo is
   pragma Elaborate_Body( Foo ) ;

   generic
      with procedure Operation is <> ;
   package Signature is end ;

   procedure Operation_Actual is null ;
   package Impl is new Signature( Operation => Operation_Actual ) ;

   generic
      with package S is new Signature( others => <> ) ;
      -- with package S is new Signature( <> ) ;
   package Module is end ;

   package M is new Module( S => Impl ) ;
end ;
=======================================================