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From: hasan@emx.utexas.edu (David A. Hasan)
Newsgroups: comp.lang.ada
Subject: layering with discriminants
Message-ID: <44963@ut-emx.uucp>
Date: 2 Mar 91 00:28:55 GMT
Sender: hasan@ut-emx.uucp
Organization: UTexas Center for Space Research
List-Id: <comp.lang.ada>

In my struggle to understand how to build layered components
in Ada, I have encountered a problem with discriminants.
Consider the following "low level" generic component:


GENERIC
   TYPE size IS (<>);
PACKAGE g_lowLevel IS
   TYPE adt (i : size) IS PRIVATE;
   -- ...other stuff...
PRIVATE
   TYPE adt (i : size) IS RECORD
    -- ...whatever...
   END RECORD;
END g_lowLevel;


Now, add a layer on top as follows:


WITH g_lowLevel;
GENERIC
   TYPE length IS (<>);
PACKAGE g_highLevel IS
   TYPE adt (i : length) IS PRIVATE;
   -- ...other stuff...
PRIVATE
   PACKAGE low_level IS NEW g_lowLevel(length);
   TYPE adt (i : length) IS NEW low_level.adt(i);   ---!!!
    -- ...whatever...
END g_highLevel;


This package might provide a different set of operations & semantics 
for its adt but implement it exactly as the lower level adt.  That's
the motivation behind what I'm doing: avoid implementing the guts of
the adt over.  This also factors the design process nicely.

My problem revolves around the fact that the DEC compiler rejects
the presence of the discriminant in the full declaration of the adt in
the private part of g_highLevel, since the declaration is not a record.
But the declaration is *derived* from a record.

My conclusion is that it is not legal to derive
types with discriminants from parents with discriminants.  
If this is true, is there anyway to achieve an equivalent effect?
--
 :   David A. Hasan
 :   WRW 402 Univ. of Texas at Austin, Austin TX 78712
 :   internet: hasan@emx.cc.utexas.edu