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Date: 11 Sep 91 18:43:18 GMT
From: eachus@mitre-bedford.arpa  (Robert I. Eachus)
Subject: Re: why are these unknown identifiers?
Message-ID: <EACHUS.91Sep11114318@Dr_No.mitre.org>
List-Id: <comp.lang.ada>

In article <9109101616.aa26718@PARIS.ICS.UCI.EDU> jduarte@liege.ICS.UCI.EDU (Jo
se Duarte) writes:

   -----------------------------------------
   package X is

	   type DIRECTIONS is (UP,DOWN,LEFT,RIGHT);

   end X;
   -----------------------------------------
   with X;
   package Y is

	   subtype DIRECTIONS is X.DIRECTIONS;

   end Y;
   -----------------------------------------
   with TEXT_IO;
   use  TEXT_IO;
   with Y;
   procedure BUG is

   V1 : Y.Directions := Y.UP;    -- Y.UP is unknown
   V2 : Y.Directions := Y.DOWN;  -- Y.DOWN is unknown

   begin
     null;
   end BUG;

   Can someone tell me why "Y.UP" and "Y.DOWN" are unknown identifiers
   within the procedure BUG? I have to "with X" and then assign X.UP
   and X.DOWN to V1 and V2 in order to compile this. Why is this the case?
   Is this an Ada pecularity or a compiler bug?

-- You could say it is an Ada pecularity, because Ada is the only
-- language with all the features necessary to run into it.  A type
-- declaration derives operations for the parent type, but a subtype
-- does not.  Therefore there is no function UP return DIRECTIONS in
-- package Y.  If you want to be able to refer to Y.UP you should say
-- either:

   with X;
   package Y is 

     type DIRECTIONS is new X.DIRECTIONS;
     -- derived functions declared here.

   end Y;

-- or:

   with X;
   package Y is

     subtype DIRECTIONS is X.DIRECTIONS;
     function UP return DIRECTIONS renames X.UP;
     function DOWN return DIRECTIONS renames X.DOWN;
     function LEFT return DIRECTIONS renames X.LEFT;
     function RIGHT return DIRECTIONS renames X.RIGHT;

   end Y;

-- The first approach will require explicit conversions when trying to
-- assign values of type X.DIRECTIONS to variables of type
-- Y.DIRECTIONS and vice-versa.  (When designing Ada, this was thought
-- to be a GOOD thing.)  In the second case cross assignments are not
-- a problem but saying use X; use Y; will result in neither
-- declaration of UP being directly visible.

					Have fun,

					Robert I. Eachus

with STANDARD_DISCLAIMER;
use  STANDARD_DISCLAIMER;
function MESSAGE (TEXT: in CLEVER_IDEAS) return BETTER_IDEAS is...
--

					Robert I. Eachus

with STANDARD_DISCLAIMER;
use  STANDARD_DISCLAIMER;
function MESSAGE (TEXT: in CLEVER_IDEAS) return BETTER_IDEAS is...