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From: Georg Bauhaus <sb463ba@l1-hrz.uni-duisburg.de>
Newsgroups: comp.lang.ada
Subject: Re: limited types in libraries
Date: Sat, 13 Dec 2003 12:43:34 +0000 (UTC)
Organization: GMUGHDU
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ada_wizard@toadmail.com wrote:
 
: Hmm. I can't see why it would ever be ok to implement "=" and "<" in a
: non-consistent way. Certainly any subprograms provided by the user
: must meet the container's requirements.

Yes, but there might be different requirements in other parts
of a program, which let the programmer _think_ that "=" and "<"
are consistent.
A database's records of persons' addresses might be sorted by
surname, for example. Suppose name(r) returns a Wide_String or
similar for the surname in a record. Then one record is sorted
before another by

  function "<" (k1, k2: DB_Record) return Boolean is
  begin
     return name(k1) < name(k2);
  end "<";

(and similarly  for ">"). However, two records indicate the same person
if they show the same name and phone number,

  function "=" (k1, k2: DB_Record) return Boolean is
  begin
     return name(k1) = name(k2) and phone(k1) = phone(k2);
  end "=";

Then it might be that for j /= 0,
    not dbrec(n) < dbrec(n + j)
and not dbrec(n) > dbrec(n + j)
but still
  not dbrec(n) = dbrec(n + j)
and this might make sense from the point of view of database records.
So I might have to change my point of view so that I can see the
requirements of the containers or sorting related algorithms, and
understand, in some details, when "=" and "<" will come into play.

(But I might just be confused.)

-- Georg