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From: Pierre Van Aswegen <pvana@iossvr.gm.hac.com>
Subject: New language feature to check record coverage?
Date: 1997/02/25
Message-ID: <33139C04.57DB@iossvr.gm.hac.com>#1/1
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Organization: MacDonald Dettwiler
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Newsgroups: comp.lang.ada
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Date: 1997-02-25T00:00:00+00:00
List-Id: <comp.lang.ada>


I would like those who collect ideas for the evolution of the language
to tell me what they think of the following idea.  It is unprecedented
in any language I know, but suggested slightly by combining the idea
that the compiler checks that you have an entry for each value in a case
statement and the use of record aggregate assigment to ensure you have a
value for each member of the aggregate.

The need I perceive stems from writing a validation function to certify 
that the values in a record are consistent.  If I change the definition
of the record (or miss a member in the original function), the compiler
is not going to remind me to update the validation function.  (As it
would remind me to fix case statements if I changed an enumeration or to
fix an aggregate assignment of said changed record.)  I have encountered
other situations where I've wished to have some guarantee that I was
addressing every part of a record.

What one needs is a kind of iterator for which the user must supply an
operation for each member of a record.  (Some members can share an
operation.)  A reasonably understandable syntax can be derived from
either case or aggregation.  

type Some_Record is
  record
    A_Member : MemberA;
    B_Member : MemberB;
    ...
  end record;

function Is_Consistent return Boolean is
  Answer : Boolean := True;
begin
  case Members'Some_Record    --* 
    when A_Member => ...;     --*
    when B_Member => ...;     --*
    ...
  end case;
  return Answer;
end Is_Consistent;

function Is_Consistent return Boolean is
  Answer : Boolean := True;
begin
  Members'Some_Record        --*
    (A_Member => ...,        --*
    B_Member => ...,         --* 
    ...)
  return Answer;
end Is_Consistent;
                            *"fantasy Ada"