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From: tmoran@acm.org
Newsgroups: comp.lang.ada
Subject: Re: [OT] Best way to isolate a GUI?
References: <b32j2a$9ie$1@slb3.atl.mindspring.net>
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>Oh, there may be cases here and there. And its always easy to construct an
>artificial scenario where variant records look better. Just that in
>practice, the places where I've used variant records have usually turned out
>to be more elegantly and less painfully dealt with by tagged records.
  Not two weeks ago I had to write code to watch data going by and
catch certain patterns, essentially a simple kind of parser.  Each pattern
required somewhat different information to be accumulated, so
  type pattern_kinds is (none, a,b,c);
  type states(kind : pattern_kinds := none) is record
    case kind is
      when none => null;
      when a    => -- data accumulators for pattern type a
      when b    => -- data accumulators for pattern type b
      etc
  current_state : states(kind=>none);
did the job nicely.  To change state takes a simple assignment, not any
freeing or allocating of classwide pointers.  The assignment has to be a
whole record assignment, so there's no chance of forgetting to initialize
some part of the data for pattern x.  If a new pattern is added it needs
no modifications to the code for existing patterns.  If new data fields
are added for pattern x, the compiler will flag any assignment that
fails to include the new field in the aggregate.  An attempt to use
a field of pattern "a" while "kind = b", will be caught.
  How would you do this with tagged records?