I do think that Traits are likely to be the solution, and I was hoping that one already existed ... especially since I see a “MooseX::TypeMap” that looks very interesting indeed but that is not quite what I want.

The motivation for my quest is that these status-codes are already present in a large database and already being used in the crufty code that my team is re-engineering.   There is, therefore, a lot of code that tests these values with eq, and we have already encountered several places in which that isolated logic is outright wrong.

So, obviously, I want to consolidate into one Moose-goodness place:  

• The value of status can be A, B, C, or D; never anything else.
• The meaning of status='B' is (whatever).
• If you want to assert that “such-and-such is now The Truth,” the way that this is represented is by changing status to 'D', but “nobody in the system except this object has to know that, or to care.”

And of course, “laziness is a virtue.”

What do I want?

has 'status' => (
  traits => ['plugh'],   # IF THIS IS NEEDED FOR THE MAGICK ...
  may_be_undef => 0,
  values => {
    'status_alive' => 'A',
  '  status_dead'  => 'D',
  },
  handles => {           # OR MAYBE SOME OTHER SUGAR-WORD ...
    'is_alive'   => 'status_alive',
    'is_dead'    => 'status_dead',
  },
);
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And my obvious thought is ... hasn’t this already been done by somebody else?   The assumption that surely somebody has, and that I just haven’t discovered it yet, is rather compelling.