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From: smize@imagin.net (Samuel Mize)
Subject: Re: private and limited private
Date: 1996/10/17
Message-ID: <DzEI6q.11tK@imagin.net>#1/1
X-Deja-AN: 189975989
references: <53dfjn$1u@rc1.vub.ac.be> <Dz3G8o.pEo@imagin.net>
 <53vmmb$560@saturn.brighton.ac.uk>
organization: ImagiNet Communications Ltd, Arlington, Texas
newsgroups: comp.lang.ada
Date: 1996-10-17T00:00:00+00:00
List-Id: <comp.lang.ada>


In article <53vmmb$560@saturn.brighton.ac.uk>,
John English <je@bton.ac.uk> wrote:
>Samuel Mize (smize@imagin.net) wrote:
>: [... snip ...]
>
>: However, you can still copy it, and use the built-in equality test.
>: This is generally OK.  For instance, if you are representing playing
>: cards, the type might be a record with two fields, one for suit and
>: one for value.  It makes sense to be able to copy a card value (e.g.,
>: for the program to remember what card was previously dealt), and to
>: compare two values to see if they refer to the same card.
>
>Actually, playing cards would be better represented by a limited type
>with a Move (as opposed to a Copy) primitive, since in the real world
>you move cards from A to B, you don't *copy* them.

But you can remember which cards have been played* (that's why casinos
use several decks at the Blackjack tables)**.  That's hard to represent
with a limited type -- your "player" program can't make a "mental copy"
of the values it's seen.

>If you allow
>assignment, you allow the creation of as many copies of the Ace of
>Spades (or whatever) that you like.  From watching Westerns, I gather
>that most people frown on this and fill you with lead/break tables
>over your head/whatever if you're caught doing it.

Proof by existence that multiple copies of the same value are possible.

* People do this at Gin Rummy and Bridge, for instance.

** It's called card counting.  It's described in various books, and there's
   probably a blackjack newsgroup.  The move "Rain Man" describes it,
   although the details are wildly inaccurate.

Samuel Mize