Re: AW: "my int( 1..31 ) $var" ?

[Christian Renz]

Now, I might be stupid, but I keep asking myself what you would need a
property for in this example. To me, it totally confuses the
underlying structure. When was the last time you asked an integer to
identify itself as a valid credit card number?
It is _not_ a property of the integer that it is a valid cc number,
rather it happens that it will be accepted as valid _by a certain
authority_. So why not go and ask the authority? Compare the case to a
phone number -- the phone number itself doesn't know if its valid. You
could only check a certain format (if e.g. in the USA, in Germany,
that would be very hard). To check the validity, query a directory
server or ask a phone to dial the number. Don't check the number
itself.

To provide even stronger evidence against using properties, consider
the fact that a credit card number will only be accepted with an
expiration date and -- with good merchants -- the three or four-digit
security code on the back of the card. Now you're up to doing
something like

# funky syntax ahead
my $cc = [ num => "8765 4321", expdate => "0799", code => "123" ];
# do magic
# ...
print "I'm rich!" if $cc.prop{CreditCard("CAMELCARD")};

Ouch! I may be conservative, but again I think you should go and ask
the authority (ie., a validation service). The authority in this case
probably is already encapsulated in a CPAN module and could look like
this:

use CreditCard::Validation;
deduct(10_000_000) if validate($number, $expdate, "PERLIAN EXPRESS");

or something like

use CreditCard::Validation qw(ISA CAMELCARD MONKSCLUB);
deduct(10_000_000) if validate($number, $expdate, $bankcode);

depending on your tastes. Yep, it doesn't use funky perl 6 syntax, but
it SWIMs (Says What I Mean, ie. it is readable).

Greetings,
Christian

--
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"If God were a Kantian, who would not have us till we came to Him from
the purest and best motives, who could be saved?"
-- C.S. Lewis, The Problem of Pain