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Postings from May 2000
Several people have developed independent solutions to readonly variables which suggests to me that there is a requirement for them: Andreas pointed out Graham Barr's tie-based solution at http://www.xray.mpe.mpg.de/mailing-lists/modules/1999-02/msg00090.html Mark-Jason Dominus pointed out his tie-based solution at http://www.plover.com/~mjd/perl/Locked/ I pointed out my (old) tie-based solution (on my perl antiques page). The constant pragma doesn't create readonly variables but rather subroutines which can often be used as readonly's, but I generally want real readonly scalars. Tom Christiansen provided XS code for the Const module which can mark scalars readonly. I am not keen on any of the tie solutions because for me constants should be fast and the tie mechanism seems an excessive overhead to get at unchanging values. The XS code that Tom gave appears to do the job, but it is runtime and means that to declare a readonly you have to do it in two steps: use Const ; use vars qw( $MYREADONLY ) ; # Declare const $MYREADONLY = 99 ; # Set r/o My proposed readonly.pm pragma declares & sets in one step: use readonly '$MYREADONLY' => 99 ; and this happens at compile time. Also its pure perl. Is it a big deal doing it in two steps? What if you have a hundred or so readonlys - writing them all out twice (once for use vars, once for the const call) isn't difficult but doesn't appeal to me. I did like the fact that const could be applied at runtime and have updated readonly.pm so that it too can do this when its needed, but it takes 3 steps -- I could have done it in two steps... see the sad conclusion. Another approach would have been to incorporate the Const XS code into the readonly pragma, but that would lead to a different syntax (and would not be pure perl) and the conclusion made it seem pointless: use readonly () ; use vars qw( $MYREADONLY ) ; # Must declare if run-time readonly->set( $MYREADONLY = 99 ) ; My sad conclusion is this: const $READONLY = 99 ; # OR use readonly '$READONLY' => 99 ; *MYREADONLY = \'splat!' ; print $READONLY ; # Yep, prints "splat!" and no warnings Looks like you can't have genuinely readonly scalars in perl unless you use ties... Maybe we should have my $READONLY : readonly = 'Initialised' ; as Barrie Slaymaker suggested after all? Unfortunately I couldn't implement it, I've only just had my first go at XS... _______________________________________________ Mark Summerfield http://www.perlpress.com