On Wed, Aug 17, 2005 at 08:47:18AM -0700, Larry Wall wrote: > : >That could be made to work by defining constant to mean you can assign > : >to it if it's undefined. But then it gets a little harder to reason > : >about it if $pi can later become undefined. I suppose we could > : >disallow undefine($pi) though.
If you can assign it when it contains an undefined value, bad
things happen:
sub f ($x is readonly) { $x = 10 }
my $a; f($a);
Compare this with:
my $x is readonly;
$x = 10;
If it is defined as "bound to a immutable value cell", both cases
above would fail, which is imho the easiest to explain.
> You could still reason about it if you can determine what the initial
> value is going to be. But certainly that's not a guarantee, which
> is one of the reasons we're now calling this write/bind-once behavior
> "readonly" and moving true constants to a separate declarator:
>
> my $pi is readonly;
> $pi = 3;
The question remains, whether you can bind the readonliness away:
my $pi is readonly; # undef at this point
my $e is rw = 2.7;
$pi := $e;
$pi = 9;
I can argue both sides -- rebindable is easier to implement, but
non-rebindable is perhaps more intuitive.
Thanks,
/Autrijus/
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