On Thu, Aug 18, 2005 at 12:02:53AM +0800, Autrijus Tang wrote:
: 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.
Though I think most people have a pretty good understanding of "worm"
these days, what with CD and DVD burners...
"Darn, I turned my third parameter into a coaster. Now what am I gonna do?"
: > 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.
English has a long history of hoisting the adjectives from the object
to its container. "I'd like a nice cup of tea" and such. So in general
I think people will expect a readonly variable to contain a readonly
value, and maybe we should do the extra work to enforce that transitively.
That being said, there's also a history of rebinding references, as
in "That woman is a man!" But we intuitively recognize that there's
something funny going on there linguistically. We can deal with it
as humans, but maybe we can allow the soulless computer to throw a
exception on such contradiction instead of laughing like we do.
That being said, it's still quite possible that
my $pi is readonly;
realizes that it's not bound to anything reasonable yet, and set
up a "worm" container of some sort that binds lazily at run time.
As with parameter defaults, it's more about whether the container's
value exists than whether it's defined.
Larry
