I think this debate is easier if you think of defaults as overriding and
auto-vivification method on a container.
On Tue, 2003-01-28 at 14:47, Paul Johnson wrote:
> Michael Lazzaro said:
> > 2a) When a cell is explicitly re-undefined, does the default value take
> > effect?
> >
> > my @a is Array( default => 'foo' ) = (1,2,3);
> >
> > @a[1] = undef;
> > @a[1]; # undef, or 'foo'?
> >
> > STRAWMAN ANSWER: 'foo'.
>
> Seems right to me. Anything else would be very confusing, I think.
This would be very confusing to me. In fact, it seems WRONG to me unless
we're saying that such arrays can simply never hold an undefined
value... which again seems wrong.
my @a is Array ( default => 'foo' );
@a[1] = undef;
Should yield the following, I would think:
('foo', undef, 'foo' x Inf)
Though, obviously there's no such thing as @a[2], but if you reference
it, that's how it would auto-vivify.
If that's not the case, I need to get my head around why, since Perl
*does* distinguish between defined and exists.
> > 2b) Primitive-typed arrays: Given the behavior of (2a), and the fact
> > that primitive-typed arrays can't store undef, what happens here?
> >
> > my int @a is Array( default => 5 );
> >
> > @a[0] = 0;
> > @a[0]; # 0, or 5?
>
> 0. Being unable to store 0 would seem to be a major limitation.
But of course.
> > @a[0] = undef;
> > @a[0]; # 0, or 5?
>
> An exception or 5.
The first assignment would attempt to convert undef to type int. That's
either an exception or zero, no? The "is Array" is a constraint on the
container, and should not be entering into the conversion at hand until
you try to STORE the result.
> Maybe undefining an element could always set it to the default value.
>
> > 3) Can the default value be a closure, based on index location?
> >
> > my @a is Array( default => { $_ ** 2 });
> >
> > STRAWMAN ANSWER: Yes, because it's cool.
>
> No, because it's unnecessary. You can always do
>
> my $value = @a[$x] //= $x ** 2;
Again, different. You're looking for something like C<ifexists=>, not
C<//=>
--
Aaron Sherman <[EMAIL PROTECTED]>
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