Yeah, you're thinking along the same lines as I was. Unfortunately, there
needs to be consensus about whether this behavior should be changed before
it can be implemented -- any idea how to make that come about?
Deven
On Sun, 21 Jan 2001, David L. Nicol wrote:
> "Deven T. Corzine" wrote:
> >
> > On Sat, 23 Dec 2000, Graham Barr wrote:
> >
> > > This has been discussed on p5p many many times. And many times
> > > I have agreed with what you wrote. However one thing you did not mention,
> > > but does need to be considered is
> > >
> > > func($x{1}{2}{3})
> > >
> > > at this point you do not know if this is a read or write access as
> > > the sub could do $_[0] = 'fred'. If this can be handled in someway
> > > so that the autoviv does not happen until the assignment then OK,
> > > but otherwise I think we would have to stick with perl5 semantics
> >
> > This is a good point. Similar arguments apply to grep, map, foreach, etc.
> > Could we have some sort of "lazy evaluation" mode for lvalues where a
> > reference to an undefined substructure sets a flag and saves the expression
> > for the lvalue, returning undef whenever evaluated in a read-only context
> > and autovivifying when necessary to write to the lvalue?
> >
> > Deven
>
>
> Lazy autovivification could work by adding an on-assignment magic to
> the previously nonexistent SV, which details the creation of the intervening
> structures.
>
> This will have to chain: What if &func defers the question
> of read or write to another layer down? An addition of a magic, so that
> values in previously undefined structures evaluate as undef but will create
> interveners as needed if and when they are assigned to (even if it is an
> assignment of undef) would work.
>
> The magic would have to consult the appropriate symbol tables when it runs,
> to avoid problems of two magics getting set up referring to the same nonexistent
> structure:
>
>
> $first = \($$$hash{one}{two}{three}); # a reference to an undefined
> # scalar, with write-magic
>
> $second = \($$$hash{one}{two}{four}); # undef with very similar write-magic
>
> $$first = "first";
> $$second = 2; # magic runs but doesn't do anything since
> # %{$$hash{one}{two}} got vivified already.
>
>
> --
> David Nicol 816.235.1187 [EMAIL PROTECTED]
> "Live fast, die young, and leave a beautiful corpse"
>
