Re: PerlHash.get_pmc_keyed of non existing key
Taking a thread from Perl 6 Internals. Will Perl 6 support this behavior? $ perl <<'EOT' my @ary; my $ref = \$ary[0]; $$ref = "value"; print '$ary[0] : ', $ary[0], "\n"; EOT $ary[0] : value Presumably the Perl 6 would be: my @ary; my $ref = [EMAIL PROTECTED]; $$ref = "value"; print '@ary[0] : ', @ary[0], "\n"; # -> @ary[0] : value If that's supported, then how will this behave? my @cows of Cow; my $slot = [EMAIL PROTECTED]; $$slot = new Dog; Do we wind up with Clarus or an exception? (Never mind whether you take exception with Clarus.) Hopefully an exception, from a user's POV. And this? my @cows of Cow: my @cats of Cat; my $ref = [EMAIL PROTECTED]; @cats[0] := $$ref; @cats[1] := @cows[0]; # Just to hammer the point home $$ref = new Dog; But then there's a question for p6i as to how all the above happens. — Gordon Henriksen [EMAIL PROTECTED]
Re: PerlHash.get_pmc_keyed of non existing key
Gordon Henriksen writes: > Taking a thread from Perl 6 Internals. Will Perl 6 support this behavior? > > $ perl <<'EOT' > my @ary; > my $ref = \$ary[0]; > $$ref = "value"; > print '$ary[0] : ', $ary[0], "\n"; > EOT > $ary[0] : value > > Presumably the Perl 6 would be: > > my @ary; > my $ref = [EMAIL PROTECTED]; > $$ref = "value"; > print '@ary[0] : ', @ary[0], "\n"; # -> @ary[0] : value That has to do with autovivification semantics. Particularly, do things autovivify when you take a nonconstant reference to them. I think they should, and that wasn't one of those cases where autovivi was broken, so it probably will. > If that's supported, then how will this behave? > > my @cows of Cow; > my $slot = [EMAIL PROTECTED]; > $$slot = new Dog; > > Do we wind up with Clarus or an exception? (Never mind whether you take > exception with Clarus.) Hopefully an exception, from a user's POV. Definitely. > And this? > > my @cows of Cow: > my @cats of Cat; > my $ref = [EMAIL PROTECTED]; > @cats[0] := $$ref; > @cats[1] := @cows[0]; # Just to hammer the point home > $$ref = new Dog; I would assume that's also an error. @cats C [1], and if you bind part of it to something that returns a non-cat, there should be an exception... or a compile error. Luke > But then there's a question for p6i as to how all the above happens. > > — > > Gordon Henriksen > [EMAIL PROTECTED] >
Re: PerlHash.get_pmc_keyed of non existing key
Luke Palmer <[EMAIL PROTECTED]> writes: > Gordon Henriksen writes: >> my $ref = [EMAIL PROTECTED]; >> $$ref = "value"; >> print '@ary[0] : ', @ary[0], "\n"; # -> @ary[0] : value > > That has to do with autovivification semantics. Particularly, do things > autovivify when you take a nonconstant reference to them. I think it's better seen as part of the semantics of references to nonexistent values. When you take a ref to a value slot that doesn't exist, you effectively create a tied variable that knows how to create that slot, and will do so only when you assign to it. /s
Re: PerlHash.get_pmc_keyed of non existing key
On Saturday, August 23, 2003, at 10:37 , Luke Palmer wrote:
Gordon Henriksen writes:
Taking a thread from Perl 6 Internals. Will Perl 6 support this
behavior?
$ perl <<'EOT'
my @ary;
my $ref = \$ary[0];
$$ref = "value";
print '$ary[0] : ', $ary[0], "\n";
EOT
$ary[0] : value
Presumably the Perl 6 would be:
my @ary;
my $ref = [EMAIL PROTECTED];
$$ref = "value";
print '@ary[0] : ', @ary[0], "\n"; # -> @ary[0] : value
That has to do with autovivification semantics. Particularly, do
things autovivify when you take a nonconstant reference to them. I
think they should, and that wasn't one of those cases where autovivi
was broken, so it probably will.
Exactly what I was arguing on p6i—autovivify in lvalue context, but not
in rvalue context. i.e., both of these programs should work as specified
in comments:
my @ary;
my $ref = [EMAIL PROTECTED];
$$ref = "value";
print '@ary[0] : ', @ary[0], "\n";
# @ary[0] : value
my %hash = { a => 1, b => 2 };
my $temp = %hash{c};
print exists %hash{c}? "exists\n" : "DNE\n";
# DNE
And this?
my @cows of Cow:
my @cats of Cat;
my $ref = [EMAIL PROTECTED];
@cats[0] := $$ref;
@cats[1] := @cows[0]; # Just to hammer the point home
$$ref = new Dog;
I would assume that's also an error. @cats C [1], and if
you bind part of it to something that returns a non-cat, there should
be an exception... or a compile error.
This particular example would be catchable at compile-time, ideally—it
would require type inference, but it's catchable at compile time. But in
the general case? Where the array, or the reference, or both are untyped
and passed in from someplace else, their types/constraints not knowable
until runtime? Where would the exception throw?
Throw here, at bind time?
@cats[0] := $$ref;
The undef value would need to know about its container (or, more
appropriately:) said container's constraints. But undef is still
compatible with both containers. To expand on that, what about this case?
my @listA is any (Cow | Cat);
my @listB is any (Cat | Dog);
@listA[0] := @listB[0];
Author could expect a Cat to be compatible and for the code to run. But
very action-at-a-distance-ey if it did work.
Or throw here, at assign time?
$$ref = new Dog;
The undef PMC would need to know about all of its containers and all of
their constraints. Yuck! With weak references, too, because @cows could
be GC'd while @cats remained valid.
—
Gordon Henriksen
[EMAIL PROTECTED]
Re: PerlHash.get_pmc_keyed of non existing key
On Saturday, August 23, 2003, at 11:14 , Sean O'Rourke wrote:
Luke Palmer <[EMAIL PROTECTED]> writes:
Gordon Henriksen writes:
my $ref = [EMAIL PROTECTED];
$$ref = "value";
print '@ary[0] : ', @ary[0], "\n"; # -> @ary[0] : value
That has to do with autovivification semantics. Particularly, do
things
autovivify when you take a nonconstant reference to them.
I think it's better seen as part of the semantics of references to
nonexistent values. When you take a ref to a value slot that doesn't
exist, you effectively create a tied variable that knows how to
create that slot, and will do so only when you assign to it.
Really? I'd think that it would create the slot immediately. Perl 5 does.
perl <<'EOT'
my %hash;
my $ref = \$hash{key};
print exists $hash{key}? "exists\n" : "DNE\n";
EOT
exists
Also means that this...
my $ref = \%hash{blah}{blort}[123]{moogle};
is just a chain of autovivifying subscript operations, rather than the
construction of some horribly complex object that knows how to create
all of those intermediate aggregates. Also means that this
%hash{blah}{blort}[123]{moogle} = 1;
work's just the same way as the reference creation (up until it assigns
a value instead of creating a reference object).
OTOH, this in Perl 5
my $val = $hash{blah}{blort}[123]{moogle};
Was always annoyingly non-orthogonal to simpler constructs, in that it
vivified all but the last container. But that's separately solvable.
—
Gordon Henriksen
[EMAIL PROTECTED]
