On Wednesday 09 July 2008 13:46:19 Bob Rogers wrote:
> From: "Patrick R. Michaud" <[EMAIL PROTECTED]>
> Date: Wed, 9 Jul 2008 08:25:52 -0500
> If I understand what you're saying, and then take it to what I see
> as its ultimate conclusion, you're basically claiming that every
> call to a subroutine involving lexicals requires a newclosure op.
>
> Yes; I believe that's a cleaner design. Doing otherwise relies on
> runtime heuristics to decide what context ought to have been closed
> over, which strikes me as much less reliable. Of course, I don't *need*
> the cleaner design, since I can just ignore the runtime heuristics -- as
> long as they don't break the newclosure case.
I read that in the lexicals PDD, and I think the current behavior is bizarre
*without* the call to newclosure. How is it even possible to close over a
lexical environment in an outer when that lexical environment was never even
created?
It's difficult to describe that as anything other than ridiculous.
Consider Bob's example Perl 5 code:
sub outer {
my $x = shift;
print "outer foo $x\n";
sub inner {
print "inner foo $x\n";
}
}
# Note that it is not illegal to call inner before outer.
inner();
There's a compile-time warning here, namely that $x will not stay shared.
Even though it looks like inner() should close over outer()'s $x per Perl 5's
scoping rules (ignoring that you can't nest subs in Perl 5), there is no $x
available at the call to inner().
If you wrote this instead:
sub outer {
my $x = shift;
print "outer foo $x\n";
return sub {
print "inner foo $x\n";
}
}
my $inner = outer( 10 );
$inner->();
... then Perl 5 effectively performs a newclosure action, attaching the active
lexpad to the new instance of the subroutine reference.
I suspect the motivation for the bizarreness of the specification is the
desire to make code like this work in Parrot:
{
my $x;
sub set_x { $x = shift }
sub get_x { return $x }
}
... except that there's no real way in Parrot right now to create an enclosing
lexical block, activate it, and attach it as the outer to one or more
Closures.
-- c
