On Fri, Mar 14, 2003 at 03:49:21PM -0500, Dan Sugalski wrote:
> At 10:14 AM -0500 3/14/03, Andrew Pimlott wrote:
>
> >A6 says that, as in Perl 5, only anonymous subs are closures. I've
> >always thought of the fact that Perl 5 named subs are not closures
> >as a bug kept for compatibility.
>
> Well... there's always the issue that closures are done by
> instantiating the sub at runtime, while named subs are instantiated
> at compile time, which causes some difficulties. (As the enclosing
> sub's lexicals instantiate at runtime, thus giving the contained sub
> nothing to close over)
I don't understand exactly what you mean, because perl's behaviour
is rather strange and I don't know how it's implemented. Currently,
named subs close over _something_, because they get the value of
lexicals from the first time the enclosing code is run.
sub foo {
my ($x) = @_;
sub bar { print ">> $x\n"; }
}
> bar() -> undef
> foo("a")
> bar() -> "a"
> foo("b")
> bar() -> "b"
Which is perfectly useless. :-)
At any rate, you're right that the fact that Perl 5 nested subs are
visible globally means that talking about closure is basically
nonsense. In my view, this is part of the bug. Nested subs should
be visible only within their scope (or at least, there should be a
way to declare them so that they are), in which case they would have
to be instantiated at run-time (right?), and there should be no
problem binding the current lexical variables.
> Now, if the named lexically scoped sub actually got re-instantiated
> every time, *that* would be different.
Am I correct that Perl 6 lexical subs would have to get
re-instantiated every time, or could you implement them such that
they are not?
Andrew
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