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Postings from November 2010
Zefram schrieb:
> Dave Mitchell wrote:
>> What do you think is the bug? It seems to be behaving the way I expect.
>
> The code looks like it closes over the lexical variable $x. That lexical
> variable is then set to 6, so when the closure is later called (through
> the&foo name) it would be expected for it to return 6. This is how
> closures normally work, both in other languages, and in Perl when that
> particular form with the () prototype is not used. For the closure
> operation to return a constant-5 sub cannot be justified by reference
> to closure theory; it can only be explained as a special exception in
> Perl semantics, which is what it is.
Sorry, I don't understand that special theory of yours.
Why on earth should
BEGIN{my $x = 5; *foo = sub(){$x}; $x=6} print foo'
ever print 6 again?
We are using lexical scope, not dynamic scope.
With dynamic scoping, i.e. local $x = 5; you could justify a 6.
Is it really that perl coders still cannot understand the difference
between lexical and dynamic scope?
> The generation of optimised constant subs is a useful facility.
> It certainly ought to be available, without having to invoke the
> installation-in-a-package behaviour of "use constant". But this exception
> to the closure rules is a poor way to make it available, since it breaks
> an otherwise logically consistent language feature. In my opinion,
> constant-sub creation should be made available via an XS function in some
> CPAN module (very easily arranged), and the magic behaviour of sub(){$x}
> should be phased out (less easy, probably requiring a deprecation cycle).
Nothing to deprecate, sorry.
Optimizing the closure to a constant sub here is also fine.
Only if it would be local var (i.e. dynamic scope) we cannot constant
fold it.
--
Reini Urban
http://phpwiki.org/ http://murbreak.at/
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