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Postings from November 2010
On 30 November 2010 18:37, Reini Urban <rurban@x-ray.at> wrote:
> 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?
Because if you take off the constant prototype () it does:
$ perl -le'BEGIN{my $x = 5; *foo = sub(){$x}; $x=6} print foo'
5
$ perl -le'BEGIN{my $x = 5; *foo = sub {$x}; $x=6} print foo'
6
> We are using lexical scope, not dynamic scope.
> With dynamic scoping, i.e. local $x = 5; you could justify a 6.
>
> Is it really that perl coders still cannot understand the difference between
> lexical and dynamic scope?
Given the above, that is a *bit* strong dont you think Reini?
>> The generation of optimised constant subs is a useful facility.
>> It certainly ought to be available, without having to invoke the
>> installation-in-a-package behaviour of "use constant". But this exception
>> to the closure rules is a poor way to make it available, since it breaks
>> an otherwise logically consistent language feature. In my opinion,
>> constant-sub creation should be made available via an XS function in some
>> CPAN module (very easily arranged), and the magic behaviour of sub(){$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 because we defined it so.
Yves
--
perl -Mre=debug -e "/just|another|perl|hacker/"
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