perl5方法测试“可能”在新版本中无法正常工作

Question

In perl 5.8

perl  -e ' print "a" if AXXXXXXXXXXXXX::->can("isa");'
perl  -e ' print "a" if AXXXXXXXXXXXXX->can("isa");'

print "nothing" , means method test failed;

In perl 5.17

perl  -e ' print "a" if AXXXXXXXXXXXXX::->can("isa");'
perl  -e ' print "a" if AXXXXXXXXXXXXX->can("isa");'

print "a", means method test succeed;

I want to know why difference ?

and why un-esixt and un-loaded module "AXXXXXXXXX" test pass in new version of perl ?

very thank you :)

Answer 1

From perldoc perl5180delta :

"$class->can", "$class->isa", and "$class->DOES" now return correct results, regardless of whether that package referred to by $class exists [perl #47113].

This change was introduced in order to fix a long-open bug report regarding UNIVERSAL::can . The change does result in arguably incompatible changes in behavior, but based on existing discussion, the change in behavior is not seen as a bug and is not likely to be reverted.

perl #47113 documents the original bug report and the recent fix

perl #118513 is a bug reporting the behavior you observed -- closed as rejected

It is very likely that there is another means to accomplish what you are attempting do with the code examples in your question. I'd recommend searching for that which you are trying to accomplish.

If you're trying to test to see if a specific module is loaded, you can use something similar to the approach adopted by Params::Validate::Dependencies in response to this change:

no strict 'refs'
unless(@{"${class}::ISA"}) [...]

There's likely another way -- again, search for what you're trying to accomplish, now that you know about this change to Perl.

Answer 2

can was buggy. As you're reported, can would report that some modules namespaces didn't have a method name isa .

>perl5.16.3 -E"say 'xxx'->can('isa') ? q{can} : q{can't}"
can't

Yet all namespaces have such a method.

>perl5.16.3 -E"say eval { 'xxx'->isa('yyy'); 1 } ? q{can} : q{can't}"
can

This was fixed in 5.18.0