package Bar;

sub new { ... }

sub foo {
  state $bar;
  $bar++;
  return $bar;
}
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$bar1 = Bar->new();
$bar2 = Bar->new();

$bar1->foo(); # returns 1, as you would expect
$bar1->foo(); # returns 2, as you would expect
$bar2->foo(); # returns 3, possibly unexpected
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jgallagher,
BrowserUk's response boils down to "don't use state variables as a replacement for class variables when doing OO". I agree with that. The argument that it will be easy to make this mistake is false though as p6 OO looks visually a lot different from p5 (method foo() vs sub foo() for instance). I am not saying caution isn't warranted, but this is just once case that doesn't negate the usefulness elsewhere.

Cheers - L~R

Update: I reworded my response to be clearer but I believe the spirit of the message was left in tact.

Yes. That is a good example of the possible surprise factor.

Now inherit Class Qux from Class Bar, and every time you call ->foo() from any instance of either class, state $bar will be incremented. This has the potential for an even bigger surprise factor I think.

---

Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error.

Lingua non convalesco, consenesco et abolesco. -- Rule 1 has a caveat! -- Who broke the cabal?

"Science is about questioning the status quo. Questioning authority".

The "good enough" maybe good enough for the now, and perfection maybe unobtainable, but that should not preclude us from striving for perfection, when time, circumstance or desire allow.

Nah, it's only surprising if foo doesn't know it's a method. Methods should never be surprised by being inherited. And Perl 6 methods always know they're methods because they have their own keyword. If foo doesn't properly manage its own state, that foo's problem, not anyone else's.

I don't think that's the type of closure BrowserUk is referring to. Think more of this perl 5 code:

sub genclosure {
  my $bar;
  return sub { $bar++; }
}
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So I would have to agree with BrowserUk's "very carefully" statement. The same as I would use your perl5 (global) closure code - very carefully. That said, I have probably about a half-dozen or so places where I need or want to cache data globally in my projects at work, so this would be very nice syntactical sugar for that. However, I also generate a few closures where this new idea could easily bite a junior programmer in the arse when they wonder why their data isn't returning properly. ("How did that counter get so high already?")

That said, I also generally avoided static in C as well, so that's not really surprising for me. After all, one generally looks at programming tasks from the basis of their experience - if it looks like a nail, that may be because all you have is a hammer.

Update: TimToady's response has cleared things up a bit, thankfully. It was looking a bit grim for state - although there is still some care to be taken. I'll have to think if I can figure out where I'd use this feature, if anywhere.

If that's what he meant, this would be the equivalent Perl6 code, using state:

sub genclosure {
  return sub { state $bar; $bar++; }
}
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I'm still not seeing a difference in safety.

That's correct--state variables clone. I see a lot of needless handwringing here based on the faulty assumption that Perl's state variables are just like C's static variables. There's a reason we gave them a different name...

I'm still not seeing a difference in safety.

If Limbic~Region's analogy with C statics is accurate, state defines an single, absolute, static piece of ram. So, this:

sub genclosure {
  my $bar;
  return sub { $bar++; }
}
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And this:

sub genclosure {
  return sub { state $bar; $bar++; }
}
[download]

are semantically quite different.

In the former case, each time you generate a coderef by calling genclosure(), that coderef will close over a different instance of the lexical $bar.

In the latter case, each coderef generated will reference the same, single, immovable, static piece of ram</code> labelled $bar.

And further, references to state $bar; in other unrelated generator functions--in the same package, or a different packages, or a different modules--will all refer to that same static piece of ram.

It is possible that state will be cleverer than that, and somehow be scoped by file or package or namespace, but then a) the analogy with C static falls through; b) the would seem to be considerable overlap with local $bar;.

---

Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error.

Lingua non convalesco, consenesco et abolesco. -- Rule 1 has a caveat! -- Who broke the cabal?

"Science is about questioning the status quo. Questioning authority".

The "good enough" maybe good enough for the now, and perfection maybe unobtainable, but that should not preclude us from striving for perfection, when time, circumstance or desire allow.