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Re: How will you use state declared variables in Perl6?

by TimToady (Parson)
on Jul 08, 2005 at 22:18 UTC ( #473584=note: print w/ replies, xml ) Need Help??


in reply to How will you use state declared variables in Perl6?

Well, they aren't really quite the same as C's statics. You should really think of them more as persistent lexicals that also happen to know how to initialize themselves on first use. Unlike globals (and like lexicals) they are by default per-thread and per-clone. (They are not per-instance, even if used in methods. In a sense, "has" variables are state variables on the instance.)

If used in a class, the state variable does not generate a public accessor unless there is a dot twigil to indicate that. Without the accessor no other class can see the variable, not even derived classes. Only your class's methods can see it, and can therefore adjudicate any use of it by derived classes. The state belongs to your class. Given that (in general) class closures are not repeatedly cloned, there's really little to choose from between "my" and "state" in the class definition itself.

A given method may certainly declare a state variable and adjudicate its use as well. The state variable functions as a class variable that no other method can see, so it could be useful in the rare case that your method doesn't happen to trust the other methods in the class. Other than that, yes, a class attribute probably makes more sense.


Comment on Re: How will you use state declared variables in Perl6?
Re^2: How will you use state declared variables in Perl6?
by BrowserUk (Pope) on Jul 08, 2005 at 22:42 UTC

    Could you also clarify the difference between state variables and local variables please? (Or have I missed the demise of the local declarator?)


    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.
      Well, technically, yes, you missed the demise of local, but only because we renamed it to temp and made it completely orthogonal to storage class declarations. You can temporize any variable, and it will restore itself at the end of the current lexical scope. You can also conditionally temporize a variable using let, and it resets at the block end only if the block fails. But these are operations on existing variables. Even in Perl 5, local never declares a variable, but merely temporarily changes the value of an existing global.

      And to reiterate, state variables are not globals. They're persistent lexicals. You can temporize them, but then you can temporize my variables as well. You can even temporize attribute accessors and not worry about whether the accessor represents a real attribute or is a wrapper around something else:

      temp $handle.autoflush = 1;

        Okay. The analogy with C statics was flawed. That makes sense.

        I should really have referred to our rather than local, but I think your explaination suffices.

        What happens if I do:

        sub bar { state $foo; ... my $code = sub qux { state $foo; }; ... }
      • Is this illegal?
      • Or do the two refer to the same thing?
      • Or does the inner $foo hide the outer $foo for the duration of that scope?

        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.

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