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Re^6: Use of "my" after Perl v5.14

by AnomalousMonk (Chancellor)
on Sep 21, 2012 at 08:51 UTC ( #994853=note: print w/replies, xml ) Need Help??

in reply to Re^5: Use of "my" after Perl v5.14
in thread Use of "my" after Perl v5.14

use v5.14; { my $count = 1; sub counter { say $count++; } } counter(); counter(); counter(); counter();

Unfortunately, this approach hides a subtle pitfall — well, subtle enough, at any rate, to catch me more than once. The problem is here illustrated:

>perl -wMstrict -le "use v5.14; ;; counter(); counter(); ;; { my $count = 42; ;; sub counter { say $count++; } } ;; counter(); counter(); " 0 1 42 43

There is a critical dependence upon order of execution. The  $count lexical variable is created at compile time, so the  counter() function has no problem accessing it; the code runs with warnings and strictures fully enabled. The variable is not initialized within its closure until some moment during run time; this moment may precede or follow any invocation of the function that quite happily accesses the variable. KaBoom!

It's pretty easy to avoid this problem: perhaps the easiest way is to just use a state variable! But the first step toward avoidance is awareness.

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