I'm writing this because I lost about 1-2 hours to debug a problem which was caused by closures.

Suppose you have a method of a class that you write using Moose(but a plain Perl object is also relevant), and you use inside that method a static variable (static in the C sense, by that I mean a variable that has scope local to the method but a lifetime that spans each call of said method). I was recommended that I use a closure and enclose the declaration of that variable and the method inside the closure to cause the variable to be static(again, the C sense of static).

What happened, was that when I created 2 objects, the value of that static variable wasn't really resetted, so most of my tests started failing, so of course the next natural move(after debugging for 1-2 hours) was to either

• enclose the ctor/dtor in the closure and reset that static(in C sense) variable so that it doesn't affect further objects
• give up the closure and the static(in C sense) variable and go for an attribute in the class that would be used for this, which if you use Moose has the advantage of being able to specify default => value , so the attribute is set to that value upon constructing a new object

I chose the latter.

Please notice that the former would attract another problem: suppose you were doing this not inside a class, but inside a role, which does not define the ctor/dtor , so now you can't reset it in the ctor/dtor because that is not specific to the role, it's specific to classes consuming that role.

I suspect there are at least 2 plausible reasons for this :

• I don't know Perl
• closures cause various side-effects not immediately noticeable that lead to bugs

I will be cautious not to draw any conclusions from this, instead, I'd like your oppinion on closures and their side-effects.

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