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RE: lexical weirdness(?)

by dchetlin (Friar)
on Oct 25, 2000 at 19:57 UTC ( #38401=note: print w/ replies, xml ) Need Help??


in reply to lexical weirdness(?)

As the anonymous but wise monk pointed out above, this is a case of the `my $x if 0' feature. For some reading material on this, start here and look through some of the p5p threads on it.

I doubt this feature will go anywhere for Perl 5 at least, because it doesn't seem to cause any problems and allows you to do something (create a static variable) which you can't really do any other way.

But it does mean you should be careful when declaring lexicals.

-dlc


Comment on RE: lexical weirdness(?)
RE: RE: lexical weirdness(?)
by jimt (Chaplain) on Oct 25, 2000 at 23:36 UTC
    Thanks for the help guys. I'll begrudgingly admit that this does make sense, though I'm not too pleased with it.

    Regardless, just to be good and point it out, my $x if 0; is most certainly not the only way to create a static variable. This is clearer, IMHO, and more widely used. (I hope)
    { my $static; sub some_function { print "Static!", $static++; }; };

    Just wrap up the function in braces and lexically scope any "static" variables you want in the enclosing braces. Much clearer, IMHO.
      There are at least two major differences between closures and static-via-my-if-0 variables.

      Here's the first. Look closely at this code and see if you can figure out what its output will be before you run it.

      #!/usr/bin/perl -w use strict; { my $closure; sub func_w_closure { print "Closure! ", $closure++, "\n"; func_w_closure($_[0]+1) if $_[0] < 3; } } sub func_w_static { my $static if 0; print "Static! ", $static++, "\n"; func_w_static($_[0]+1) if $_[0] < 3; } func_w_closure(0); func_w_static(0);

      I'll use tilly's spoiler trick:

      [~] $ dev/test/scope.pl Closure! 0 Closure! 1 Closure! 2 Closure! 3 Static! 0 Static! 0 Static! 0 Static! 0

      In other words, the closure is the same variable through all calls, including recursive ones, while each recursive call to the func_w_static gets its own copy.

      The cool thing about that, of course, is that if you call func_w_closure again, you see that each recursive instance keeps its value!

      The other difference is that closures can be shared among functions -- if you stick another function into the lexical scope of `$static' above, some_function and your new function will refer to the same variable.

      -dlc

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