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All the Perl that's Practical to Extract and Report

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  • Is there any way to get the caller's $_, like with Perl 6 $CALLER::_?
    • Do you mean the caller's lexical $_, if it has one ?

      Well, lexical $_ is no different from other lexical variables. So I imagine PadWalker [cpan.org] will work with it.

      If you write:
      my $_; sub mylc { lc } then the lc will act on the lexical $_. (I.e. you'll have a closure.)

      • So the gap between built-ins and user defined functions gets even larger than it already is. With no clean way to access the caller's lexical $_ (PadWalker's nice, but not something I'd like to use on a regular basis) and built-ins that do work on that lexical $_, creating functions that work like built-ins gets hard.

        Closures aren't good enough, because you can't import them and many people do like to have utility functions in modules.

        The lexical $_ sounded like a good idea, but having a special variable
        • A few points:

          "my $_" is an addition, not a replacement. It's 100% backwards-compatible.

          You can import closures, as long as they're not anonymous. But by doing so you "import" their lexicals as well.

          What you want is (as I understand it) to define a function that defaults to $_, as in :

          sub f(;$){my $arg = @_ ? $_[0] : $CALLER::_;...}

          This is unrelated to the ability of my'ing $_, which is introduced to solve side-effect and scoping problems, as well as problems with "local $_" versus magic. May I point

          • If I read p5p correctly, "lc" will still operate on the current $_, even if it is lexical.

            But my contrived "mylc" would no longer work as with a global $_.

            sub mylc { lc }
             
            our $_ = "FOO";
            print lc;    # foo
            print mylc;  # foo
             
            my $_ = "FOO";
            print lc;    # foo
            print mylc;  # FOO

            The point of having a default variable is not having to type it! But you now have to pass it to "mylc" explicitly if it is lexical.

            I understand that you want a lexical variable because you do

            • But my contrived "mylc" would no longer work as with a global $_. No, it will indeed. Just try it :) Look:

              $ bleadperl -le 'sub f{print}$_=1;my$_=2;f'
              1

              Here f()'s body is not in a scope where a lexical $_ is defined. Hence it will not use it. Really the difference between $_ and my $_ is the same than the difference between $x and my $x. You don't need to learn new things. And this new feature doesn't make you loose anything.

              • Thank you for explaining how lexicals work. I am aware of scoping in Perl.

                All I'm asking for is a cheat to get the $_ in the caller's lexical scope.

                I am assuming that my $_ = 3; print; will actually output 3. Builtins often use $_ as the default value if no argument is given. My own utility functions also do that. I'd like to use lexical $_, but I'd also like to keep using utility functions the way I'm using builtins. Without scary stuff like PadWalker.

                I'm not familiar with perl's guts, but is it hard to
                • Maybe you're too excited about the change that you don't see my point :) Thanks for your patience!

                  CALLER:: is feasible, I think it won't be difficult. So, assuming it gets implemented, the idiom to get the current default variable would be :

                  my $arg = exists $CALLER::{_} ? $CALLER::_ : $_;

                  This has the advantage of being valid in perl 5.8 and below. Assuming none of your modules is called CALLER.pm !

                  You're most welcome to post your thoughts to P5P. You definitively have a point about CALLER an d I'd like to see it discussed.