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Re^2: Perl cheat sheet

by ikegami (Pope)
on Aug 21, 2006 at 19:32 UTC ( #568659=note: print w/replies, xml ) Need Help??

in reply to Re: Perl cheat sheet
in thread Perl cheat sheet

"$foo" creates a stringified copy of $foo. $foo will get stringified when it needs to get stringified. There's no reason to do it prematurely. For example, it prevents people from using object with stringification overloaded.

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Re^3: Perl cheat sheet
by ursus (Acolyte) on Jan 07, 2009 at 00:50 UTC
    Globals are an important exception here. If you log($!), you may easily have a clobbered $! by the time you use it. log("$!") gets a snapshot of the variable at the time of the call.

      That's something most people will never need to know.

      The problem is that new programmers love to put *everything* in quotes. While it's a bad idea to pass global (lexical and package) variables as arguments, that is probably not a problem you'll ever run into. Just about every sub already creates a copy of the value before it can change.

      sub logger { my ($arg) = @_; # The copying happens here, so ... # no need to do it in the caller. }

      One could even argue it's the sub's responsibility to protect global variables it changes if it uses @_ at any other point.

        Sometimes I forget that "never" means "you probably don't want to". ;-) Next time I'm in that code, I'll make it the responsibility of the called function. Thanks.

      $! was an unfortunate choice of variable. :-) It's a dualvar so it has different values if treated as a string or as a numerical.

      open my $fh, '<', 'this does not exist'; printf "%s (%d)\n", $!, $!; __END__ No such file or directory (2)
      It's better to just create a copy and pass that, if the subroutine doesn't copy the argument.


        The example was fine. It doesn't make sense for logger to need the dualvar.

        • If you wanted to pass the error string to the sub, the caller would use "$!".
        • If you wanted to pass the error number to sub, the caller would use 0+$!.
        • If the sub needs both an error string and an error number, it would take two arguments.
        • If the sub wants to look at $!, it wouldn't take it as an argument.

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