in reply to Re: just how special are $a and $b these days?
in thread just how special are $a and $b these days?

What I'm trying to do is write a binary search module that takes the same comparator function that was used to sort the array being search.

I'm getting the idea here that since $a and $b are package variables, sort comparators need to be declared in the same package as their use. I had thought that, for instance 

# the following is dumb and is written strictly as an example
package backwards;
sub pmc { $b cmp $a };
package main;
print sort backwards::pmc ( split /\s+/, `cat $0` )

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would work as intended.

it doesn't

so it looks like ${caller().'::a'} is going to be where the search key goes, and ${caller().'::b'} is going to be where the guesses go for evaluation. Done with glob aliasing, of course.

Thank you everyone

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Re^3: just how special are $a and $b these days?
by BrowserUk (Patriarch) on Mar 04, 2010 at 19:53 UTC

    This does: 

    package backwards;
sub pmc { $::b cmp $::a };
package main;
print sort backwards::pmc ( split /\s+/, `cat $0` )

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    c:\test>junk66
};{subsplitsortprintpmcpackagepackagemain;cmpbackwards;backwards::pmc`
+cat/\s+/,)($::b$::a$0`

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    Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error.
    "Science is about questioning the status quo. Questioning authority".
    In the absence of evidence, opinion is indistinguishable from prejudice.
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[d/l]
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      This does:

      Not really. That only works for the special case where you are performing your sort in main::. It doesn't work if you are in yet another package.

      In other words, this fails:

      package backwards;
sub pmc { $::b cmp $::a };
package blah;
sub foo { print for sort backwards::pmc ( split /\s+/, `cat $0` ) }
package main;
blah::foo();

      [download]

      The right way to do it is to prototype your comparator function with ($$) and to use @_ thusly:

      package backwards;
sub pmc($$) { $_[1] cmp $_[0] };
package blah;
sub foo { print for sort backwards::pmc ( split /\s+/, `cat $0` ) }
package main;
blah::foo();

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      -sauoq
      "My two cents aren't worth a dime.";
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[d/l]
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        True. Good call.

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