http://www.perlmonks.org?node_id=1072250


in reply to Dualvars besides $!

Not a variable, but a value: the value for false, which you can generate with !1.

It return '' in string context and 0 in numeric context, and doesn't warn in numeric context like the empty string normally does:

$ perl -wE 'say 0+""'
Argument "" isn't numeric in addition (+) at -e line 1.
0
$ perl -wE 'say 0+!1'
0

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My version of Scalar::Util doesn't export an is_dual function, so I can't check. It helps to type that without the _ :-)

Update: I should have mentioned it earlier: even though the thing is called a "dualvar", it's not actually variables that are tested; values are being tested. It makes no difference if those values are return values, stored in array, or are stored in a scalar variable.

Perl 6 - the future is here, just unevenly distributed

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Re^2: Dualvars besides $!
by davido (Cardinal) on Jan 27, 2014 at 18:05 UTC

    A value for "true" is also dualvar:

    perl -MScalar::Util -E 'say Scalar::Util::isdual(!0)'

    ...outputs '1', indicating the test was true.


    Dave

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Re^2: Dualvars besides $!
by szabgab (Priest) on Jan 27, 2014 at 17:36 UTC
    wow !1. I have not heard of that earlier. greping the pods of 5.18.2 - it is not mentioned there either. But it works as you showed.
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