Re: "defined" function fails to evaluate expression; feature or fault?

It's not a feature or bug, it just is :)

defined() returns a bool, (either false/true) when it evaluates something, so when you do this: defined ($x || $y);, it's not doing what you think it does.

Here's an example that illustrates the issue:

perl -wMstrict -E 'my $x = defined $ARGV[5]; say defined $x'
1
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Now, $ARGV[5] is not set in any way, but since we're returning the value of the call to $x, $x will most definitely be defined, because it now evaluates to false as the variable was not defined (define() returns false, if something wasn't defined).

You need to call a define on each side of the comparison operator:

my $x = defined $x || defined $y;
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...as per your solutions that work. Then, check $x for truth:

die "blah...\n" if ! $x;
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...or combined:

die "blech...\n" if ! defined $x || defined $y;
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Comment on Re: "defined" function fails to evaluate expression; feature or fault? Select or Download Code

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Re^2: "defined" function fails to evaluate expression; feature or fault? by AnomalousMonk (Archbishop) on Jan 26, 2017 at 21:08 UTC
Not that it makes any difference in this case, but defined returns `''` (the empty string; false) or `1` (true): `c:\@Work\Perl>perl -wMstrict -le "print 'perl version ', $]; ;; print 'undefined A: >', defined(), '<'; print 'undefined B: >', defined(undef), '<'; print 'defined: >', defined(42), '<'; " perl version 5.008009 undefined A: >< undefined B: >< defined: >1< perl version 5.014004 undefined A: >< undefined B: >< defined: >1<` [download] Give a man a fish: `<%-{-{-{-<`	[reply] [d/l] [select]
Re^3: "defined" function fails to evaluate expression; feature or fault? by haukex (Archbishop) on Jan 27, 2017 at 07:44 UTC
Hi AnomalousMonk, defined returns `''` (the empty string; false) or `1` (true) To be even more specific, Perl has a "special" value for "false" - see Truth and Falsehood. The difference between an empty string and the special "false" value: `$ perl -wMstrict -le 'my $x=""; print 3+$x;' Argument "" isn't numeric in addition (+) at -e line 1. 3 $ perl -wMstrict -le 'my $x=!!""; print 3+$x;' 3 $ perl -wMstrict -le 'my $x=defined(); print 3+$x;' 3` [download] And looking even more closely with Devel::Peek: Read more... (862 Bytes) Regards, -- Hauke D Update 2019-08-17: Updated the link to "Truth and Falsehood".	[reply] [d/l] [select]
Re^3: "defined" function fails to evaluate expression; feature or fault? by stevieb (Canon) on Jan 26, 2017 at 21:13 UTC
Yep, I caught that after doing some testing, and had updated my reply (at least I thought I fixed it all). Thanks for the heads-up, as if someone is expecting a zero and is getting the empty-string type false instead, it could cause head-scratching.	[reply]