defined defined -- Bug or "Feature"?

roho has asked for the wisdom of the Perl Monks concerning the following question:

I just finished tracking down a very elusive bug in my code. The bug was caused by two defined's side by side (a copy/paste error). Perl doesn't object at either compile time or at run time, but two defined's reverses the logic of the expression. In the sample code below the first "if" statement is False (as expected), but the second "if" statement is True because of the two defined's.

I suppose there may be a use case somewhere where the result of one "defined" is fed into another, but my first reaction was that this is a bug. What do other monks think about this? Is this a bug in Perl or a "feature"?

#!/usr/bin/perl
use strict;
use warnings;

my $x;

if (defined $x) {
        print "True\n";
} else {
        print "False\n";
}

if (defined defined $x) {
        print "True\n";
} else {
        print "False\n";
}
[download]

"Its not how hard you work, its how much you get done."

Comment on defined defined -- Bug or "Feature"? Download Code

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Re: defined defined -- Bug or "Feature"? by dave_the_m (Monsignor) on Dec 24, 2015 at 07:45 UTC
`defined($x)` returns a false, but defined value, so `defined(defined($x))` is true. Dave.	[reply] [d/l] [select]
Re^2: defined defined -- Bug or "Feature"? by GrandFather (Saint) on Dec 24, 2015 at 08:00 UTC
You state that like you know it to be the case, but I couldn't see that behavior described in the documentation. Do you know this from the code? Premature optimization is the root of all job security	[reply]
Re^3: defined defined -- Bug or "Feature"? by dave_the_m (Monsignor) on Dec 24, 2015 at 09:33 UTC
The docs say that it returns a boolean value, i.e. a true or false value. It would be surprising for a false value to also be undefined, as you would get 'undefined' warnings if you tried using operators on the results. This shouldn't warn: `$x = defined($y) ^ defined($z);` [download] Dave.	[reply] [d/l]
Re^3: defined defined -- Bug or "Feature"? by Anonymous Monk on Dec 24, 2015 at 09:29 UTC
I guess he didn't (guess). It's easily tested... `print defined( undef ) // 'n/a'; print defined( undef ) \|\| 'n/a'; n/a` [download]	[reply] [d/l]
Re: defined defined -- Bug or "Feature"? by GrandFather (Saint) on Dec 24, 2015 at 07:56 UTC
The most likely answer is that defined returns an empty string as a false value. An empty string is defined so the left most defined sees a defined value - the result from the right most defined. So, not really a bug, just an interesting edge case. Premature optimization is the root of all job security	[reply]
Re^2: defined defined -- Bug or "Feature"? by Anonymous Monk on Dec 24, 2015 at 08:33 UTC
Probably not, Pl_sv_false ought to be same as undef, there were some bugs regarding this, but i'm afk & my mem is fuzzy. Devel::Peek...:)	[reply]
Re^3: defined defined -- Bug or "Feature"? by dave_the_m (Monsignor) on Dec 24, 2015 at 09:36 UTC
PL_sv_undef, PL_sv_no and PL_sv_yes are three distinct values with distinct properties. Only PL_sv_undef is undef. Dave.	[reply]
Re^4: defined defined -- Bug or "Feature"? by Anonymous Monk on Dec 24, 2015 at 10:21 UTC
Re: defined defined -- Bug or "Feature"? by dsheroh (Monsignor) on Dec 24, 2015 at 08:39 UTC
two defined's reverses the logic of the expression No, it doesn't reverse the logic. It's always true, regardless of the single `defined`'s result: `$ perl -E 'say ((defined $x) ? "defined" : "undef")' undef $ perl -E 'say ((defined defined $x) ? "defined" : "undef")' defined $ perl -E '$x = 1; say ((defined $x) ? "defined" : "undef")' defined $ perl -E '$x = 1; say ((defined defined $x) ? "defined" : "undef")' defined` [download] This is because, as stated in previous comments, the first (rightmost) `defined` will always return a defined result, so the second (leftmost) `defined` always receives a defined input value.	[reply] [d/l] [select]
Re: defined defined -- Bug or "Feature"? by Anonymous Monk on Dec 24, 2015 at 10:23 UTC
perlsyn: Truth and Falsehood The number 0, the strings `'0'` and `""` , the empty list `()`, and `undef` are all false in a boolean context. All other values are true. Negation of a true value by `!` or `not` returns a special false value. When evaluated as a string it is treated as `""`, but as a number, it is treated as 0. Most Perl operators that return true or false behave this way. I'm assuming this is the missing piece of documentation that explains why defined always returns a defined value.	[reply] [d/l] [select]