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Re^3: What is true and false in Perl? (undef,undef)

by tye (Cardinal)
on Oct 18, 2012 at 03:50 UTC ( #999663=note: print w/ replies, xml ) Need Help??


in reply to Re^2: What is true and false in Perl?
in thread What is true and false in Perl?

It isn't a list nor an array in scalar context that your example is making use of. It is a list assignment (in scalar context).

A list assignment in scalar context returns the number of items in the right-hand list.

And, in your example, 'x' =~ m/(.)(.)/, in a list context, returns an empty list, not a list of two 'undef's. An empty list (on the right-hand side of the list assignment) contains zero items so the assignment in scalar context returns 0, which is one of the documented false values.

But there is a kernel of truth hidden behind your mistakes.

There have been some exceptions (as near as I can remember, on an inconsistent basis) to the rule about what a list assignment returns in scalar context. Those exceptions were meant to handle cases very similar to what you described.

sub undefs { my( $count ) = @_; return ( undef ) x $count; } if( my( $x, $y ) = undefs(2) ) { print "true\n"; } else { print "false\n"; }

In some versions of Perl, that code would print "false". In the relatively modern Perl I had handy, it prints "true".

I even recall having a very short on-line conversation with Larry years ago about what I considered to be bugs such as ( undef, undef )[0] being an empty list instead of being a list containing a single 'undef' and he was worried about breaking the fact that the above code was supposed to print "false". I argued that that wasn't a feature worth keeping.

But I didn't follow-up to see what decisions were made back then. And, just now, I didn't test the current behavior except minimally. I think there was at least one thread about this stuff on PerlMonks around the time Larry and I talked, but I haven't looked for it. It might've just been in the chatterbox.

- tye        


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Re^4: What is true and false in Perl? (undef,undef)
by amartyaj (Initiate) on Nov 22, 2013 at 18:00 UTC
    Hi can somebody explain to me the reason behind the following: test.pl: #!/usr/bin/perl my $a = (1==0); print "__".ref($a)."__\n"; Output: ____

      What do you think ref does?

      In your example code, $a doesn't contain a reference. It contains a false value. ref does exactly this: "Returns a non-empty string if EXPR is a reference, the empty string otherwise."


      Dave

        Got it. Thanks a lot Dave.

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