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Re: [perl #22027] undef appears as scalar return value for a list

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Rick Delaney
April 24, 2003 10:10
Re: [perl #22027] undef appears as scalar return value for a list
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On Thu, Apr 24, 2003 at 02:02:50PM -0000, Mike Stok wrote:
> sub show {
>     print defined $_[0] ? $_[0] : '(undef)', "\n";
> }
> my $rv = ('a', 'b', 'c');
> show $rv;
> my $rv2 = ('a', 'b', 'c', ());
> show $rv2;
> I would have expected $rv2 to be 'c' as I thought ('a', 'b', 'c', ()) should
> be flattened to ('a', 'b', 'c').  The warnings indicate that in the first 
> case two values are useless, and in the second three values are useless.

I think it's reasonable for $rv2 to be undef since that's what you get

   $rv2 = ();

The number of warnings makes sense to me too, since in the first case
there are 3 comma-separated expressions and in the second there are 4.
Flattening should only happen when you have list context.

I'd expect C's comma operator to behave the same, except C doesn't allow
an empty expression like "()".

> Things like
>   @array = ('a', 'b', 'c', ());         # => ('a', 'b', 'c')
>   $scalar = ('a', 'b', 'c', ())[-1];    # => 'c'
> seem to work as I would expect.

These are both list context so the expression "()" is treated as an
empty list (and flattened).  In the scalar context above the expression
"()" is not a list.

Rick Delaney

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