[pm-h] Interesting Blog Posts on Context & Array vs List

[G. Wade Johnson]

On Wed, 19 Feb 2014 12:02:25 -0800 (PST)
"Michael R. Davis" <mrdvt92 at yahoo.com> wrote:

> > Personally I find context one of the most fascinating aspects of
> > Perl (Maybe someone should do a presentation!) and I must admit
> > I've been guilty of referring to things as being in "Array
> > Context".  Does anyone disagree with these postings or have any
> > additional knowledge to share?
> 
> The only case that should not be used is caught by use warnings.
> Maybe I just think in "Perl context" nowadays...
> 
> perl -Mwarnings -e ' $a =      (5,6,7); print "$a\n"' #scalar
> assignment of list  => 7 #never use this "feature"... perl -Mwarnings
> -e '($a)=      (5,6,7); print "$a\n"' #array  assignment of list  =>
> 5 perl -Mwarnings -e ' $a = @a = (5,6,7); print "$a\n"' #scalar
> assignment of array => 3 perl -Mwarnings -e '($a)= @a = (5,6,7);
> print "$a\n"' #array  assignment of array => 5 perl -Mwarnings -e '
> $a =    @{[5,6,7]};print "$a\n"' #scalar assignment of defref anon
> array => 3 perl -Mwarnings -e '($a)=    @{[5,6,7]};print "$a\n"'
> #array  assignment of defref anon array => 5

Great example.

To be a bit pedantic, the first is not a "scalar assignment of a list"
it is "comma operator in scalar context". Putting parens around
something only makes it a "list" on the left side of an assignment.
And, that is an odd special case.

For those who can't imagine why you would do such a thing, it often
happens when a sub returns a "list" (which it is in list context) and
the calling code assigns it to a scalar. This completely changes how
the return is interpreted.

> I actually don't like the current popular practice of doing
> 
> my ($self, $a, $b, $c)=@_;
> 
> I much prefer 
> 
> my $self = shift;
> my $a    = shift;
> my $b    = shift;
> 
> as I can
> 
> my $self = shift;
> my $a    = shift || "default";
> my $b    = shift // 0;
> 
> and actually understand what I did months later.

Although it only matters in a few circumstances, a single 'my'
statement is marginally faster than multiples. I used to do the
separate statement thing, but now I tend to:

my ($self, $foo, $bar, $baz) = @_;
$foo ||= "default";
$baz //= 0;

I vaguely remember something about a cost for the shift, but that may
have been fixed.

The context concept is definitely critical to getting the most out of
Perl.

G. Wade
-- 
The greatest enemy of knowledge is not ignorance, it is the illusion of
knowledge.                                       -- Stephen Hawking