I am currently documenting all
of Perl 5's operators (see the perlopref GitHub project) and I have decided to include Perl 5's pseudo-operators as well. To me, a pseudo-operator in Perl is anything that looks like an operator, but is really more than one operator or a some other piece
of syntax. I have documented the four I am familiar with already:
()= the countof operator
=()= the goatse/countof operator
~~ the scalar context operator
}{ the Eskimo-kiss operator
What other names exist for these pseudo-operators, and do you know
of any pseudo-operators I have missed?
=head1 Pseudo-operators
There are idioms in Perl 5 that appear to be operators, but are really a
combination
of several operators or pieces
of syntax. These pseudo-operators
have the precedence
of the constituent parts.
=head2 ()= X
=head3 Description
This pseudo-operator is the list assignment operator (aka the countof
operator). It is made up
of two items C<()>, and C<=>. In scalar context
it returns the number
of items in the list X. In list context it returns an
empty list. It is useful when you have something that returns a list and
you want to know the number
of items in that list and don't care about the
list's contents. It is needed because the comma operator returns the last
item in the sequence rather than the number
of items in the sequence when it
is placed in scalar context.
It works because the assignment operator returns the number
of items
available to be assigned when its left hand side has list context. In the
following example there are five values in the list being assigned to the
list C<($x, $y, $z)>, so C<$count> is assigned C<5>.
my $count = my ($x, $y, $z) = qw/a b c d e/;
The empty list (the C<()> part
of the pseudo-operator) triggers this
behavior.
=head3 Example
sub f { return qw/a b c d e/ }
my $count = ()= f(); #$count is now 5
my $string = "cat cat dog cat";
my $cats = ()= $string =~ /cat/g; #$cats is now 3
print scalar( ()= f() ), "\n"; #prints "5\n"
=head3 See also
L</X = Y> and L</X =()= Y>
=head2 X =()= Y
This pseudo-operator is often called the goatse operator for reasons better
left unexamined; it is also called the list assignment or countof operator.
It is made up
of three items C<=>, C<()>, and C<=>. When X is a scalar
variable, the number
of items in the list Y is returned. If X is an array
or a hash it it returns an empty list. It is useful when you have something
that returns a list and you want to know the number
of items in that list
and don't care about the list's contents. It is needed because the comma
operator returns the last item in the sequence rather than the number
of
items in the sequence when it is placed in scalar context.
It works because the assignment operator returns the number
of items
available to be assigned when its left hand side has list context. In the
following example there are five values in the list being assigned to the
list C<($x, $y, $z)>, so C<$count> is assigned C<5>.
my $count = my ($x, $y, $z) = qw/a b c d e/;
The empty list (the C<()> part
of the pseudo-operator) triggers this
behavior.
=head3 Example
sub f { return qw/a b c d e/ }
my $count =()= f(); #$count is now 5
my $string = "cat cat dog cat";
my $cats =()= $string =~ /cat/g; #$cats is now 3
=head3 See also
L</=> and L</()=>
=head2 ~~X
=head3 Description
This pseudo-operator is named the scalar context operator. It is made up
of
two bitwise negation operators. It provides scalar context to the
expression X. It works because the first bitwise negation operator provides
scalar context to X and performs a bitwise negation
of the result; since the
result
of two bitwise negations is the original item, the value
of the
original expression is preserved.
With the addition
of the Smart match operator, this pseudo-operator is even
more confusing. The C<scalar> function is much easier to understand and you
are encouraged to use it instead.
=head3 Example
my @a = qw/a b c d/;
print ~~@a, "\n"; #prints 4
=head3 See also
L</~X>, L</X ~~ Y>, and L<perlfunc/scalar>
=head2 X }{ Y
=head3 Description
This pseudo-operator is called the Eskimo-kiss operator because it looks
like two faces touching noses. It is made up
of an closing brace and an
opening brace. It is used when using C<perl> as a command-line program with
the C<-n> or C<-p> options. It has the effect
of running X inside
of the
loop created by C<-n> or C<-p> and running Y at the end
of the program. It
works because the closing brace closes the loop created by C<-n> or C<-p>
and the opening brace creates a new bare block that is closed by the loop's
original ending. You can see this behavior by using the L<B::Deparse>
module. Here is the command C<perl -ne 'print $_;'> deparsed:
LINE: while (defined($_ = <ARGV>)) {
print $_;
}
Notice how the original code was wrapped with the C<while> loop. Here is
the deparsing
of C<perl -ne '$count++ if /foo/; }{ print "$count\n"'>:
LINE: while (defined($_ = <ARGV>)) {
++$count if /foo/;
}
{
print "$count\n";
}
Notice how the C<while> loop is closed by the closing brace we added and the
opening brace starts a new bare block that is closed by the closing brace
that was originally intended to close the C<while> loop.
=head3 Example
# count unique lines in the file FOO
perl -nle '$seen{$_}++ }{ print "$_ => $seen{$_}" for keys %seen' FOO
# sum all
of the lines until the user types control-d
perl -nle '$sum += $_ }{ print $sum'
=head3 See also
L<perlrun> and L<perlsyn>
=cut
