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
