=NAME Yet Another S<Perl 6> Operator: Comparisons - Part II

=VERSION

  Maintainer: Adriano Ferreira <ferreira@cpan.org>
  Date: 29 Sep 2007
  Last Modified: 2 Oct 2007
  Number: 6
  Version: 3

  Status: Draft

=head1 Body

In the L<the last article| compare1.html >, we've seen
some of the usual relational operators in S<Perl 6> and 
their enhanced syntax through chaining (which allows
expressions like C<<a < b < c>>).

Another kind of comparison operators are those that,
instead of true/false returns, identify the relative
order between its operands: before, equal, or after.

For numbers, there is the I<spaceship operator> (just like
in S<Perl 5>):

    0 <=> 1    # returns that 0 is before 1
    0 <=> 0    # returns that 0 equals 0
    1 <=> 0    # returns that 1 is after 0

But there is a change: instead of C<-1>, C<0>, C<1>, the
return is C<Order::Increase>, C<Order::Same>, C<Order::Decrease>,
the values of an enumeration, which in numeric context
reduces into C<-1>, C<0>, C<1>.

For strings, the sort comparator was renamed. After
the triad C<le>, C<eq>, and C<gt>, it is now called the B<leg> operator.

    'a' leg 'b'   # Order::Increase
    'a' leg 'a'   # Order::Same
    'b' leg 'a'   # Order::Decrease

That was done on purpose, to leave the S<Perl 5> string
sort comparator C<cmp> free to be a generic sort
comparison, in the same way that C<before> and C<after>
were conceived.

    $dt_today cmp $dt_tomorrow  # Order::Increase
    $dt_today cmp $dt_today     # Order::Same
    $dt_today cmp $dt_yesterday # Order::Decrease

The C<cmp> operator is the operator of choice when
comparing B<any> items: numbers, strings, and objects.
With respect to numbers and strings, its default
is to act just like C<'<=>'> and C<leg>, respectively. In turn,
objects may have their comparison behavior
customized by the author of the code.

Chaining does not make sense to these operators.
But for ordering operations like sorting, finding
the minimum and maximum, they are just perfect.
Some examples of sorting expressions follow.
(Notice that these could be made shorter using
the new signatures of the C<sort> method and
other S<Perl 6> novelties.)

=begin code
    @values.sort; # use default 'cmp'
    @values.sort: { $^b cmp $^a }; # reverse order

    @values.sort: {    $^a<name> leg $^b<name>
                    || $^a<id>   <=> $^b<id>   };
        # a sort based on values keyed by 'name' and 'id'
        # respectively

=end code



$Revision: 73 $

=begin comment

Keywords:
  Perl
  Perl 6
  operators
  comparisons
  sort comparisons

=end comment