=NAME Yet Another S<Perl 6> Operator: The Conditional Operator

=VERSION

  Maintainer: Adriano Ferreira <ferreira@cpan.org>
  Date: 17 Dec 2007
  Last Modified: 18 Dec 2007
  Number: 11
  Version: 2

  Status: Draft

=head1 Body

The syntax of an if-then-else expression in S<Perl 6>
is composed by the conditional operator.

  say "My answer is: ", $maybe ?? 'yes' !! 'no';

The expression above is equivalent to that, which
uses the if-then-else statement within a C<do>.

  say "My answer is: ", do {
      if $maybe {
          'yes';
      } 
      else {
          'no';
      }
  };

The operator C<'?? !!'> syntactically breaks
the expression into three subexpresssions. The first
is evaluated in boolean context and, based on that result,
one of the two parts are evaluated. (It never evaluates
both of them.) If the conditional is true, it evaluates
and returns the middle part; if false, the right part.

The choice for the strong markers C<'??'> and C<'!!'>
was done to enhance the visibility of the construction.
Contrast this to the typical C expression (which S<Perl 5>
also adopted):

  $maybe ? 'yes' : 'no'

where the tokens C<'?'> and C<':'> easily blend with
other symbols in complex expressions, making harder
to distinguish what's going on without extra spaces
and layout.

To stop some common errors, it is a syntax error
to use an operator with looser precedence (such as C<'='>)
in the middle part.

  my $x;
  hmm() ??  $x = 1  !!  $x = 2;      # ERROR
  hmm() ?? ($x = 1) !! ($x = 2);     # works

Be aware that both sides have to be parenthesized. A partial
fix is even wronger:

  hmm() ?? ($x = 1) !!  $x = 2;      # parses, but WRONG

which actually means

  (
    hmm() ?? ($x = 1) !! $x
  ) = 2;

always assigning C<2> to C<$x> (because C<($x = 1)> is a valid lvalue).

Also to help catch errors by programmers used to C-derived languages
(like S<Perl 5> itself), the S<Perl 6> parser will produce an error
when a bare question mark is seen in infix position and suggest
that C<?? !!> should be used instead. That is an example
of pseudo operators that will be introduced to catch migratory
brainos at compile time, helping the transition into the new
language and common first-time mistakes by the would-be
S<Perl 6> programmers. A non-exhaustive list of such operators is
C<< postfix:{'->'} >>, C<< infix<?> >>, and C<< infix:<=~> >>.

I<Note.> That article is a mere rewriting of
L< the section on "Conditional operator precedence" from Synopsis 03
   | http://perlcabal.org/syn/S03.html#Conditional_operator_precedence >.

$Revision: 94 $

=begin comment

Keywords:
  Perl
  Perl 6
  operators
  ternary operator
  trinary operator
  conditional operator

=end comment