=head1 NAME

perldelta - what is new for perl v5.11.0

=head1 DESCRIPTION

This document describes differences between the 5.10.0 and the 5.11.0
development releases.

=head1 Incompatible Changes

=head2 Switch statement changes

The handling of complex expressions by the C<given>/C<when> switch
statement has been enhanced. There are two new cases where C<when> now
interprets its argument as a boolean, instead of an expression to be used
in a smart match:

=over 4

=item flip-flop operators

The C<..> and C<...> flip-flop operators are evaluated in boolean context,
following their usual semantics; see L<perlop/"Range Operators">.

=item defined-or operator

A compound expression involving the defined-or operator, as in
C<when (expr1 // expr2)>, will be treated as boolean if the first
expression is boolean. (This just extends the existing rule that applies
to the regular or operator, as in C<when (expr1 || expr2)>.)

=back

The next paragraph details more changes brought to the semantics to
the smart match operator, that naturally also modify the behaviour
of the switch statements where smart matching is implicitly used.

=head2 Smart match changes

=head3 Changes to type-based dispatch

The smart match operator C<~~> is no longer commutative. The behaviour of
a smart match now depends primarily on the type of its right hand
argument. Moreover, its semantics has been adjusted for greater
consistency or usefulness in several cases. While the general backwards
compatibility is maintained, several changes must be noted:

=over 4

=item *

Code references with an empty prototype are no longer treated specially.
They are passed an argument like the other code references (even if they
choose to ignore it).

=item *

C<%hash ~~ sub {}> and C<@array ~~ sub {}> now test that the subroutine
returns a true value for each key of the hash (or element of the
array), instead of passing the whole hash or array as a reference to
the subroutine.

=item *

Due to the commutativity breakage, code references are no longer
treated specially when appearing on the left of the C<~~> operator,
but like any vulgar scalar.

=item *

C<undef ~~ %hash> is always false (since C<undef> can't be a key in a
hash). No implicit conversion to C<""> is done (as was the case in perl
5.10.0).

=item *

C<$scalar ~~ @array> now always distributes the smart match across the
elements of the array. It's true if one element in @array verifies
C<$scalar ~~ $element>. This is a generalization of the old behaviour
that tested whether the array contained the scalar.

=back

The full dispatch table for the smart match operator is given in
L<perlsyn/"Smart matching in detail">.

=head3 Smart match and overloading

According to the rule of dispatch based on the rightmost argument type,
when an object overloading C<~~> appears on the right side of the
operator, the overload routine will always be called (with a 3rd argument
set to a true value, see L<overload>.) However, when the object will
appear on the left, the overload routine will be called only when the
rightmost argument is a simple scalar. This way distributivity of smart match
across arrays is not broken, as well as the other behaviours with complex
types (coderefs, hashes, regexes). Thus, writers of overloading routines
for smart match mostly need to worry only with comparing against a scalar,
and possibly with stringification overloading; the other common cases
will be automatically handled consistently.

C<~~> will now refuse to work on objects that do not overload it (in order
to avoid relying on the object's underlying structure).

=head1 Core Enhancements

=head1 The C<overloading> pragma

This pragma allows you to lexically disable or enable overloading
for some or all operations. (Yuval Kogman)

=head1 Modules and Pragmata

=head2 Pragmata Changes

=over 4

=item C<overloading>

See L</"The C<overloading> pragma"> above.

=back

=head1 Utility Changes

=head1 Documentation

=head1 Performance Enhancements

=head1 Installation and Configuration Improvements

=head1 Selected Bug Fixes

=over 4

=item C<-I> on shebang line now adds directories in front of @INC

as documented, and as does C<-I> when specified on the command-line.
(Renée Bäcker)

=back

=head1 New or Changed Diagnostics

=head1 Changed Internals

=head1 Known Problems

=head2 Platform Specific Problems

=head1 Reporting Bugs

If you find what you think is a bug, you might check the articles
recently posted to the comp.lang.perl.misc newsgroup and the perl
bug database at http://bugs.perl.org/ .  There may also be
information at http://www.perl.org/ , the Perl Home Page.

If you believe you have an unreported bug, please run the B<perlbug>
program included with your release.  Be sure to trim your bug down
to a tiny but sufficient test case.  Your bug report, along with the
output of C<perl -V>, will be sent off to perlbug@perl.org to be
analysed by the Perl porting team.

=head1 SEE ALSO

The F<Changes> file for exhaustive details on what changed.

The F<INSTALL> file for how to build Perl.

The F<README> file for general stuff.

The F<Artistic> and F<Copying> files for copyright information.

=cut