=encoding utf-8

=head1 NAME

perlexperiment - A listing of experimental features in Perl

=head1 DESCRIPTION

This document lists the current and past experimental features in the perl
core. Although all of these are documented with their appropriate topics,
this succinct listing gives you an overview and basic facts about their
status.

So far we've merely tried to find and list the experimental features and infer
their inception, versions, etc. There's a lot of speculation here.

=head2 Current experiments

=over 8

=item Pluggable keywords

Introduced in Perl 5.11.2

See L<perlapi/PL_keyword_plugin> for the mechanism.

The ticket for this experiment is
L<[perl #13199]|https://github.com/Perl/perl5/issues/13199>.

=item Aliasing via reference

Introduced in Perl 5.22.0

Using this feature triggers warnings in the category
C<experimental::refaliasing>.

The ticket for this experiment is
L<[perl #14150]|https://github.com/Perl/perl5/issues/14150>.

See also: L<perlref/Assigning to References>

=item use re 'strict';

Introduced in Perl 5.22.0

Using this feature triggers warnings in the category
C<experimental::re_strict>.

The ticket for this experiment is
L<[perl #18755]|https://github.com/Perl/perl5/issues/18755>

See L<re/'strict' mode>

=item Declaring a reference to a variable

Introduced in Perl 5.26.0

Using this feature triggers warnings in the category
C<experimental::declared_refs>.

The ticket for this experiment is
L<[perl #15458]|https://github.com/Perl/perl5/issues/15458>.

See also: L<perlref/Declaring a Reference to a Variable>

=item There is an C<installhtml> target in the Makefile.

The ticket for this experiment is
L<[perl #12726]|https://github.com/Perl/perl5/issues/12726>.

=item (Limited) Variable-length look-behind

Introduced in Perl 5.30.0.

Variability of up to 255 characters is handled.

Using this feature triggers warnings in the category
C<experimental::vlb>.

The ticket for this experiment is
L<[perl #18756]|https://github.com/Perl/perl5/issues/18756>.

See also: L<perlre/(*positive_lookbehind:I<pattern>)> and
L<perlre/(*negative_lookbehind:I<pattern>)>

=item Unicode private use character hooks

Introduced in Perl 5.30.0.

This feature is part of an interface intended for internal and experimental
use by the perl5 developers.  You are unlikely to encounter it in the wild.

Using this feature triggers warnings in the category
C<experimental::private_use>.

The ticket for this experiment is
L<[perl #18758]|https://github.com/Perl/perl5/issues/18758>.

=item Unicode property wildcards

Introduced in Perl 5.30.0.

This feature allows regular expression matching against Unicode character
properties to be expressed more concisely.

Using this feature triggers warnings in the category
C<experimental::uniprop_wildcards>.

The ticket for this experiment is
L<[perl #18759]|https://github.com/Perl/perl5/issues/18759>.

=item try/catch control structure

Introduced in Perl 5.34.0.

Using the optional C<finally> block part of this feature triggers warnings in
the category C<experimental::try>.

The ticket for this experiment is
L<[perl #18760]|https://github.com/Perl/perl5/issues/18760>

=item Use of C<@_> within subroutine signatures

Introduced in Perl 5.36.0 as part of a reduction in the scope of experimental
subroutine signatures.

Using the default arguments array (C<@_>) within a subroutine that uses
signatures will emit a warning in the category
C<experimental::args_array_with_signatures>. This includes C<@_> directly,
elements of it such as C<$_[$index]>, or situations where the default
arguments array is accessed implicitly such as C<shift> or C<pop> without
arguments.

=item The builtin namespace

Introduced in Perl 5.36.0.

Using certain functions of this feature triggers warnings in the category
C<experimental::builtin>.

In Perl 5.36.0, a new namespace, C<builtin>, was created for new core functions
that will not be present in every namespace, but will be available for
importing.  The namespace itself was considered experimental until Perl 5.39.2.
Some specific functions within it remain experimental.

The ticket for this experiment is
L<[perl #19764]|https://github.com/Perl/perl5/issues/19764>.

=item The defer block modifier

Introduced in Perl 5.36.0

Using this feature triggers warnings in the category C<experimental::defer>.

This feature adds a new kind of block, a C<defer> block, which will not be
executed until the containing block is being exited.

The ticket for this experiment is
L<[perl #17949]|https://github.com/Perl/perl5/issues/17949>.

=item Extra paired delimiters for quote-like operators

Introduced in Perl 5.36.0

Using this feature triggers warnings in the category
C<experimental::extra_paired_delimiters>.

This feature allows for many non-ASCII pairs of mirroring delimiters, for
example:

    my @array = qw« tinker tailor soldier spy »;

The ticket for this experiment is
L<[perl #19765]|https://github.com/Perl/perl5/issues/19765>.

=item New object system and C<class> syntax

Introduced in Perl 5.37.9.

Using this feature triggers warnings in the category C<experimental::class>.

This feature enables the new object system and its associated keywords
C<class>, C<field>, C<method>.

The ticket for this experiment is
L<[perl #22139]|https://github.com/Perl/perl5/issues/22139>.

=item C<any> and C<all> list processing operators

Introduced in Perl 5.41.7.

Using these features triggers warnings in the categories C<experimental::any>
or C<experiment::all>.

This adds new list processing operators named C<any> and C<all>, each as their
own named feature.

The ticket for this experiment is
L<[perl #22794]|https://github.com/Perl/perl5/issues/22794>.

=back

=head2 Accepted features

These features were so wildly successful and played so well with others that
we decided to remove their experimental status and admit them as full, stable
features in the world of Perl, lavishing all the benefits and luxuries thereof.
They are also awarded +5 Stability and +3 Charisma.

=over 8

=item 64-bit support

Introduced in Perl 5.005

=item die accepts a reference

Introduced in Perl 5.005

=item DB module

Introduced in Perl 5.6.0

See also L<perldebug>, L<perldebtut>

=item Weak references

Introduced in Perl 5.6.0

=item Internal file glob

Introduced in Perl 5.6.0

=item fork() emulation

Introduced in Perl 5.6.1

See also L<perlfork>

=item -Dusemultiplicity -Duseithreads

Introduced in Perl 5.6.0

Accepted in Perl 5.8.0

=item Support for long doubles

Introduced in Perl 5.6.0

Accepted in Perl 5.8.1

=item The C<\N> regex character class

The C<\N> character class, not to be confused with the named character
sequence C<\N{NAME}>, denotes any non-newline character in a regular
expression.

Introduced in Perl 5.12

Exact version of acceptance unclear, but no later than Perl 5.18.

=item C<(?{code})> and C<(??{ code })>

Introduced in Perl 5.6.0

Accepted in Perl 5.20.0

See also L<perlre>

=item Linux abstract Unix domain sockets

Introduced in Perl 5.9.2

Accepted before Perl 5.20.0.  The Socket library is now primarily maintained
on CPAN, rather than in the perl core.

See also L<Socket>

=item Lvalue subroutines

Introduced in Perl 5.6.0

Accepted in Perl 5.20.0

See also L<perlsub>

=item Backtracking control verbs

C<(*ACCEPT)>

Introduced in Perl 5.10

Accepted in Perl 5.20.0

=item The C<:pop> IO pseudolayer

See also L<perlrun/PERLIO>

Accepted in Perl 5.20.0

=item C<\s> in regexp matches vertical tab

Accepted in Perl 5.22.0

=item Postfix dereference syntax

Introduced in Perl 5.20.0

Accepted in Perl 5.24.0

=item Lexical subroutines

Introduced in Perl 5.18.0

Accepted in Perl 5.26.0

=item String- and number-specific bitwise operators

Introduced in Perl 5.22.0

Accepted in Perl 5.28.0

=item Alphabetic assertions

Introduced in Perl 5.28.0

Accepted in Perl 5.32.0

=item Script runs

Introduced in Perl 5.28.0

Accepted in Perl 5.32.0

=item The infix C<isa> operator

Introduced in Perl 5.32.0

Accepted in Perl 5.36.0

=item Subroutine signatures

Introduced in Perl 5.20.0

Accepted in Perl 5.36.0

=item Regular Expression Set Operations

Introduced in Perl 5.18

Accepted in Perl 5.36

See : L<perlrecharclass/Extended Bracketed Character Classes>

=item try/catch control structure

Introduced in Perl 5.34.0.

Accepted in Perl 5.40 when not using the optional C<finally> block.

=item The "const" attribute

Introduced in Perl 5.22.0

Accepted in Perl 5.40

See also: L<perlsub/Constant Functions>

=item for loop with multiple iteration variables

Introduced in Perl 5.36.0.

Accepted in Perl 5.40.

This feature enables a parenthesized list of iteration variables for C<for>
rather than a single variable.

=back

=head2 Removed features

These features are no longer considered experimental and their functionality
has disappeared. It's your own fault if you wrote production programs using
these features after we explicitly told you not to (see L<perlpolicy>).

=over 8

=item 5.005-style threading

Introduced in Perl 5.005

Removed in Perl 5.10

=item perlcc

Introduced in Perl 5.005

Moved from Perl 5.9.0 to CPAN

=item The pseudo-hash data type

Introduced in Perl 5.6.0

Removed in Perl 5.9.0

=item GetOpt::Long Options can now take multiple values at once (experimental)

C<Getopt::Long> upgraded to version 2.35

Removed in Perl 5.8.8

=item Assertions

The C<-A> command line switch

Introduced in Perl 5.9.0

Removed in Perl 5.9.5

=item Test::Harness::Straps

Moved from Perl 5.10.1 to CPAN

=item C<legacy>

The experimental C<legacy> pragma was swallowed by the C<feature> pragma.

Introduced in Perl 5.11.2

Removed in Perl 5.11.3

=item Lexical C<$_>

Using this feature triggered warnings in the category
C<experimental::lexical_topic>.

Introduced in Perl 5.10.0

Removed in Perl 5.24.0

=item Array and hash container functions accept references

Using this feature triggered warnings in the category
C<experimental::autoderef>.

Superseded by L</Postfix dereference syntax>.

Introduced in Perl 5.14.0

Removed in Perl 5.24.0

=item C<our> can have an experimental optional attribute C<unique>

Introduced in Perl 5.8.0

Deprecated in Perl 5.10.0

Removed in Perl 5.28.0

=item The C<:win32> IO pseudolayer

Introduced in Perl 5.8.0 (or before)

Removed in Perl 5.36.0

=back

=head2 Features abandoned in place

These are experiments that are considered failures, but due to mass
use have been left in place.

The switch and smartmatch features are likely to be the only ones
which will ever fall into this category.  They were introduced before
we had a well-defined experimental feature process.

These are unlikely to be updated with behavioural changes.

=over

=item Smart match (C<~~>)

Introduced in Perl 5.10.0

Modified in Perl 5.10.1, 5.12.0

Deprecated in 5.38.0

Scheduled for removal in 5.42.0, but this was reversed.

Enabled by default, and in all feature bundles up to 5.40, disabled by
default in feature bundle 5.42, but can be enabled with C<use feature
"smartmatch">.

The ticket for this experiment is
L<[perl #13173]|https://github.com/Perl/perl5/issues/13173>.

=item Switch (given/when)

Introduced in Perl 5.10.0

Deprecated in 5.38.0

Scheduled for removal in 5.42.0, but this was reversed.

Enabled by feature bundles 5.10 through 5.34, but can be enabled with
C<use feature "switch">.

The ticket for this experiment is
L<[perl #13173]|https://github.com/Perl/perl5/issues/13173>.

=back

=head1 SEE ALSO

For a complete list of features check L<feature>.

=head1 AUTHORS

brian d foy C<< <briandfoy@pobox.com> >>

SE<eacute>bastien Aperghis-Tramoni C<< <saper@cpan.org> >>

=head1 COPYRIGHT

Copyright 2010, brian d foy C<< <briandfoy@pobox.com> >>

=head1 LICENSE

You can use and redistribute this document under the same terms as Perl
itself.

=cut
