perldelta: Add to-do note for hash changes

[perl5.git] / pod / perldelta.pod

=encoding utf8

=for to-do
23b7025ebc definitely needs to be summarised.

=head1 NAME

[ this is a template for a new perldelta file.  Any text flagged as XXX needs
to be processed before release. ]

perldelta - what is new for perl v5.17.7

=head1 DESCRIPTION

This document describes differences between the 5.17.6 release and the 5.17.7
release.

If you are upgrading from an earlier release such as 5.17.5, first read
L<perl5176delta>, which describes differences between 5.17.5 and 5.17.6.

=head1 Notice

XXX Any important notices here

=head1 Core Enhancements

=head2 $&, $` and $' are no longer slow

These three infamous variables have been redeemed and no longer slow down
your program when used.  Hence, the /p regular expression flag now does
nothing.

=head1 Security

XXX Any security-related notices go here.  In particular, any security
vulnerabilities closed should be noted here rather than in the
L</Selected Bug Fixes> section.

[ List each security issue as a =head2 entry ]

=head1 Incompatible Changes

=head2 readline() with C<$/ = \N> now reads N characters, not N bytes

Previously, when reading from a stream with I/O layers such as
C<encoding>, the readline() function, otherwise known as the C<< <> >>
operator, would read I<N> bytes from the top-most layer. [perl #79960]

Now, I<N> characters are read instead.

There is no change in behaviour when reading from streams with no
extra layers, since bytes map exactly to characters.

=head2 Lexical subroutine warnings have moved

The warning about the use of an experimental feature emitted when lexical
subroutines (added in 5.17.4) are used now happens when the subroutine
itself is declared, not when the "lexical_subs" feature is activated via
C<use feature>.

This stops C<use feature ':all'> from warning, but causes
C<my sub foo; my sub bar> to warn twice.

=head2 Overridden C<glob> is now passed one argument

C<glob> overrides used to be passed a magical undocumented second argument
that identified the caller.  Nothing on CPAN was using this, and it got in
the way of a bug fix, so it was removed.  If you really need to identify
the caller, see L<Devel::Callsite> on CPAN.

=head1 Deprecations

XXX Any deprecated features, syntax, modules etc. should be listed here.  In
particular, deprecated modules should be listed here even if they are listed as
an updated module in the L</Modules and Pragmata> section.

[ List each deprecation as a =head2 entry ]

=head2 Lexical $_ is now deprecated

Since it was introduced in Perl 5.10, it has caused much confusion with no
obvious solution:

=over

=item *

Various modules (e.g., List::Util) expect callback routines to use the
global $_.  C<use List::Util 'first'; my $_; first { $_ == 1 } @list> does
not work as one would expect.

=item *

A C<my $_> declaration earlier in the same file can cause confusing closure
warnings.

=item *

The "_" subroutine prototype character allows called subroutines to access
your lexical $_, so it is not really private after all.

=item *

Nevertheless, subroutines with a "(@)" prototype and methods cannot access
the caller's lexical $_, unless they are written in XS.

=item *

But even XS routines cannot access a lexical $_ declared, not in the
calling subroutine, but in an outer scope, iff that subroutine happened not
to mention $_ or use any operators that default to $_.

=back

=head2 Various XS-callable functions are now deprecated

The following functions will be removed from a future version of Perl,
and should not be used.  With participating C compilers (e.g., gcc),
compiling any file that uses any of these will generate a warning.
These were not intended for public use; there are equivalent, faster,
macros for most of them.  See L<perlapi/Character classes>:
C<is_uni_ascii>,
C<is_uni_ascii_lc>,
C<is_uni_blank>,
C<is_uni_blank_lc>,
C<is_uni_cntrl>,
C<is_uni_cntrl_lc>,
C<is_uni_idfirst_lc>,
C<is_uni_space>,
C<is_uni_space_lc>,
C<is_uni_xdigit>,
C<is_uni_xdigit_lc>,
C<is_utf8_ascii>,
C<is_utf8_blank>,
C<is_utf8_cntrl>,
C<is_utf8_idcont>,
C<is_utf8_idfirst>,
C<is_utf8_perl_space>,
C<is_utf8_perl_word>,
C<is_utf8_posix_digit>,
C<is_utf8_space>,
C<is_utf8_xdigit>.
C<is_utf8_xidcont>,
C<is_utf8_xidfirst>,
C<to_uni_lower_lc>,
C<to_uni_title_lc>,
and
C<to_uni_upper_lc>.

=head1 Performance Enhancements

XXX Changes which enhance performance without changing behaviour go here.
There may well be none in a stable release.

[ List each enhancement as a =item entry ]

=over 4

=item *

Perl has a new copy-on-write mechanism that avoids the need to copy the
internal string buffer when assigning from one scalar to another.  This
makes copying large strings appear much faster.  Modifying one of the two
(or more) strings after an assignment will force a copy internally.  This
makes it unnecessary to pass strings by reference for efficiency.

=back

=head1 Modules and Pragmata

XXX All changes to installed files in F<cpan/>, F<dist/>, F<ext/> and F<lib/>
go here.  If Module::CoreList is updated, generate an initial draft of the
following sections using F<Porting/corelist-perldelta.pl>, which prints stub
entries to STDOUT.  Results can be pasted in place of the '=head2' entries
below.  A paragraph summary for important changes should then be added by hand.
In an ideal world, dual-life modules would have a F<Changes> file that could be
cribbed.

[ Within each section, list entries as a =item entry ]

=head2 New Modules and Pragmata

=over 4

=item *

XXX

=back

=head2 Updated Modules and Pragmata

=over 4

=item *

L<File::DosGlob> has been upgraded from version 1.08 to 1.09.  The internal
cache of file names that it keeps for each caller is now freed when that
caller is freed.  This means
C<< use File::DosGlob 'glob'; eval 'scalar <*>' >> no longer leaks memory.

=item *

L<File::Glob> has been upgraded from version 1.18 to 1.19.  File::Glob has
had exactly the same fix as File::DosGlob.  Since it is what Perl's own
C<glob> operator itself uses (except on VMS), this means
C<< eval 'scalar <*>' >> no longer leaks.

=item *

L<GDBM_File> has been upgraded from version 1.14 to 1.15. The undocumented
optional fifth parameter to C<TIEHASH> has been removed. This was intended
to provide control of the callback used by C<gdbm*> functions in case of
fatal errors (such as filesystem problems), but did not work (and could
never have worked). No code on CPAN even attempted to use it. The callback
is now always the previous default, C<croak>. Problems on some platforms with
how the C<C> C<croak> function is called have also been resolved.

=back

=head2 Removed Modules and Pragmata

=over 4

=item *

XXX

=back

=head1 Documentation

XXX Changes to files in F<pod/> go here.  Consider grouping entries by
file and be sure to link to the appropriate page, e.g. L<perlfunc>.

=head2 New Documentation

XXX Changes which create B<new> files in F<pod/> go here.

=head3 L<XXX>

XXX Description of the purpose of the new file here

=head2 Changes to Existing Documentation

XXX Changes which significantly change existing files in F<pod/> go here.
However, any changes to F<pod/perldiag.pod> should go in the L</Diagnostics>
section.

=head3 L<perlapi/Character classes>

=over 4

=item *

There are quite a few macros callable from XS modules that classify
characters into things like alphabetic, punctuation, etc.  More of these
are now documented, including ones which work on characters whose code
points are outside the Latin-1 range.

=back

=head1 Diagnostics

The following additions or changes have been made to diagnostic output,
including warnings and fatal error messages.  For the complete list of
diagnostic messages, see L<perldiag>.

XXX New or changed warnings emitted by the core's C<C> code go here.  Also
include any changes in L<perldiag> that reconcile it to the C<C> code.

=head2 New Diagnostics

XXX Newly added diagnostic messages go under here, separated into New Errors
and New Warnings

=head3 New Errors

=over 4

=item *

XXX L<message|perldiag/"message">

=back

=head3 New Warnings

=over 4

=item *

XXX L<message|perldiag/"message">

=back

=head2 Changes to Existing Diagnostics

XXX Changes (i.e. rewording) of diagnostic messages go here

=over 4

=item *

L<Constant(%s): Call to &{$^H{%s}} did not return a defined value|perldiag/Constant(%s): Call to &{$^H{%s}} did not return a defined value>

Constant overloading that returns C<undef> results in this error message.
For numeric constants, it used to say "Constant(undef)".  "undef" has been
replaced with the number itself. 

=back

=head1 Utility Changes

XXX Changes to installed programs such as F<perlbug> and F<xsubpp> go here.
Most of these are built within the directories F<utils> and F<x2p>.

[ List utility changes as a =head3 entry for each utility and =item
entries for each change
Use L<XXX> with program names to get proper documentation linking. ]

=head3 L<XXX>

=over 4

=item *

XXX

=back

=head1 Configuration and Compilation

XXX Changes to F<Configure>, F<installperl>, F<installman>, and analogous tools
go here.  Any other changes to the Perl build process should be listed here.
However, any platform-specific changes should be listed in the
L</Platform Support> section, instead.

[ List changes as a =item entry ].

=over 4

=item *

XXX

=back

=head1 Testing

XXX Any significant changes to the testing of a freshly built perl should be
listed here.  Changes which create B<new> files in F<t/> go here as do any
large changes to the testing harness (e.g. when parallel testing was added).
Changes to existing files in F<t/> aren't worth summarizing, although the bugs
that they represent may be covered elsewhere.

[ List each test improvement as a =item entry ]

=over 4

=item *

XXX

=back

=head1 Platform Support

XXX Any changes to platform support should be listed in the sections below.

[ Within the sections, list each platform as a =item entry with specific
changes as paragraphs below it. ]

=head2 New Platforms

XXX List any platforms that this version of perl compiles on, that previous
versions did not.  These will either be enabled by new files in the F<hints/>
directories, or new subdirectories and F<README> files at the top level of the
source tree.

=over 4

=item XXX-some-platform

XXX

=back

=head2 Discontinued Platforms

=over 4

=item BeOS

Support for BeOS has been removed.

=back

=head2 Platform-Specific Notes

XXX List any changes for specific platforms.  This could include configuration
and compilation changes or changes in portability/compatibility.  However,
changes within modules for platforms should generally be listed in the
L</Modules and Pragmata> section.

=over 4

=item XXX-some-platform

XXX

=back

=head1 Internal Changes

XXX Changes which affect the interface available to C<XS> code go here.  Other
significant internal changes for future core maintainers should be noted as
well.

[ List each change as a =item entry ]

=over 4

=item *

SvUPGRADE() is no longer an expression. Originally this macro (and its
underlying function, sv_upgrade()) were documented as boolean, although
in reality they always croaked on error and never returned false. In 2005
the documentation was updated to specify a void return value, but
SvUPGRADE() was left always returning 1 for backwards compatibility. This
has now been removed, and SvUPGRADE() is now a statement with no return
value.

So this is now a syntax error:

    if (!SvUPGRADE(sv)) { croak(...); }

If you have code like that, simply replace it with

    SvUPGRADE(sv);

or to to avoid compiler warnings with older perls, possibly

    (void)SvUPGRADE(sv);

=item *

Perl has a new copy-on-write mechanism that allows any SvPOK scalar to be
upgraded to a copy-on-write scalar.  A reference count on the string buffer
is stored in the string buffer itself.

This breaks a few XS modules by allowing copy-on-write scalars to go
through code paths that never encountered them before.

This behaviour can still be disabled by running F<Configure> with
B<-Accflags=-DPERL_NO_COW>.  This option will probably be removed in Perl
5.20.

=item *

Copy-on-write no longer uses the SvFAKE and SvREADONLY flags.  Hence,
SvREADONLY indicates a true read-only SV.

Use the SvIsCOW macro (as before) to identify a copy-on-write scalar.

=item *

C<PL_sawampersand> is now a constant.  The switch this variable provided
(to enable/disable the pre-match copy depending on whether C<$&> had been
seen) has been removed and replaced with copy-on-write, eliminating a few
bugs.

The previous behaviour can still be enabled by running F<Configure> with
B<-Accflags=-DPERL_SAWAMPERSAND>.

=item *

PL_glob_index is gone.

=back

=head1 Selected Bug Fixes

XXX Important bug fixes in the core language are summarized here.  Bug fixes in
files in F<ext/> and F<lib/> are best summarized in L</Modules and Pragmata>.

[ List each fix as a =item entry ]

=over 4

=item *

C<sort {undef} ...> under fatal warnings no longer crashes.  It started
crashing in Perl 5.16.

=item *

Stashes blessed into each other
(C<bless \%Foo::, 'Bar'; bless \%Bar::, 'Foo'>) no longer result in double
frees.  This bug started happening in Perl 5.16.

=item *

Numerous memory leaks have been fixed, mostly involving fatal warnings and
syntax errors.

=item *

Lexical constants (C<my sub answer () { 42 }>) no longer cause double
frees.

=item *

Constant subroutine redefinition warns by default, but lexical constants
were accidentally exempt from default warnings.  This has been corrected.

=item *

Some failed regular expression matches such as C<'f' =~ /../g> were not
resetting C<pos>.  Also, "match-once" patterns (C<m?...?g>) failed to reset
it, too, when invoked a second time [perl #23180].

=item *

Accessing C<$&> after a pattern match now works if it had not been seen
before the match.  I.e., this applies to C<${'&'}> (under C<no strict>) and
C<eval '$&'>.  The same applies to C<$'> and C<$`> [perl #4289].

=item *

Several bugs involving C<local *ISA> and C<local *Foo::> causing stale
MRO caches have been fixed.  

=item *

Defining a subroutine when its typeglob has been aliased no longer results
in stale method caches.  This bug was introduced in Perl 5.10.

=item *

Localising a typeglob containing a subroutine when the typeglob's package
has been deleted from its parent stash no longer produces an error.  This
bug was introduced in Perl 5.14.

=item *

Under some circumstances, C<local *method=...> would fail to reset method
caches upon scope exit.

=item *

C</[.foo.]/> is no longer an error, but produces a warning (as before) and
is treated as C</[.fo]/> [perl #115818].

=item *

C<goto $tied_var> now calls FETCH before deciding what type of goto
(subroutine or label) this is.

=item *

Renaming packages through glob assignment
(C<*Foo:: = *Bar::; *Bar:: = *Baz::>) in combination with C<m?...?> and
C<reset> no longer makes threaded builds crash.

=item *

An earlier release in the 5.17.x series could crash if user code prevented
_charnames from loading via C<$INC{'_charnames.pm'}++>.

=back

=head1 Known Problems

XXX Descriptions of platform agnostic bugs we know we can't fix go here.  Any
tests that had to be C<TODO>ed for the release would be noted here.  Unfixed
platform specific bugs also go here.

[ List each fix as a =item entry ]

=over 4

=item *

XXX

=back

=head1 Obituary

XXX If any significant core contributor has died, we've added a short obituary
here.

=head1 Acknowledgements

XXX Generate this with:

  perl Porting/acknowledgements.pl v5.17.6..HEAD

=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://rt.perl.org/perlbug/ .  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 L<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.

If the bug you are reporting has security implications, which make it
inappropriate to send to a publicly archived mailing list, then please send it
to perl5-security-report@perl.org.  This points to a closed subscription
unarchived mailing list, which includes all the core committers, who will be
able to help assess the impact of issues, figure out a resolution, and help
co-ordinate the release of patches to mitigate or fix the problem across all
platforms on which Perl is supported.  Please only use this address for
security issues in the Perl core, not for modules independently distributed on
CPAN.

=head1 SEE ALSO

The F<Changes> file for an explanation of how to view 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