Re: Optree Generation

[p5sagit/p5-mst-13.2.git] / pod / perltodo.pod

=head1 NAME

perltodo - Perl TO-DO List

=head1 DESCRIPTION

This is a list of wishes for Perl.  Send updates to
I<perl5-porters@perl.org>.  If you want to work on any of these
projects, be sure to check the perl5-porters archives for past ideas,
flames, and propaganda.  This will save you time and also prevent you
from implementing something that Larry has already vetoed.  One set
of archives may be found at:

    http://www.xray.mpe.mpg.de/mailing-lists/perl5-porters/

=head1 assertions

Clean up and finish support for assertions. See L<assertions>.

=head1 iCOW

Sarathy and Arthur have a proposal for an improved Copy On Write which
specifically will be able to COW new ithreads. If this can be implemented
it would be a good thing.

=head1 (?{...}) closures in regexps

Fix (or rewrite) the implementation of the C</(?{...})/> closures.

=head1 A re-entrant regexp engine

This will allow the use of a regex from inside (?{ }), (??{ }) and
(?(?{ })|) constructs.

=head1 pragmata

=head2 lexical pragmas

Reimplement the mechanism of lexical pragmas to be more extensible. Fix
current pragmas that don't work well (or at all) with lexical scopes or in
run-time eval(STRING) (C<sort>, C<re>, C<encoding> for example). MJD has a
preliminary patch that implements this.

=head2 use less 'memory'

Investigate trade offs to switch out perl's choices on memory usage.
Particularly perl should be able to give memory back.

=head1 prototypes and functions

=head2 _ prototype character

Study the possibility of adding a new prototype character, C<_>, meaning
"this argument defaults to $_".

=head2 inlining autoloaded constants

Currently the optimiser can inline constants when expressed as subroutines
with prototype ($) that return a constant. Likewise, many packages wrapping
C libraries export lots of constants as subroutines which are AUTOLOADed on
demand. However, these have no prototypes, so can't be seen as constants by
the optimiser. Some way of cheaply (low syntax, low memory overhead) to the
perl compiler that a name is a constant would be great, so that it knows to
call the AUTOLOAD routine at compile time, and then inline the constant.

=head2 Finish off lvalue functions

The old perltodo notes "They don't work in the debugger, and they don't work for
list or hash slices."

=head1 Unicode and UTF8

=head2 Implicit Latin 1 => Unicode translation

Conversions from byte strings to UTF-8 currently map high bit characters
to Unicode without translation (or, depending on how you look at it, by
implicitly assuming that the byte strings are in Latin-1). As perl assumes
the C locale by default, upgrading a string to UTF-8 may change the
meaning of its contents regarding character classes, case mapping, etc.
This should probably emit a warning (at least).

=head2 UTF8 caching code

The string position/offset cache is not optional. It should be.

=head2 Unicode in Filenames

chdir, chmod, chown, chroot, exec, glob, link, lstat, mkdir, open,
opendir, qx, readdir, readlink, rename, rmdir, stat, symlink, sysopen,
system, truncate, unlink, utime, -X.  All these could potentially accept
Unicode filenames either as input or output (and in the case of system
and qx Unicode in general, as input or output to/from the shell).
Whether a filesystem - an operating system pair understands Unicode in
filenames varies.

Known combinations that have some level of understanding include
Microsoft NTFS, Apple HFS+ (In Mac OS 9 and X) and Apple UFS (in Mac
OS X), NFS v4 is rumored to be Unicode, and of course Plan 9.  How to
create Unicode filenames, what forms of Unicode are accepted and used
(UCS-2, UTF-16, UTF-8), what (if any) is the normalization form used,
and so on, varies.  Finding the right level of interfacing to Perl
requires some thought.  Remember that an OS does not implicate a
filesystem.

(The Windows -C command flag "wide API support" has been at least
temporarily retired in 5.8.1, and the -C has been repurposed, see
L<perlrun>.)

=head2 Unicode in %ENV

Currently the %ENV entries are always byte strings.

=head1 Regexps

=head2 regexp optimiser optional

The regexp optimiser is not optional. It should configurable to be, to allow
its performance to be measured, and its bugs to be easily demonstrated.

=head1 POD

=head2 POD -> HTML conversion still sucks

Which is crazy given just how simple POD purports to be, and how simple HTML
can be. It's not actually I<as> simple as it sounds, particularly with the
flexibility POD allows for C<=item>, but it would be good to improve the
visual appeal of the HTML generated, and to avoid it having any validation
errors. See also L</make HTML install work>, as the layout of installation tree
is needed to improve the cross-linking.

=head1 Misc medium sized projects

=head2 UNITCHECK

Introduce a new special block, UNITCHECK, which is run at the end of a
compilation unit (module, file, eval(STRING) block). This will correspond to
the Perl 6 CHECK. Perl 5's CHECK cannot be changed or removed because the
O.pm/B.pm backend framework depends on it.

=head2 optional optimizer

Make the peephole optimizer optional.

=head2 You WANT *how* many

Currently contexts are void, scalar and list. split has a special mechanism in
place to pass in the number of return values wanted. It would be useful to
have a general mechanism for this, backwards compatible and little speed hit.
This would allow proposals such as short circuiting sort to be implemented
as a module on CPAN.

=head2 lexical aliases

Allow lexical aliases (maybe via the syntax C<my \$alias = \$foo>.

=head2 IPv6

Clean this up. Check everything in core works

=head2 entersub XS vs Perl

At the moment pp_entersub is huge, and has code to deal with entering both
perl and XS subroutines. Subroutine implementations rarely change between 
perl and XS at run time, so investigate using 2 ops to enter subs (one for
XS, one for perl) and swap between if a sub is redefined.

=head2 @INC source filter to Filter::Simple

The second return value from a sub in @INC can be a source filter. This isn't
documented. It should be changed to use Filter::Simple, tested and documented.

=head2 bincompat functions

There are lots of functions which are retained for binary compatibility.
Clean these up. Move them to mathom.c, and don't compile for blead?

=head2 Constant folding

The peephole optimiser should trap errors during constant folding, and give
up on the folding, rather than bailing out at compile time.  It is quite
possible that the unfoldable constant is in unreachable code, eg something
akin to C<$a = 0/0 if 0;>

=head1 Tests

=head2 Make Schwern poorer

Tests for everything, At which point Schwern coughs up $500 to TPF.

=head2 test B

A test suite for the B module would be nice.

=head2 common test code for timed bailout

Write portable self destruct code for tests to stop them burning CPU in
infinite loops. Needs to avoid using alarm, as some of the tests are testing
alarm/sleep or timers.

=head1 Installation

=head2 compressed man pages

Be able to install them. This would probably need a configure test to see how
the system does compressed man pages (same directory/different directory?
same filename/different filename), as well as tweaking the F<installman> script
to compress as necessary.

=head2 Make Config.pm cope with differences between build and installed perl

Quite often vendors ship a perl binary compiled with their (pay-for)
compilers.  People install a free compiler, such as gcc. To work out how to
build extensions, Perl interrogates C<%Config>, so in this situation
C<%Config> describes compilers that aren't there, and extension building
fails. This forces people into chosing between re-compiling perl themselves
using the compiler they have, or only using modules that the vendor ships.

It would be good to find a way teach C<Config.pm> about the installation setup,
possibly involving probing at install time or later, so that the C<%Config> in
a binary distruction better describes the installed machine, when the installed
machine differs from the build machine in some significant way.

=head2 Relocatable perl

Make it possible to create a relocatable perl binary. Will need some collusion
with Config.pm. We could use a syntax of ... for location of current binary?

=head2 make HTML install work

There is an C<installhtml> target in the Makefile. It's marked as
"experimental". It would be good to get this tested, make it work reliably, and
remove the "experimental" tag. This would include

=over 4

=item 1

Checking that cross linking between various parts of the documentation works.
In particular that links work between the modules (files with POD in F<lib/>)
and the core documentation (files in F<pod/>)

=item 2

Work out how to split perlfunc into chunks, preferably one per function group,
preferably with general case code that could be used elsewhere. Challenges
here are correctly identifying the groups of functions that go together, and
making the right named external cross-links point to the right page. Things to
be aware of are C<-X>, groups such as C<getpwnam> to C<endservent>, two or
more C<=items> giving the different parameter lists, such as

    =item substr EXPR,OFFSET,LENGTH,REPLACEMENT
    
    =item substr EXPR,OFFSET,LENGTH
    
    =item substr EXPR,OFFSET

and different parameter lists having different meanings. (eg C<select>)

=back

=head2 put patchlevel in -v

Currently perl from p4/rsync ships with a patchlevel.h file that usually
defines one local patch, of the form "MAINT12345" or "RC1". The output of
perl -v doesn't report that a perl isn't an official release, and this
information can get lost in bugs reports. Because of this, the minor version
isn't bumped up until RC time, to minimise the possibility of versions of perl
escaping that believe themselves to be newer than they actually are.

It would be useful to find an elegant way to have the "this is an interim
maintenance release" or "this is a release candidate" in the terse -v output,
and have it so that it's easy for the pumpking to remove this just as the
release tarball is rolled up. This way the version pulled out of rsync would
always say "I'm a development release" and it would be safe to bump the
reported minor version as soon as a release ships, which would aid perl
developers.

=head1 Incremental things

Some tasks that don't need to get done in one big hit.

=head2 autovivification

Make all autovivification consistent w.r.t LVALUE/RVALUE and strict/no strict;

=head2 fix tainting bugs

Fix the bugs revealed by running the test suite with the C<-t> switch (via
C<make test.taintwarn>).

=head2 Make tainting consistent

Tainting would be easier to use if it didn't take documented shortcuts and allow
taint to "leak" everywhere within an expression.

=head2 Dual life everything

As part of the "dists" plan, anything that doesn't belong in the smallest perl
distribution needs to be dual lifed. Anything else can be too.

=head1 Vague things

Some more nebulous ideas

=head2 threads

=over 4

=item *

Re-implement C<:unique> in a way that is actually thread-safe

=item *

Make C<threads::shared> share aggregates properly

(these two may actually share approach, if not implementation

=back

Generally make threads more robust. See also L<iCOW>

=head2 POSIX memory footprint

Ilya observed that use POSIX; eats memory like there's no tomorrow, and at
various times worked to cut it down. There is probably still fat to cut out -
for example POSIX passes Exporter some very memory hungry data structures.

=head2 Optimize away @_

The old perltodo notes "Look at the "reification" code in C<av.c>".

=head2 switch ops

The old perltodo notes "Although we have C<Switch.pm> in core, Larry points to
the dormant C<nswitch> and C<cswitch> ops in F<pp.c>; using these opcodes would
be much faster."

=head2 Attach/detach debugger from running program

The old perltodo notes "With C<gdb>, you can attach the debugger to a running
program if you pass the process ID. It would be good to do this with the Perl
debugger on a running Perl program, although I'm not sure how it would be done."
ssh and screen do this with named pipes in tmp. Maybe we can too.

=head2 A decent benchmark

perlbench seems impervious to any recent changes made to the perl core. It would
be useful to have a reasonable general benchmarking suite that roughly
represented what current perl programs do, and measurably reported whether
tweaks to the core improve, degrade or don't really affect performance, to
guide people attempting to optimise the guts of perl.

=head2 readpipe(LIST)

system() accepts a LIST syntax (and a PROGRAM LIST syntax) to avoid
running a shell. readpipe() (the function behind qx//) could be similarly
extended.

=head2 Self ties

self ties are currently illegal because they caused too many segfaults. Maybe
the causes of these could be tracked down and self-ties on all types re-
instated.