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1=head1 NAME
2
3perltodo - Perl TO-DO List
4
5=head1 DESCRIPTION
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7This is a list of wishes for Perl. The tasks we think are smaller or easier
8are listed first. Anyone is welcome to work on any of these, but it's a good
9idea to first contact I<perl5-porters@perl.org> to avoid duplication of
10effort. By all means contact a pumpking privately first if you prefer.
e50bb9a1 11
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12Whilst patches to make the list shorter are most welcome, ideas to add to
13the list are also encouraged. Check the perl5-porters archives for past
14ideas, and any discussion about them. One set of archives may be found at:
e50bb9a1 15
0bdfc961 16 http://www.xray.mpe.mpg.de/mailing-lists/perl5-porters/
938c8732 17
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18What can we offer you in return? Fame, fortune, and everlasting glory? Maybe
19not, but if your patch is incorporated, then we'll add your name to the
20F<AUTHORS> file, which ships in the official distribution. How many other
21programming languages offer you 1 line of immortality?
938c8732 22
4e577f8b 23=head1 The roadmap to 5.10
938c8732 24
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25The roadmap to 5.10 envisages feature based releases, as various items in this
26TODO are completed.
27
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28=head2 Needed for a 5.9.4 release
29
30=over
31
32=item *
c1f116f6 33
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34Implement L</state variables> (mostly done currently)
35
36=item *
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37
38Review assertions. Review syntax to combine assertions. Assertions could take
39advantage of the lexical pragmas work. L</What hooks would assertions need?>
4e577f8b 40
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41=item *
42
25441cc9 43C<encoding> should be turned into a lexical pragma (probably).
860f190d 44
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45=back
46
47=head2 Needed for a 5.9.5 release
48
49=over
50
51=item *
c1f116f6 52
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53Implement L</_ prototype character>
54
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55=item *
56
57Review smart match semantics in light of Perl 6 developments.
58
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59=back
60
61=head2 Needed for a 5.9.6 release
62
63Stabilisation. If all goes well, this will be the equivalent of a 5.10-beta.
e50bb9a1 64
0bdfc961 65=head1 Tasks that only need Perl knowledge
e50bb9a1 66
0bdfc961 67=head2 common test code for timed bail out
e50bb9a1 68
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69Write portable self destruct code for tests to stop them burning CPU in
70infinite loops. This needs to avoid using alarm, as some of the tests are
71testing alarm/sleep or timers.
e50bb9a1 72
0bdfc961 73=head2 POD -> HTML conversion in the core still sucks
e50bb9a1 74
938c8732 75Which is crazy given just how simple POD purports to be, and how simple HTML
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76can be. It's not actually I<as> simple as it sounds, particularly with the
77flexibility POD allows for C<=item>, but it would be good to improve the
78visual appeal of the HTML generated, and to avoid it having any validation
79errors. See also L</make HTML install work>, as the layout of installation tree
80is needed to improve the cross-linking.
938c8732 81
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82The addition of C<Pod::Simple> and its related modules may make this task
83easier to complete.
84
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85=head2 Parallel testing
86
87The core regression test suite is getting ever more comprehensive, which has
88the side effect that it takes longer to run. This isn't so good. Investigate
89whether it would be feasible to give the harness script the B<option> of
90running sets of tests in parallel. This would be useful for tests in
91F<t/op/*.t> and F<t/uni/*.t> and maybe some sets of tests in F<lib/>.
92
93Questions to answer
94
95=over 4
96
97=item 1
98
99How does screen layout work when you're running more than one test?
100
101=item 2
102
103How does the caller of test specify how many tests to run in parallel?
104
105=item 3
106
107How do setup/teardown tests identify themselves?
108
109=back
110
111Pugs already does parallel testing - can their approach be re-used?
112
0bdfc961 113=head2 Make Schwern poorer
e50bb9a1 114
613bd4f7 115We should have tests for everything. When all the core's modules are tested,
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116Schwern has promised to donate to $500 to TPF. We may need volunteers to
117hold him upside down and shake vigorously in order to actually extract the
118cash.
3958b146 119
0bdfc961 120See F<t/lib/1_compile.t> for the 3 remaining modules that need tests.
e50bb9a1 121
0bdfc961 122=head2 Improve the coverage of the core tests
e50bb9a1 123
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124Use Devel::Cover to ascertain the core's test coverage, then add tests that
125are currently missing.
30222c0f 126
0bdfc961 127=head2 test B
e50bb9a1 128
0bdfc961 129A full test suite for the B module would be nice.
e50bb9a1 130
0bdfc961 131=head2 A decent benchmark
e50bb9a1 132
617eabfa 133C<perlbench> seems impervious to any recent changes made to the perl core. It
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134would be useful to have a reasonable general benchmarking suite that roughly
135represented what current perl programs do, and measurably reported whether
136tweaks to the core improve, degrade or don't really affect performance, to
137guide people attempting to optimise the guts of perl. Gisle would welcome
138new tests for perlbench.
6168cf99 139
0bdfc961 140=head2 fix tainting bugs
6168cf99 141
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142Fix the bugs revealed by running the test suite with the C<-t> switch (via
143C<make test.taintwarn>).
e50bb9a1 144
0bdfc961 145=head2 Dual life everything
e50bb9a1 146
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147As part of the "dists" plan, anything that doesn't belong in the smallest perl
148distribution needs to be dual lifed. Anything else can be too. Figure out what
149changes would be needed to package that module and its tests up for CPAN, and
150do so. Test it with older perl releases, and fix the problems you find.
e50bb9a1 151
0bdfc961 152=head2 Improving C<threads::shared>
722d2a37 153
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154Investigate whether C<threads::shared> could share aggregates properly with
155only Perl level changes to shared.pm
722d2a37 156
0bdfc961 157=head2 POSIX memory footprint
e50bb9a1 158
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159Ilya observed that use POSIX; eats memory like there's no tomorrow, and at
160various times worked to cut it down. There is probably still fat to cut out -
161for example POSIX passes Exporter some very memory hungry data structures.
e50bb9a1 162
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163=head2 embed.pl/makedef.pl
164
165There is a script F<embed.pl> that generates several header files to prefix
166all of Perl's symbols in a consistent way, to provide some semblance of
167namespace support in C<C>. Functions are declared in F<embed.fnc>, variables
168in F<interpvar.h> and F<thrdvar.h>. Quite a few of the functions and variables
169are conditionally declared there, using C<#ifdef>. However, F<embed.pl>
170doesn't understand the C macros, so the rules about which symbols are present
171when is duplicated in F<makedef.pl>. Writing things twice is bad, m'kay.
172It would be good to teach C<embed.pl> to understand the conditional
173compilation, and hence remove the duplication, and the mistakes it has caused.
e50bb9a1 174
e50bb9a1 175
e50bb9a1 176
e50bb9a1 177
adebf063 178
adebf063 179
0bdfc961 180=head1 Tasks that need a little sysadmin-type knowledge
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182Or if you prefer, tasks that you would learn from, and broaden your skills
183base...
e50bb9a1 184
cd793d32 185=head2 make HTML install work
e50bb9a1 186
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187There is an C<installhtml> target in the Makefile. It's marked as
188"experimental". It would be good to get this tested, make it work reliably, and
189remove the "experimental" tag. This would include
190
191=over 4
192
193=item 1
194
195Checking that cross linking between various parts of the documentation works.
196In particular that links work between the modules (files with POD in F<lib/>)
197and the core documentation (files in F<pod/>)
198
199=item 2
200
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201Work out how to split C<perlfunc> into chunks, preferably one per function
202group, preferably with general case code that could be used elsewhere.
203Challenges here are correctly identifying the groups of functions that go
204together, and making the right named external cross-links point to the right
205page. Things to be aware of are C<-X>, groups such as C<getpwnam> to
206C<endservent>, two or more C<=items> giving the different parameter lists, such
207as
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208
209 =item substr EXPR,OFFSET,LENGTH,REPLACEMENT
adebf063 210 =item substr EXPR,OFFSET,LENGTH
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211 =item substr EXPR,OFFSET
212
213and different parameter lists having different meanings. (eg C<select>)
214
215=back
3a89a73c 216
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217=head2 compressed man pages
218
219Be able to install them. This would probably need a configure test to see how
220the system does compressed man pages (same directory/different directory?
221same filename/different filename), as well as tweaking the F<installman> script
222to compress as necessary.
223
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224=head2 Add a code coverage target to the Makefile
225
226Make it easy for anyone to run Devel::Cover on the core's tests. The steps
227to do this manually are roughly
228
229=over 4
230
231=item *
232
233do a normal C<Configure>, but include Devel::Cover as a module to install
234(see F<INSTALL> for how to do this)
235
236=item *
237
238 make perl
239
240=item *
241
242 cd t; HARNESS_PERL_SWITCHES=-MDevel::Cover ./perl -I../lib harness
243
244=item *
245
246Process the resulting Devel::Cover database
247
248=back
249
250This just give you the coverage of the F<.pm>s. To also get the C level
251coverage you need to
252
253=over 4
254
255=item *
256
257Additionally tell C<Configure> to use the appropriate C compiler flags for
258C<gcov>
259
260=item *
261
262 make perl.gcov
263
264(instead of C<make perl>)
265
266=item *
267
268After running the tests run C<gcov> to generate all the F<.gcov> files.
269(Including down in the subdirectories of F<ext/>
270
271=item *
272
273(From the top level perl directory) run C<gcov2perl> on all the C<.gcov> files
274to get their stats into the cover_db directory.
275
276=item *
277
278Then process the Devel::Cover database
279
280=back
281
282It would be good to add a single switch to C<Configure> to specify that you
283wanted to perform perl level coverage, and another to specify C level
284coverage, and have C<Configure> and the F<Makefile> do all the right things
285automatically.
286
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287=head2 Make Config.pm cope with differences between build and installed perl
288
289Quite often vendors ship a perl binary compiled with their (pay-for)
290compilers. People install a free compiler, such as gcc. To work out how to
291build extensions, Perl interrogates C<%Config>, so in this situation
292C<%Config> describes compilers that aren't there, and extension building
293fails. This forces people into choosing between re-compiling perl themselves
294using the compiler they have, or only using modules that the vendor ships.
295
296It would be good to find a way teach C<Config.pm> about the installation setup,
297possibly involving probing at install time or later, so that the C<%Config> in
298a binary distribution better describes the installed machine, when the
299installed machine differs from the build machine in some significant way.
300
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301=head2 linker specification files
302
303Some platforms mandate that you provide a list of a shared library's external
304symbols to the linker, so the core already has the infrastructure in place to
305do this for generating shared perl libraries. My understanding is that the
306GNU toolchain can accept an optional linker specification file, and restrict
307visibility just to symbols declared in that file. It would be good to extend
308F<makedef.pl> to support this format, and to provide a means within
309C<Configure> to enable it. This would allow Unix users to test that the
310export list is correct, and to build a perl that does not pollute the global
311namespace with private symbols.
312
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313=head2 Cross-compile support
314
315Currently C<Configure> understands C<-Dusecrosscompile> option. This option
316arranges for building C<miniperl> for TARGET machine, so this C<miniperl> is
317assumed then to be copied to TARGET machine and used as a replacement of full
318C<perl> executable.
319
320This should be done litle differently. Namely C<miniperl> should be built for
321HOST and then full C<perl> with extensions should be compiled for TARGET.
8523e164 322
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323
324
325=head1 Tasks that need a little C knowledge
326
327These tasks would need a little C knowledge, but don't need any specific
328background or experience with XS, or how the Perl interpreter works
329
330=head2 Make it clear from -v if this is the exact official release
89007cb3 331
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332Currently perl from C<p4>/C<rsync> ships with a F<patchlevel.h> file that
333usually defines one local patch, of the form "MAINT12345" or "RC1". The output
334of perl -v doesn't report that a perl isn't an official release, and this
89007cb3 335information can get lost in bugs reports. Because of this, the minor version
fa11829f 336isn't bumped up until RC time, to minimise the possibility of versions of perl
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337escaping that believe themselves to be newer than they actually are.
338
339It would be useful to find an elegant way to have the "this is an interim
340maintenance release" or "this is a release candidate" in the terse -v output,
341and have it so that it's easy for the pumpking to remove this just as the
342release tarball is rolled up. This way the version pulled out of rsync would
343always say "I'm a development release" and it would be safe to bump the
344reported minor version as soon as a release ships, which would aid perl
345developers.
346
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347This task is really about thinking of an elegant way to arrange the C source
348such that it's trivial for the Pumpking to flag "this is an official release"
349when making a tarball, yet leave the default source saying "I'm not the
350official release".
351
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352=head2 Ordering of "global" variables.
353
354F<thrdvar.h> and F<intrpvarh> define the "global" variables that need to be
355per-thread under ithreads, where the variables are actually elements in a
356structure. As C dictates, the variables must be laid out in order of
357declaration. There is a comment
358C</* Important ones in the first cache line (if alignment is done right) */>
359which implies that at some point in the past the ordering was carefully chosen
360(at least in part). However, it's clear that the ordering is less than perfect,
361as currently there are things such as 7 C<bool>s in a row, then something
362typically requiring 4 byte alignment, and then an odd C<bool> later on.
363(C<bool>s are typically defined as C<char>s). So it would be good for someone
364to review the ordering of the variables, to see how much alignment padding can
365be removed.
366
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367It's also worth checking that all variables are actually used. Perl 5.8.0
368shipped with C<PL_nrs> still defined in F<thrdvar.h>, despite it being unused
369since a change over a year earlier. Had this been spotted before release, it
370could have been removed, but now it has to remain in the 5.8.x releases to
371keep the structure the same size, to retain binary compatibility.
372
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373It's probably worth checking if all need to be the types they are. For example
374
375 PERLVAR(Ierror_count, I32) /* how many errors so far, max 10 */
376
377might work as well if stored in a signed (or unsigned) 8 bit value, if the
378comment is accurate. C<PL_multi_open> and C<PL_multi_close> can probably
379become C<char>s. Finding variables to downsize coupled with rearrangement
380could shrink the interpreter structure; a size saving which is multiplied by
381the number of threads running.
382
fee0a0f7 383=head2 Profile Perl - am I hot or not?
62403a3c 384
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385The Perl source code is stable enough that it makes sense to profile it,
386identify and optimise the hotspots. It would be good to measure the
387performance of the Perl interpreter using free tools such as cachegrind,
388gprof, and dtrace, and work to reduce the bottlenecks they reveal.
389
390As part of this, the idea of F<pp_hot.c> is that it contains the I<hot> ops,
391the ops that are most commonly used. The idea is that by grouping them, their
392object code will be adjacent in the executable, so they have a greater chance
393of already being in the CPU cache (or swapped in) due to being near another op
394already in use.
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395
396Except that it's not clear if these really are the most commonly used ops. So
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397as part of exercising your skills with coverage and profiling tools you might
398want to determine what ops I<really> are the most commonly used. And in turn
399suggest evictions and promotions to achieve a better F<pp_hot.c>.
62403a3c 400
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401=head2 Shrink struct context
402
403In F<cop.h>, we have
404
405 struct context {
406 U32 cx_type; /* what kind of context this is */
407 union {
408 struct block cx_blk;
409 struct subst cx_subst;
410 } cx_u;
411 };
412
413There are less than 256 values for C<cx_type>, and the constituent parts
414C<struct block> and C<struct subst> both contain some C<U8> and C<U16> fields,
415so it should be possible to move them to the first word, and share space with
416a C<U8> C<cx_type>, saving 1 word.
417
418=head2 Allocate OPs from arenas
419
420Currently all new OP structures are individually malloc()ed and free()d.
421All C<malloc> implementations have space overheads, and are now as fast as
422custom allocates so it would both use less memory and less CPU to allocate
423the various OP structures from arenas. The SV arena code can probably be
424re-used for this.
425
a229ae3b 426=head2 Improve win32/wince.c
0bdfc961 427
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428Currently, numerous functions look virtually, if not completely,
429identical in both C<win32/wince.c> and C<win32/win32.c> files, which can't be good.
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430
431
0bdfc961 432=head1 Tasks that need a knowledge of XS
e50bb9a1 433
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434These tasks would need C knowledge, and roughly the level of knowledge of
435the perl API that comes from writing modules that use XS to interface to
436C.
437
f23930d5 438=head2 shrink C<PVBM>s
4a750395 439
35b64ab6 440By removing unused elements and careful re-ordering, the structures for C<AV>s,
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441C<HV>s, C<CV>s and C<GV>s have recently been shrunk considerably. C<PVIO>s
442probably aren't worth it, as typical programs don't use more than 8, and
443(at least) C<Filter::Util::Call> uses C<SvPVX>/C<SvCUR>/C<SvLEN> on a C<PVIO>,
444so it would mean code changes to modules on CPAN. C<PVBM>s might have some
445savings to win.
4a750395 446
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447=head2 Implicit Latin 1 => Unicode translation
448
449Conversions from byte strings to UTF-8 currently map high bit characters
450to Unicode without translation (or, depending on how you look at it, by
451implicitly assuming that the byte strings are in Latin-1). As perl assumes
452the C locale by default, upgrading a string to UTF-8 may change the
453meaning of its contents regarding character classes, case mapping, etc.
454This should probably emit a warning (at least).
455
456This task is incremental - even a little bit of work on it will help.
e50bb9a1 457
cd793d32 458=head2 autovivification
e50bb9a1 459
cd793d32 460Make all autovivification consistent w.r.t LVALUE/RVALUE and strict/no strict;
e50bb9a1 461
0bdfc961 462This task is incremental - even a little bit of work on it will help.
e50bb9a1 463
0bdfc961 464=head2 Unicode in Filenames
e50bb9a1 465
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466chdir, chmod, chown, chroot, exec, glob, link, lstat, mkdir, open,
467opendir, qx, readdir, readlink, rename, rmdir, stat, symlink, sysopen,
468system, truncate, unlink, utime, -X. All these could potentially accept
469Unicode filenames either as input or output (and in the case of system
470and qx Unicode in general, as input or output to/from the shell).
471Whether a filesystem - an operating system pair understands Unicode in
472filenames varies.
e50bb9a1 473
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474Known combinations that have some level of understanding include
475Microsoft NTFS, Apple HFS+ (In Mac OS 9 and X) and Apple UFS (in Mac
476OS X), NFS v4 is rumored to be Unicode, and of course Plan 9. How to
477create Unicode filenames, what forms of Unicode are accepted and used
478(UCS-2, UTF-16, UTF-8), what (if any) is the normalization form used,
479and so on, varies. Finding the right level of interfacing to Perl
480requires some thought. Remember that an OS does not implicate a
481filesystem.
e50bb9a1 482
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483(The Windows -C command flag "wide API support" has been at least
484temporarily retired in 5.8.1, and the -C has been repurposed, see
485L<perlrun>.)
969e704b 486
0bdfc961 487=head2 Unicode in %ENV
969e704b 488
0bdfc961 489Currently the %ENV entries are always byte strings.
e50bb9a1 490
0bdfc961 491=head2 use less 'memory'
e50bb9a1 492
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493Investigate trade offs to switch out perl's choices on memory usage.
494Particularly perl should be able to give memory back.
e50bb9a1 495
0bdfc961 496This task is incremental - even a little bit of work on it will help.
0abe3f7c 497
0bdfc961 498=head2 Re-implement C<:unique> in a way that is actually thread-safe
0abe3f7c 499
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500The old implementation made bad assumptions on several levels. A good 90%
501solution might be just to make C<:unique> work to share the string buffer
502of SvPVs. That way large constant strings can be shared between ithreads,
503such as the configuration information in F<Config>.
0abe3f7c 504
0bdfc961 505=head2 Make tainting consistent
0abe3f7c 506
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507Tainting would be easier to use if it didn't take documented shortcuts and
508allow taint to "leak" everywhere within an expression.
0abe3f7c 509
0bdfc961 510=head2 readpipe(LIST)
0abe3f7c 511
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512system() accepts a LIST syntax (and a PROGRAM LIST syntax) to avoid
513running a shell. readpipe() (the function behind qx//) could be similarly
514extended.
0abe3f7c 515
e50bb9a1 516
e50bb9a1 517
e50bb9a1 518
f86a8bc5 519
0bdfc961 520=head1 Tasks that need a knowledge of the interpreter
3298bd4d 521
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522These tasks would need C knowledge, and knowledge of how the interpreter works,
523or a willingness to learn.
3298bd4d 524
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525=head2 lexical pragmas
526
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527Document the new support for lexical pragmas in 5.9.3 and how %^H works.
528Maybe C<re>, C<encoding>, maybe other pragmas could be made lexical.
0562c0e3 529
d10fc472 530=head2 Attach/detach debugger from running program
1626a787 531
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532The old perltodo notes "With C<gdb>, you can attach the debugger to a running
533program if you pass the process ID. It would be good to do this with the Perl
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534debugger on a running Perl program, although I'm not sure how it would be
535done." ssh and screen do this with named pipes in /tmp. Maybe we can too.
1626a787 536
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537=head2 LVALUE functions for lists
538
539The old perltodo notes that lvalue functions don't work for list or hash
540slices. This would be good to fix.
541
542=head2 LVALUE functions in the debugger
543
544The old perltodo notes that lvalue functions don't work in the debugger. This
545would be good to fix.
546
547=head2 _ prototype character
548
549Study the possibility of adding a new prototype character, C<_>, meaning
550"this argument defaults to $_".
551
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552=head2 state variables
553
554C<my $foo if 0;> is deprecated, and should be replaced with
555C<state $x = "initial value\n";> the syntax from Perl 6.
16fc99ce 556Rafael has sent a first cut patch to perl5-porters.
4e577f8b 557
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558=head2 regexp optimiser optional
559
560The regexp optimiser is not optional. It should configurable to be, to allow
561its performance to be measured, and its bugs to be easily demonstrated.
562
563=head2 UNITCHECK
564
565Introduce a new special block, UNITCHECK, which is run at the end of a
566compilation unit (module, file, eval(STRING) block). This will correspond to
567the Perl 6 CHECK. Perl 5's CHECK cannot be changed or removed because the
568O.pm/B.pm backend framework depends on it.
569
570=head2 optional optimizer
571
572Make the peephole optimizer optional. Currently it performs two tasks as
573it walks the optree - genuine peephole optimisations, and necessary fixups of
574ops. It would be good to find an efficient way to switch out the
575optimisations whilst keeping the fixups.
576
577=head2 You WANT *how* many
578
579Currently contexts are void, scalar and list. split has a special mechanism in
580place to pass in the number of return values wanted. It would be useful to
581have a general mechanism for this, backwards compatible and little speed hit.
582This would allow proposals such as short circuiting sort to be implemented
583as a module on CPAN.
584
585=head2 lexical aliases
586
587Allow lexical aliases (maybe via the syntax C<my \$alias = \$foo>.
588
589=head2 entersub XS vs Perl
590
591At the moment pp_entersub is huge, and has code to deal with entering both
592perl and XS subroutines. Subroutine implementations rarely change between
593perl and XS at run time, so investigate using 2 ops to enter subs (one for
594XS, one for perl) and swap between if a sub is redefined.
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595
596=head2 Self ties
597
598self ties are currently illegal because they caused too many segfaults. Maybe
599the causes of these could be tracked down and self-ties on all types re-
600instated.
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601
602=head2 Optimize away @_
603
604The old perltodo notes "Look at the "reification" code in C<av.c>".
605
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606=head2 What hooks would assertions need?
607
608Assertions are in the core, and work. However, assertions needed to be added
609as a core patch, rather than an XS module in ext, or a CPAN module, because
610the core has no hooks in the necessary places. It would be useful to
611investigate what hooks would need to be added to make it possible to provide
612the full assertion support from a CPAN module, so that we aren't constraining
613the imagination of future CPAN authors.
614
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615=head2 Properly Unicode safe tokeniser and pads.
616
617The tokeniser isn't actually very UTF-8 clean. C<use utf8;> is a hack -
618variable names are stored in stashes as raw bytes, without the utf-8 flag
619set. The pad API only takes a C<char *> pointer, so that's all bytes too. The
620tokeniser ignores the UTF-8-ness of C<PL_rsfp>, or any SVs returned from
621source filters. All this could be fixed.
622
d7889f52 623=head2 Integrate Russ Allbery's strlcat/strlcpy implementation
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625And remove the last remaining uses of strcat() and strcpy(). Also, add
626my_strlcat() and my_strlcpy() to Devel::PPPort so previous versions of Perl can
627use these APIs.
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629=head1 Big projects
630
631Tasks that will get your name mentioned in the description of the "Highlights
632of 5.10"
633
634=head2 make ithreads more robust
635
4e577f8b 636Generally make ithreads more robust. See also L</iCOW>
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637
638This task is incremental - even a little bit of work on it will help, and
639will be greatly appreciated.
640
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641One bit would be to write the missing code in sv.c:Perl_dirp_dup.
642
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643=head2 iCOW
644
645Sarathy and Arthur have a proposal for an improved Copy On Write which
646specifically will be able to COW new ithreads. If this can be implemented
647it would be a good thing.
648
649=head2 (?{...}) closures in regexps
650
651Fix (or rewrite) the implementation of the C</(?{...})/> closures.
652
653=head2 A re-entrant regexp engine
654
655This will allow the use of a regex from inside (?{ }), (??{ }) and
656(?(?{ })|) constructs.