more given nonsense: what's a topicalizer?

[p5pRT]

Migrated from rt.perl.org#90182 (status was 'open')

Searchable as RT90182$

[p5pRT]

From tchrist@perl.com

When you run this​:

  use 5.010;
  while (<DATA>) {
  when (/foo/) { say "meta" }
  when (/\d/) { say "number" }
  default { say "other" }
  }
  __END__
  fee fie
  23 skiddoo
  foo fum

You get​:

  other
  Can't use when() outside a topicalizer at /tmp/s0 line 5, <DATA> line 1.
  Exit 255

In other words, it does the right thing, and then blows up.
The work around is​:

  use 5.010;
  while (<DATA>) {
  eval { # ...protect the loop from senseless nannying
  when (/foo/) { say "meta" }
  when (/\d/) { say "number" }
  default { say "other" }
  }
  }
  __END__
  fee fie
  23 skiddoo
  foo fum

Which correctly prints​:

  other
  number
  meta

which is just ridiculous. What does it think

  while (<DATA>)

is if not a topicalizer??

BTW, this *does* work correctly in Perl 6.

--tom

Summary of my perl5 (revision 5 version 12 subversion 3) configuration​:
 
  Platform​:
  osname=openbsd, osvers=4.4, archname=OpenBSD.i386-openbsd
  uname='openbsd chthon 4.4 generic#0 i386 '
  config_args='-des'
  hint=recommended, useposix=true, d_sigaction=define
  useithreads=undef, usemultiplicity=undef
  useperlio=define, d_sfio=undef, uselargefiles=define, usesocks=undef
  use64bitint=undef, use64bitall=undef, uselongdouble=undef
  usemymalloc=n, bincompat5005=undef
  Compiler​:
  cc='cc', ccflags ='-fno-strict-aliasing -pipe -fstack-protector -I/usr/local/include',
  optimize='-O2',
  cppflags='-fno-strict-aliasing -pipe -fstack-protector -I/usr/local/include'
  ccversion='', gccversion='3.3.5 (propolice)', gccosandvers='openbsd4.4'
  intsize=4, longsize=4, ptrsize=4, doublesize=8, byteorder=1234
  d_longlong=define, longlongsize=8, d_longdbl=define, longdblsize=12
  ivtype='long', ivsize=4, nvtype='double', nvsize=8, Off_t='off_t', lseeksize=8
  alignbytes=4, prototype=define
  Linker and Libraries​:
  ld='cc', ldflags ='-Wl,-E -fstack-protector -L/usr/local/lib'
  libpth=/usr/local/lib /usr/lib
  libs=-lgdbm -lm -lutil -lc
  perllibs=-lm -lutil -lc
  libc=/usr/lib/libc.so.48.0, so=so, useshrplib=false, libperl=libperl.a
  gnulibc_version=''
  Dynamic Linking​:
  dlsrc=dl_dlopen.xs, dlext=so, d_dlsymun=undef, ccdlflags=' '
  cccdlflags='-DPIC -fPIC ', lddlflags='-shared -fPIC -L/usr/local/lib -fstack-protector'

Characteristics of this binary (from libperl)​:
  Compile-time options​: PERL_DONT_CREATE_GVSV PERL_MALLOC_WRAP
  USE_LARGE_FILES USE_PERLIO USE_PERL_ATOF
  Built under openbsd
  Compiled at Feb 14 2011 07​:32​:03
  %ENV​:
  PERL_UNICODE="SA"
  @​INC​:
  /usr/local/lib/perl5/site_perl/5.12.3/OpenBSD.i386-openbsd
  /usr/local/lib/perl5/site_perl/5.12.3
  /usr/local/lib/perl5/5.12.3/OpenBSD.i386-openbsd
  /usr/local/lib/perl5/5.12.3
  /usr/local/lib/perl5/site_perl/5.11.3
  /usr/local/lib/perl5/site_perl/5.10.1
  /usr/local/lib/perl5/site_perl/5.10.0
  /usr/local/lib/perl5/site_perl/5.8.7
  /usr/local/lib/perl5/site_perl/5.8.0
  /usr/local/lib/perl5/site_perl/5.6.0
  /usr/local/lib/perl5/site_perl/5.005
  /usr/local/lib/perl5/site_perl
  .

[p5pRT]

From @ikegami

On Mon, May 9, 2011 at 9​:50 AM, tchrist1 <perlbug-followup@​perl.org> wrote​:

which is just ridiculous. What does it think

while (<DATA>)

is if not a topicalizer??

So what would qualify?

while ($_ = ...) { ... }
while (defined($_ = ...)) { ... }
while (($_ = ...) RELOP ...) { ... }
while (..., $_ = ..., ...) { ... }
while (..., defined($_ = ...), ...) { ... }
while (..., ($_ = ...) RELOP ..., ...) { ... }

Same for C<for(;;)>?

[p5pRT]

The RT System itself - Status changed from 'new' to 'open'

[p5pRT]

From tchrist@perl.com

"Eric Brine via RT" <perlbug-followup@​perl.org> wrote
  on Tue, 10 May 2011 09​:41​:45 PDT​:

which is just ridiculous. What does it think

while (<DATA>)

is if not a topicalizer??

So what would qualify?

while ($_ = ...) { ... }
while (defined($_ = ...)) { ... }
while (($_ = ...) RELOP ...) { ... }
while (..., $_ = ..., ...) { ... }
while (..., defined($_ = ...), ...) { ... }
while (..., ($_ = ...) RELOP ..., ...) { ... }

Same for C<for(;;)>?

In Perl6, when works wherever $_ is around to test against.
This seems perfectly straightforward. I don't know how this
very complicated set of behaviors found their way into Perl5.

--tom

[p5pRT]

From @ikegami

In Perl6, when works wherever $_ is around to test against.

$_ is always around in Perl5, even when there's nothing to next/break out
of.

[p5pRT]

From tchrist@perl.com

Eric Brine <ikegami@​adaelis.com> wrote
  on Tue, 10 May 2011 13​:01​:44 EDT​:

In Perl6, when works wherever $_ is around to test against.

$_ is always around in Perl5, even when there's nothing to
next/break out of.

Just let them try. :)

I would be quite happy to have it in

  while (<>) {
  ... when ...;
  ... when ...;
  ... when ...;
  ... when ...;
  }

--tom

[p5pRT]

From @ap

* tchrist1 <perlbug-followup@​perl.org> [2011-05-09 15​:55]​:

which is just ridiculous. What does it think

while \(\<DATA>\)

is if not a topicalizer??

BTW, this *does* work correctly in Perl 6.

I think this is the consequence of `break` being added as
a `given`-specific special form of `next`. I am guessing wildly
that the thought behind adding `break` was that doing `next`
inside `given` should abort the iteration of a surrounding loop
instead of just the `given` block, maybe in (mistaken) analogy to
what `next` does inside `if`. At least that is the only plausible
reason I can imagine why someone thought `break` was a good idea.

And once you have `break` you have to come up with a special set
of rules for how `when` will examine scope, which gets you into
this mess.

Needless to say I don’t see the point since it’s possible to
do `next LABEL` and this is how one has always been supposed to
break out of nested loops. Half amusing and half maddening is
that `given` has *extra* special rules to do `next` instead of
`break` when inside a `foreach` rather than a `given`.

I think `when` shouldn’t just do an implicit `next` in all cases.
That already has clear rules about what it will complain of, and
it would make `when` far more useful.

In fact if `when` lost the implicit smart match and became
*nothing else/more* than an `if` with implicit `next`, I’d find
it *even better*. That’s probably not feasible, alas. But we
could at least lose this `break` silliness.

Regards,
--
Aristotle Pagaltzis // <http​://plasmasturm.org/>

[p5pRT]

From @ikegami

On Tue, May 10, 2011 at 3​:10 PM, Tom Christiansen <tchrist@​perl.com> wrote​:

Eric Brine <ikegami@​adaelis.com> wrote
on Tue, 10 May 2011 13​:01​:44 EDT​:

In Perl6, when works wherever $_ is around to test against.

$_ is always around in Perl5, even when there's nothing to
next/break out of.

Just let them try. :)

That's what's happening. It's trying to C<break> and finds neither C<given>
or C<foreach> (for C<next>).

It could start looking for any loop. Is there any problems (compatibility
issues) with doing so?

[p5pRT]

From @ap

* Aristotle Pagaltzis <pagaltzis@​gmx.de> [2011-05-11 00​:15]​:

I think `when` shouldn’t just do an implicit `next` in all cases.

No, I don’t think that, I just ended up saying it because of too
much editing. I think it *should* just do an implicit `next`.

Regards,
--
Aristotle Pagaltzis // <http​://plasmasturm.org/>

[p5pRT]

From tchrist@perl.com

* Aristotle Pagaltzis <pagaltzis@​gmx.de> [2011-05-11 00​:15]​:

I think `when` shouldn�t just do an implicit `next` in all cases.

No, I don�t think that, I just ended up saying it because of too
much editing. I think it *should* just do an implicit `next`.

Me, I was surprised it didn't.

--tom

[p5pRT]

From @cpansprout

On Mon May 09 06​:50​:09 2011, tom christiansen wrote​:

When you run this​:

use 5\.010;
while \(\<DATA>\) \{
when \(/foo/\) \{ say "meta"   \}
when \(/\\d/\)  \{ say "number" \}
default      \{ say "other"  \}
\}
\_\_END\_\_
fee fie
23 skiddoo
foo fum

You get​:

other
Can't use when\(\) outside a topicalizer at /tmp/s0 line 5\, \<DATA>

line 1.
Exit 255

It’s even stranger than you think​:

$ perl -E 'given(1) { for my $x (2) { break } } warn ok'
ok at -e line 1.
$ perl -E 'given(1) { for (2) { break } }'
Can't "break" in a loop topicalizer at -e line 1.

[p5pRT]

From perl-diddler@tlinx.org

Created by perl-diddler@tlinx.org

It seems like perl is being a bit over zealous about it's policing
the use of "when". Technically, a topicalizer is something that
defines the "topic" (or subject).. like given and when automatically
use '$_'. But I can write my own 'topicalizer' that funnels data
into $_ -- trivially.

A prime example would be 'when(<>){}', where you process lines like
/filt/ and do {xxyz; continue};
/filter/ and do {xxyz+1; next};
...

Sometimes an if/elsif/else/ construct is useful, but only if you
don't have exceptions.

Another example I ran into today​:

In a trivial prog, I had a foreach loop to parse args.
Later had an arg that took a param -- how to skip a 'each',
in foreach? Well​:

for ($_=$ARGV[$n = 0]; $n<@​ARGV; $_=$ARGV[$n]) {
  ...
  when(arg (-rc) with param) {
  my $m=$n+1;
  $m&gt;=$#ARGV or die "-rc needs param\n";
  $rc=$ARGV[$m]; ++$n
  }
  when (next case)...
}

---
Course now, this doesn't work -- to make it work I'll
have to add an extra 2 lines inside the forloop,
which I hope won't have any side effects...i.e.

for ($_=each(@​ARGV); $_<=lastmem(@​ARGV); $_=next(@​ARGV) ) {
  given ($_) {
  when ...
  ...
  }
}

Can someone explain to me the rational for the interpreter
to 'restrict' the usage of 'when' to a smaller subset than
where 'last/next/continue' would work or why it not working
in the same places shouldn't be considered a bug?

Thanks!
(p.s. can you think of any side effects of tossing in the
extra given{} in the for loop to satisfy the interp's warnings?)

Perl Info

Flags:
    category=core
    severity=low

This perlbug was built using Perl 5.12.3 - Fri May  6 13:31:41 UTC 2011
It is being executed now by  Perl 5.12.3 - Fri May  6 13:26:30 UTC 2011.

Site configuration information for perl 5.12.3:

Configured by abuild at Fri May  6 13:26:30 UTC 2011.

Summary of my perl5 (revision 5 version 12 subversion 3) configuration:
   
  Platform:
    osname=linux, osvers=2.6.36, archname=x86_64-linux-thread-multi
    uname='linux build33 2.6.36 #1 smp 2011-02-21 10:34:10 +0100 x86_64 x86_64 x86_64 gnulinux '
    config_args='-ds -e -Dprefix=/usr -Dvendorprefix=/usr -Dinstallusrbinperl -Dusethreads -Di_db -Di_dbm -Di_ndbm -Di_gdbm -Duseshrplib=true -Doptimize=-fmessage-length=0 -O2 -Wall -D_FORTIFY_SOURCE=2 -fstack- protector -funwind-tables -fasynchronous-unwind-tables -g -Wall -pipe -Accflags=-DPERL_USE_SAFE_PUTENV'
    hint=recommended, useposix=true, d_sigaction=define
    useithreads=define, usemultiplicity=define
    useperlio=define, d_sfio=undef, uselargefiles=define, usesocks=undef
    use64bitint=define, use64bitall=define, uselongdouble=undef
    usemymalloc=n, bincompat5005=undef
  Compiler:
    cc='cc', ccflags ='-D_REENTRANT -D_GNU_SOURCE -DPERL_USE_SAFE_PUTENV -DDEBUGGING -fno-strict-aliasing -pipe -fstack-protector -D_LARGEFILE_SOURCE -D_FILE_OFFSET_BITS=64',
    optimize='-fmessage-length=0 -O2 -Wall -D_FORTIFY_SOURCE=2 -fstack-protector -funwind-tables -fasynchronous-unwind-tables -g -Wall -pipe',
    cppflags='-D_REENTRANT -D_GNU_SOURCE -DPERL_USE_SAFE_PUTENV -DDEBUGGING -fno-strict-aliasing -pipe -fstack-protector'
    ccversion='', gccversion='4.5.1 20101208 [gcc-4_5-branch revision 167585]', gccosandvers=''
    intsize=4, longsize=8, ptrsize=8, doublesize=8, byteorder=12345678
    d_longlong=define, longlongsize=8, d_longdbl=define, longdblsize=16
    ivtype='long', ivsize=8, nvtype='double', nvsize=8, Off_t='off_t', lseeksize=8
    alignbytes=8, prototype=define
  Linker and Libraries:
    ld='cc', ldflags =' -L/usr/local/lib64 -fstack-protector'
    libpth=/lib64 /usr/lib64 /usr/local/lib64
    libs=-lm -ldl -lcrypt -lpthread
    perllibs=-lm -ldl -lcrypt -lpthread
    libc=/lib64/libc-2.11.3.so, so=so, useshrplib=true, libperl=libperl.so
    gnulibc_version='2.11.3'
  Dynamic Linking:
    dlsrc=dl_dlopen.xs, dlext=so, d_dlsymun=undef, ccdlflags='-Wl,-E -Wl,-rpath,/usr/lib/perl5/5.12.3/x86_64-linux-thread-multi/CORE'
    cccdlflags='-fPIC', lddlflags='-shared -L/usr/local/lib64 -fstack-protector'

Locally applied patches:
    


@INC for perl 5.12.3:
    /usr/lib/perl5/site_perl/5.12.3/x86_64-linux-thread-multi
    /usr/lib/perl5/site_perl/5.12.3
    /usr/lib/perl5/vendor_perl/5.12.3/x86_64-linux-thread-multi
    /usr/lib/perl5/vendor_perl/5.12.3
    /usr/lib/perl5/5.12.3/x86_64-linux-thread-multi
    /usr/lib/perl5/5.12.3
    .


Environment for perl 5.12.3:
    HOME=/home/law
    LANG=en_US.UTF-8
    LANGUAGE (unset)
    LC_CTYPE=en_US.UTF-8
    LD_LIBRARY_PATH (unset)
    LOGDIR (unset)
    PATH=.:/sbin:/usr/local/sbin:/opt/lsb/bin:/home/law/bin:/usr/local/bin:/usr/bin:/bin:/usr/X11R6/bin:/usr/games:/opt/kde3/bin:/usr/lib/mit/bin:/usr/lib/mit/sbin:/usr/lib/qt3/bin:/usr/sbin
    PERL_BADLANG (unset)
    SHELL=/bin/bash

[p5pRT]

From @ikegami

On Sun, Jan 22, 2012 at 12​:21 AM, Linda Walsh <perlbug-followup@​perl.org>wrote​:

Can someone explain to me the rational for the interpreter
to 'restrict' the usage of 'when' to a smaller subset than
where 'last/next/continue' would work

C<when> was primary written to compare to $_ (e.g. C<< when ("foo") >>), so
it's only allowed in flow control statements that set $_ (e.g. C<< for
(...) >>, but not C<< for my $x (...) >> and C<< for (...; ...; ...) >>).

[p5pRT]

The RT System itself - Status changed from 'new' to 'open'

[p5pRT]

From @cpansprout

On Sat Jan 21 22​:11​:07 2012, ikegami@​adaelis.com wrote​:

On Sun, Jan 22, 2012 at 12​:21 AM, Linda Walsh <perlbug-
followup@​perl.org>wrote​:

Can someone explain to me the rational for the interpreter
to 'restrict' the usage of 'when' to a smaller subset than
where 'last/next/continue' would work

C<when> was primary written to compare to $_ (e.g. C<< when ("foo")

), so
it's only allowed in flow control statements that set $_ (e.g. C<< for
(...) >>, but not C<< for my $x (...) >> and C<< for (...; ...; ...)
).

Except even that doesn’t work properly. Since this is basically the
same issue, I’m merging it with #90182.

--

Father Chrysostomos