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[perl #116020] toggle operator missing reset: proposal

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December 9, 2012 23:05
[perl #116020] toggle operator missing reset: proposal
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This is a bug report for perl from,
generated with the help of perlbug 1.39 running under perl 5.14.3.

As others have noted, the perl toggle (a.k.a. range) operator 
has the odd property that it has global state, which can't be reset.

This makes its use difficult where the input is external.

Consider the case where we want to extract lines from a file
between marker --a and --b.  We code something like (reduced):
sub extract {
   my $out = '';
   while( <F> ) {
        $out .= $_ if /^--a$/ ... /^--b/;
   return $out;
print extract("filea"), extract("second");

This is great, until we encounter a filea that is missing the (last) --b marker.
When we call extract with the second file, the ... operator is looking for --b, so
the first block of the second file isn't included.

Various work-arounds have been used; typically arranging for an "extra" end marker to be
passed through the toggle.  This is awkward, since it has to be the same operator in the same
line of code.  So we end up reading a file into an array of lines, or putting the test in a subroutine
or ...

My proposal:
  Since we can label any statement (now), why not have the reset operator take a label argument?
  Then reset can act on the toggle(s) in that statement.

  So the minimum implemention would permit:
sub extract {
   my $out = '';
   reset TOGGLE;
   while( <F> ) {
        $out .= $_ if /^--a$/ ... /^--b/;
   return $out;

   Here, reset will initialize every toggle operator in the labeled statement.

   Extra credit:

   if the label is indexed, initialize the n'th toggle.  E.g. The LABEL has the semantics of
   an array of the operators in the following statement.

   A somewhat contrived example:

   reset TOGGLES[0,2];

    $x = /^--a/ ... /^--b/ || /^--c/ ... /^--d/ || /^--e/ ... /^--f/;

    initializes the first and third toggles, but doesn't touch the second.

This seems useful, consistent with Perl syntax and yet new syntax that won't break any existing code.

I think it would solve this long-standing divot in Perl's semantics.

[Please do not change anything below this line]
This perlbug was built using Perl 5.14.3 in the Fedora build system.
It is being executed now by Perl 5.14.3 - Thu Oct 18 13:30:29 UTC 2012.

Site configuration information for perl 5.14.3:

Configured by Red Hat, Inc. at Thu Oct 18 13:30:29 UTC 2012.

Summary of my perl5 (revision 5 version 14 subversion 3) configuration:
    osname=linux, osvers=2.6.32-279.9.1.el6.x86_64, archname=x86_64-linux-thread-multi
    uname='linux 2.6.32-279.9.1.el6.x86_64 #1 smp fri aug 31 09:04:24 edt 2012 x86_64 x86_64 x86_64 gnulinux '
    config_args='-des -Doptimize=-O2 -g -pipe -Wall -Wp,-D_FORTIFY_SOURCE=2 -fexceptions -fstack-protector --param=ssp-buffer-size=4  -m64 -mtune=generic -Dccdlflags=-Wl,--enable-new-dtags -Dlddlflags=-shared -O2 -g -pipe -Wall -Wp,-D_FORTIFY_SOURCE=2 -fexceptions -fstack-protector --param=ssp-buffer-size=4  -m64 -mtune=generic -Wl,-z,relro  -DDEBUGGING=-g -Dversion=5.14.3 -Dmyhostname=localhost -Dperladmin=root@localhost -Dcc=gcc -Dcf_by=Red Hat, Inc. -Dprefix=/usr -Dvendorprefix=/usr -Dsiteprefix=/usr/local -Dsitelib=/usr/local/share/perl5 -Dsitearch=/usr/local/lib64/perl5 -Dprivlib=/usr/share/perl5 -Dvendorlib=/usr/share/perl5/vendor_perl -Darchlib=/usr/lib64/perl5 -Dvendorarch=/usr/lib64/perl5/vendor_perl -Darchname=x86_64-linux-thread-multi -Dlibpth=/usr/local/lib64 /lib64 /usr/lib64 -Duseshrplib -Dusethreads -Duseithreads -Dusedtrace=/usr/bin/dtrace -Duselargefiles -Dd_semctl_semun -Di_db -Ui_ndbm -Di_gdbm -Di_shadow -Di_syslog -Dman3ext=3pm -Duseperlio -Dinstallusrbin!
 perl=n -Ubincompat5005 -Uversiononly -Dpager=/usr/bin/less -isr -Dd_gethostent_r_proto -Ud_endhostent_r_proto -Ud_sethostent_r_proto -Ud_endprotoent_r_proto -Ud_setprotoent_r_proto -Ud_endservent_r_proto -Ud_setservent_r_proto -Dscriptdir=/usr/bin'
    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
    cc='gcc', ccflags ='-D_REENTRANT -D_GNU_SOURCE -fno-strict-aliasing -pipe -fstack-protector -I/usr/local/include -D_LARGEFILE_SOURCE -D_FILE_OFFSET_BITS=64',
    optimize='-O2 -g -pipe -Wall -Wp,-D_FORTIFY_SOURCE=2 -fexceptions -fstack-protector --param=ssp-buffer-size=4 -m64 -mtune=generic',
    cppflags='-D_REENTRANT -D_GNU_SOURCE -fno-strict-aliasing -pipe -fstack-protector -I/usr/local/include'
    ccversion='', gccversion='4.7.2 20120921 (Red Hat 4.7.2-2)', 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='gcc', ldflags =' -fstack-protector'
    libpth=/usr/local/lib64 /lib64 /usr/lib64
    libs=-lresolv -lnsl -lgdbm -ldb -ldl -lm -lcrypt -lutil -lpthread -lc -lgdbm_compat
    perllibs=-lresolv -lnsl -ldl -lm -lcrypt -lutil -lpthread -lc
    libc=, so=so, useshrplib=true,
  Dynamic Linking:
    dlsrc=dl_dlopen.xs, dlext=so, d_dlsymun=undef, ccdlflags='-Wl,--enable-new-dtags -Wl,-rpath,/usr/lib64/perl5/CORE'
    cccdlflags='-fPIC', lddlflags='-shared -O2 -g -pipe -Wall -Wp,-D_FORTIFY_SOURCE=2 -fexceptions -fstack-protector --param=ssp-buffer-size=4 -m64 -mtune=generic -Wl,-z,relro '

Locally applied patches:

@INC for perl 5.14.3:

Environment for perl 5.14.3:
    LANGUAGE (unset)
    LD_LIBRARY_PATH (unset)
    LOGDIR (unset)
    PERL_BADLANG (unset)

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