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chained comparison #16317

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p5pRT opened this issue Dec 19, 2017 · 6 comments
Closed

chained comparison #16317

p5pRT opened this issue Dec 19, 2017 · 6 comments

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@p5pRT
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@p5pRT p5pRT commented Dec 19, 2017

Migrated from rt.perl.org#132607 (status was 'open')

Searchable as RT132607$

@p5pRT
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@p5pRT p5pRT commented Dec 19, 2017

From zefram@fysh.org

Created by zefram@fysh.org

It would be nice for Perl to understand chained comparisons such as "$a
le $b lt $c", which would mean "$a le $b && $b lt $c" but with only a
single evaluation of the middle comparand. The comparison operators are
currently non-associative, so most of what ought to be chained comparisons
are currently syntax errors. Introducing chained comparisons would not
change the meaning of any currently-legal expression.

One wrinkle​: we currently have two levels of precedence for comparison
operators, one for "lt" et al, and a looser one for "eq" et al. So "$a lt
$b eq $c" is currently legal. This is imitating the C precedence levels,
I think mainly so that "!=" can be used to mean "xor". It's already a
bit of a gotcha, and we'd have to warn that things like that don't make
chained comparisons.

Perl Info

Flags:
    category=core
    severity=wishlist

Site configuration information for perl 5.27.6:

Configured by zefram at Tue Nov 21 05:42:59 GMT 2017.

Summary of my perl5 (revision 5 version 27 subversion 6) configuration:
   
  Platform:
    osname=linux
    osvers=3.16.0-4-amd64
    archname=x86_64-linux-thread-multi
    uname='linux barba.rous.org 3.16.0-4-amd64 #1 smp debian 3.16.43-2+deb8u2 (2017-06-26) x86_64 gnulinux '
    config_args='-des -Dprefix=/home/zefram/usr/perl/perl_install/perl-5.27.6-i64-f52 -Duselargefiles -Dusethreads -Uafs -Ud_csh -Uusesfio -Uusenm -Duseshrplib -Dusedevel -Uversiononly -Ui_db'
    hint=recommended
    useposix=true
    d_sigaction=define
    useithreads=define
    usemultiplicity=define
    use64bitint=define
    use64bitall=define
    uselongdouble=undef
    usemymalloc=n
    default_inc_excludes_dot=define
    bincompat5005=undef
  Compiler:
    cc='cc'
    ccflags ='-D_REENTRANT -D_GNU_SOURCE -fwrapv -fno-strict-aliasing -pipe -fstack-protector-strong -I/usr/local/include -D_LARGEFILE_SOURCE -D_FILE_OFFSET_BITS=64 -D_FORTIFY_SOURCE=2'
    optimize='-O2'
    cppflags='-D_REENTRANT -D_GNU_SOURCE -fwrapv -fno-strict-aliasing -pipe -fstack-protector-strong -I/usr/local/include'
    ccversion=''
    gccversion='4.9.2'
    gccosandvers=''
    intsize=4
    longsize=8
    ptrsize=8
    doublesize=8
    byteorder=12345678
    doublekind=3
    d_longlong=define
    longlongsize=8
    d_longdbl=define
    longdblsize=16
    longdblkind=3
    ivtype='long'
    ivsize=8
    nvtype='double'
    nvsize=8
    Off_t='off_t'
    lseeksize=8
    alignbytes=8
    prototype=define
  Linker and Libraries:
    ld='cc'
    ldflags =' -fstack-protector-strong -L/usr/local/lib'
    libpth=/usr/local/lib /usr/lib/gcc/x86_64-linux-gnu/4.9/include-fixed /usr/include/x86_64-linux-gnu /usr/lib /lib/x86_64-linux-gnu /lib/../lib /usr/lib/x86_64-linux-gnu /usr/lib/../lib /lib
    libs=-lpthread -lnsl -ldb -ldl -lm -lcrypt -lutil -lc
    perllibs=-lpthread -lnsl -ldl -lm -lcrypt -lutil -lc
    libc=libc-2.19.so
    so=so
    useshrplib=true
    libperl=libperl.so
    gnulibc_version='2.19'
  Dynamic Linking:
    dlsrc=dl_dlopen.xs
    dlext=so
    d_dlsymun=undef
    ccdlflags='-Wl,-E -Wl,-rpath,/home/zefram/usr/perl/perl_install/perl-5.27.6-i64-f52/lib/5.27.6/x86_64-linux-thread-multi/CORE'
    cccdlflags='-fPIC'
    lddlflags='-shared -O2 -L/usr/local/lib -fstack-protector-strong'



@INC for perl 5.27.6:
    /home/zefram/usr/perl/perl_install/perl-5.27.6-i64-f52/lib/site_perl/5.27.6/x86_64-linux-thread-multi
    /home/zefram/usr/perl/perl_install/perl-5.27.6-i64-f52/lib/site_perl/5.27.6
    /home/zefram/usr/perl/perl_install/perl-5.27.6-i64-f52/lib/5.27.6/x86_64-linux-thread-multi
    /home/zefram/usr/perl/perl_install/perl-5.27.6-i64-f52/lib/5.27.6


Environment for perl 5.27.6:
    HOME=/home/zefram
    LANG (unset)
    LANGUAGE (unset)
    LD_LIBRARY_PATH (unset)
    LOGDIR (unset)
    PATH=/home/zefram/usr/perl/perl_install/perl-5.27.6-i64-f52/bin:/home/zefram/usr/perl/util:/home/zefram/pub/x86_64-unknown-linux-gnu/bin:/home/zefram/pub/common/bin:/usr/bin:/bin:/usr/local/bin:/usr/games
    PERL_BADLANG (unset)
    SHELL=/usr/bin/zsh

@p5pRT
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@p5pRT p5pRT commented Dec 19, 2017

From @demerphq

I so look forward to this, both as a patch i can learn from and as a
feature I would enjoy. ++Zefram.

Yves

On 19 Dec 2017 7​:38 am, "Zefram" <perlbug-followup@​perl.org> wrote​:

# New Ticket Created by Zefram
# Please include the string​: [perl #132607]
# in the subject line of all future correspondence about this issue.
# <URL​: https://rt-archive.perl.org/perl5/Ticket/Display.html?id=132607 >

This is a bug report for perl from zefram@​fysh.org,
generated with the help of perlbug 1.41 running under perl 5.27.6.

-----------------------------------------------------------------
[Please describe your issue here]

It would be nice for Perl to understand chained comparisons such as "$a
le $b lt $c", which would mean "$a le $b && $b lt $c" but with only a
single evaluation of the middle comparand. The comparison operators are
currently non-associative, so most of what ought to be chained comparisons
are currently syntax errors. Introducing chained comparisons would not
change the meaning of any currently-legal expression.

One wrinkle​: we currently have two levels of precedence for comparison
operators, one for "lt" et al, and a looser one for "eq" et al. So "$a lt
$b eq $c" is currently legal. This is imitating the C precedence levels,
I think mainly so that "!=" can be used to mean "xor". It's already a
bit of a gotcha, and we'd have to warn that things like that don't make
chained comparisons.

[Please do not change anything below this line]
-----------------------------------------------------------------
---
Flags​:
category=core
severity=wishlist
---
Site configuration information for perl 5.27.6​:

Configured by zefram at Tue Nov 21 05​:42​:59 GMT 2017.

Summary of my perl5 (revision 5 version 27 subversion 6) configuration​:

Platform​:
osname=linux
osvers=3.16.0-4-amd64
archname=x86_64-linux-thread-multi
uname='linux barba.rous.org 3.16.0-4-amd64 #1 smp debian
3.16.43-2+deb8u2 (2017-06-26) x86_64 gnulinux '
config_args='-des -Dprefix=/home/zefram/usr/
perl/perl_install/perl-5.27.6-i64-f52 -Duselargefiles -Dusethreads -Uafs
-Ud_csh -Uusesfio -Uusenm -Duseshrplib -Dusedevel -Uversiononly -Ui_db'
hint=recommended
useposix=true
d_sigaction=define
useithreads=define
usemultiplicity=define
use64bitint=define
use64bitall=define
uselongdouble=undef
usemymalloc=n
default_inc_excludes_dot=define
bincompat5005=undef
Compiler​:
cc='cc'
ccflags ='-D_REENTRANT -D_GNU_SOURCE -fwrapv -fno-strict-aliasing
-pipe -fstack-protector-strong -I/usr/local/include -D_LARGEFILE_SOURCE
-D_FILE_OFFSET_BITS=64 -D_FORTIFY_SOURCE=2'
optimize='-O2'
cppflags='-D_REENTRANT -D_GNU_SOURCE -fwrapv -fno-strict-aliasing
-pipe -fstack-protector-strong -I/usr/local/include'
ccversion=''
gccversion='4.9.2'
gccosandvers=''
intsize=4
longsize=8
ptrsize=8
doublesize=8
byteorder=12345678
doublekind=3
d_longlong=define
longlongsize=8
d_longdbl=define
longdblsize=16
longdblkind=3
ivtype='long'
ivsize=8
nvtype='double'
nvsize=8
Off_t='off_t'
lseeksize=8
alignbytes=8
prototype=define
Linker and Libraries​:
ld='cc'
ldflags =' -fstack-protector-strong -L/usr/local/lib'
libpth=/usr/local/lib /usr/lib/gcc/x86_64-linux-gnu/4.9/include-fixed
/usr/include/x86_64-linux-gnu /usr/lib /lib/x86_64-linux-gnu /lib/../lib
/usr/lib/x86_64-linux-gnu /usr/lib/../lib /lib
libs=-lpthread -lnsl -ldb -ldl -lm -lcrypt -lutil -lc
perllibs=-lpthread -lnsl -ldl -lm -lcrypt -lutil -lc
libc=libc-2.19.so
so=so
useshrplib=true
libperl=libperl.so
gnulibc_version='2.19'
Dynamic Linking​:
dlsrc=dl_dlopen.xs
dlext=so
d_dlsymun=undef
ccdlflags='-Wl,-E -Wl,-rpath,/home/zefram/usr/
perl/perl_install/perl-5.27.6-i64-f52/lib/5.27.6/x86_64-
linux-thread-multi/CORE'
cccdlflags='-fPIC'
lddlflags='-shared -O2 -L/usr/local/lib -fstack-protector-strong'

---
@​INC for perl 5.27.6​:
/home/zefram/usr/perl/perl_install/perl-5.27.6-i64-f52/
lib/site_perl/5.27.6/x86_64-linux-thread-multi
/home/zefram/usr/perl/perl_install/perl-5.27.6-i64-f52/
lib/site_perl/5.27.6
/home/zefram/usr/perl/perl_install/perl-5.27.6-i64-f52/
lib/5.27.6/x86_64-linux-thread-multi
/home/zefram/usr/perl/perl_install/perl-5.27.6-i64-f52/lib/5.27.6

---
Environment for perl 5.27.6​:
HOME=/home/zefram
LANG (unset)
LANGUAGE (unset)
LD_LIBRARY_PATH (unset)
LOGDIR (unset)
PATH=/home/zefram/usr/perl/perl_install/perl-5.27.6-i64-
f52/bin​:/home/zefram/usr/perl/util​:/home/zefram/pub/x86_64-
unknown-linux-gnu/bin​:/home/zefram/pub/common/bin​:/usr/
bin​:/bin​:/usr/local/bin​:/usr/games
PERL_BADLANG (unset)
SHELL=/usr/bin/zsh

@p5pRT
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@p5pRT p5pRT commented Dec 19, 2017

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

@p5pRT
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@p5pRT p5pRT commented Dec 20, 2017

From @leonerd

On Tue, 19 Dec 2017 09​:10​:57 +0100
demerphq <demerphq@​gmail.com> wrote​:

I so look forward to this, both as a patch i can learn from and as a
feature I would enjoy. ++Zefram.

+1

I'd be happy to accept some new warnings, in exchange for being able to
e.g.​:

  grep { $min <= $_ <= $max } @​numbers

--
Paul "LeoNerd" Evans

leonerd@​leonerd.org.uk | https://metacpan.org/author/PEVANS
http​://www.leonerd.org.uk/ | https://www.tindie.com/stores/leonerd/

@p5pRT
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@p5pRT p5pRT commented Dec 20, 2017

From @davidnicol

how would this be done without looking ahead more than one parse-time
element?

Perhaps a "Comparison Expression" type would be introduced to the grammar.
Except the fact of the current syntax error implies there is already a
place the new thing could go.

Expressions in the middle should only get evaluated once, and later ones
should short-circuit out​:

  $min <= &lookup1 < &lookup2 <= $max

should be equivalent to

  do { my $tmp1 = &lookup1; $min <= $tmp1 and do { my $tmp2 =
&lookup2; $tmp1 < $tmp2 and $tmp2 <= $max } }

rather than the double-calling

  ( $min <= &lookup1 and &lookup1 < &lookup2 and &lookup2 <= $max )

for any expression &lookupN more complicated than an unblessed scalar.

I think I'm willing to write a test case for this if it would help.

On Wed, Dec 20, 2017 at 10​:39 AM, Paul "LeoNerd" Evans <
leonerd@​leonerd.org.uk> wrote​:

On Tue, 19 Dec 2017 09​:10​:57 +0100
demerphq <demerphq@​gmail.com> wrote​:

I so look forward to this, both as a patch i can learn from and as a
feature I would enjoy. ++Zefram.

+1

I'd be happy to accept some new warnings, in exchange for being able to
e.g.​:

grep { $min <= $_ <= $max } @​numbers

--
Paul "LeoNerd" Evans

leonerd@​leonerd.org.uk | https://metacpan.org/author/PEVANS
http​://www.leonerd.org.uk/ | https://www.tindie.com/stores/leonerd/

--
“no man should be compelled to do what the laws do not require; nor to
refrain from acts which the laws permit.” Calder v. Bull (U.S. 1798)

@xenu
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@xenu xenu commented Aug 15, 2020

This feature was implemented in perl 5.32, see 02b85d3 and the corresponding perldelta entry.

@xenu xenu closed this as completed Aug 15, 2020
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