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canonical truth values from overloaded predicates #16273

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p5pRT opened this issue Nov 22, 2017 · 3 comments
Open

canonical truth values from overloaded predicates #16273

p5pRT opened this issue Nov 22, 2017 · 3 comments

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@p5pRT
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p5pRT commented Nov 22, 2017

Migrated from rt.perl.org#132489 (status was 'open')

Searchable as RT132489$

@p5pRT
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p5pRT commented Nov 22, 2017

From zefram@fysh.org

Created by zefram@fysh.org

Operator overloading methods are allowed to return any value. Like this​:

$ perl -lwe 'package Five { use overload "<" => sub { 5 }; } print bless({}, "Five") < 3'
5

It makes sense for operators like "+" to be able to return any value,
but it doesn't make sense for predicate operators, such as "!" and
comparisons. To have these operators sometimes able to return values
that are not canonical truth values just makes them surprising,
and interferes with optimisations that we could potentially make.
Operators that ordinarily return only truth values should canonicalise
the truth value returned by overload methods. Note that we already have
such canonicalisation in the tie interfaces, for example for EXISTS in
the tied hash interface.

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 commented Dec 14, 2017

From @csjewell

On Wed, 22 Nov 2017 13​:52​:12 -0800, zefram@​fysh.org wrote​:

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]

Operator overloading methods are allowed to return any value. Like
this​:

$ perl -lwe 'package Five { use overload "<" => sub { 5 }; } print
bless({}, "Five") < 3'
5

It makes sense for operators like "+" to be able to return any value,
but it doesn't make sense for predicate operators, such as "!" and
comparisons. To have these operators sometimes able to return values
that are not canonical truth values just makes them surprising,
and interferes with optimisations that we could potentially make.
Operators that ordinarily return only truth values should canonicalise
the truth value returned by overload methods. Note that we already
have
such canonicalisation in the tie interfaces, for example for EXISTS in
the tied hash interface.

The problem is that there are modules (at least one​: IO​::All) that abuse the fact that overloaded comparators are NOT canonicalized in a useful way.

@p5pRT
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p5pRT commented Dec 14, 2017

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

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