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Report information
Id: 132489
Status: open
Priority: 0/
Queue: perl5

Owner: Nobody
Requestors: zefram [at] fysh.org
Cc:
AdminCc:

Operating System: (no value)
PatchStatus: (no value)
Severity: Wishlist
Type: core
Perl Version: 5.27.6
Fixed In: (no value)



Date: Wed, 22 Nov 2017 21:51:59 +0000
From: zefram [...] fysh.org
To: perlbug [...] perl.org
Subject: canonical truth values from overloaded predicates
CC: zefram [...] fysh.org
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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. [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
RT-Send-CC: perl5-porters [...] perl.org
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On Wed, 22 Nov 2017 13:52:12 -0800, zefram@fysh.org wrote: Show quoted text
> > 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.


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