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Id: 132490
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 22:04:54 +0000
From: zefram [...] fysh.org
To: perlbug [...] perl.org
Subject: context consistency for overloaded operators
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] Should context be passed through when operators invoke overloaded methods? Here's the current situation: $ perl -lwe 'package Foo { use overload "!" => sub { print wantarray ? "list" : defined(wantarray) ? "scalar" : "void"; "foo" }; } $f = bless({}, "Foo"); $a = !$f; @a = (!$f); !$f' Useless use of not in void context at -e line 1. scalar scalar void It's not consistent. Void context is passed through, but the method always gets scalar context if the operator is in a non-void context. This seems like a bad arrangement. We should not make list context be propagated. The purpose of overloading isn't to turn operator invocations into arbitrary method calls; allowing an operator that normally yields a scalar result to return a list would complicate all sorts of op processing. The purpose of overloading is to allow an object to provide behaviour that fits into the general concept of the operator. Propagating void context doesn't advance that aim. The general concept of these operators includes that they perform the same operation regardless of context. To that end, an overload method should be aided in its aim of consistent behaviour by being called with a consistent context. Note that we have no need to propagate void context for the purpose of warnings. The warning arises from the *operator* appearing in void context. [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
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Show quoted text
>Should context be passed through when operators invoke overloaded methods
Because operators implemented as subroutines this subroutines should know the context they are called. Show quoted text
>The purpose of overloading isn't to turn operator invocations into arbitrary method calls;
Why not? Operator call is just subroutine call. This is simple and did not force user to know something special. Show quoted text
>allowing an operator that normally yields a scalar result to return a list would complicate all sorts of op processing
I think here we should just distinguish unary and list operators/subroutines. Unary should always produce the scalar: %h = ( x => !$obj, # call in list context, but we return scalar y => scalar $obj->method, # call in list context, but we return scalar ) I will agree that list result should be prohibited for calls parsed as unary operators: sub myrand (;$)/sub xxx($) Show quoted text
>The purpose of overloading is to allow an object to provide behaviour that fits into the general concept of the operator. Propagating void context doesn't advance that aim
I do not see any reason to prohibit context check. For example we can **skip hard work** when somebody call operator in void context (Not just emit warning) Show quoted text
>The general concept of these operators includes that they perform the same operation regardless of context
These always lead to confusion when # https://perldoc.perl.org/perlop.html#Logical-Defined-Or @a = @b || @c; # This doesn't do the right thing Also without context propagation we will not be able to implement sets multiplication: @multi = @z * @y; @multi = (1,2,3)*(3,4); print @multi # [1,3],[1,4],[2,3],[2,4],[3,3],[3,4]


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