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context consistency for overloaded operators #16274

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

context consistency for overloaded operators #16274

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#132490 (status was 'open')

Searchable as RT132490$

@p5pRT
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p5pRT commented Nov 22, 2017

From zefram@fysh.org

Created by zefram@fysh.org

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.

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 11, 2017

From @KES777

Should context be passed through when operators invoke overloaded methods

Because operators implemented as subroutines this subroutines should know the context they are called.

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.

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($)

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)

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]

@p5pRT
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p5pRT commented Dec 11, 2017

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

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