Chained comparison evaluates tied scalar twice

[briandfoy]

Description

The perlop docs describe the current behavior, so this doesn't contradict anything.

A chained comparison in 5.31.10 appears to evaluate the middle condition twice for a tied scalar but doesn't warn about it. Since a programmer might not realize they are using a tied variable, they might see odd behavior.

Steps to Reproduce

use v5.31.10;

package UnstableScalar {
	use parent qw(Tie::Scalar);
	sub TIESCALAR { bless {}, $_[0] }
	sub FETCH {
		my $value = int rand 10;
		say "Fetching scalar with $value";
		return $value;
		}
	}

tie my $unstable, 'UnstableScalar';

if( 5 < $unstable < 9 ) {
	say "Found a value between 5 and 9";
	}

The comparison short circuits just fine, but if the first comparison is true, the tied scalar is evaluated again (so things such as Tie::Cycle will unexpectedly progress or skip values):

$ perl5.31.10 ~/Desktop/test.pl
Fetching scalar with 2

$ perl5.31.10 ~/Desktop/test.pl
Fetching scalar with 0

$ perl5.31.10 ~/Desktop/test.pl
Fetching scalar with 9
Fetching scalar with 2
Found a value between 5 and 9

$ perl5.31.10 ~/Desktop/test.pl
Fetching scalar with 6
Fetching scalar with 9

Expected behavior

I expect that the scalar would only call FETCH once and reuse the result in the second comparison. perlop says this may happen, but perhaps a warning would be useful here. Ideally, tied variables would be evaluated once.

Is there a chance that the internals could recognize this and use the tied object instead of the scalar itself? This works as expected:

	if( 5 < tied($unstable)->FETCH < 9 ) {
		say "Found a value between 5 and 9";
		}

Perl configuration

Summary of my perl5 (revision 5 version 31 subversion 10) configuration:

  Platform:
	osname=darwin
	osvers=19.3.0
	archname=darwin-2level
	uname='darwin otter.local 19.3.0 darwin kernel version 19.3.0: thu jan 9 20:58:23 pst 2020; root:xnu-6153.81.5~1release_x86_64 x86_64 '
	config_args='-des -Dprefix=/usr/local/perls/perl-5.31.10 -Dusedevel'
	hint=recommended
	useposix=true
	d_sigaction=define
	useithreads=undef
	usemultiplicity=undef
	use64bitint=define
	use64bitall=define
	uselongdouble=undef
	usemymalloc=n
	default_inc_excludes_dot=define
	bincompat5005=undef
  Compiler:
	cc='cc'
	ccflags ='-fno-common -DPERL_DARWIN -mmacosx-version-min=10.15 -fno-strict-aliasing -pipe -fstack-protector-strong -I/usr/local/include -DPERL_USE_SAFE_PUTENV'
	optimize='-O3'
	cppflags='-fno-common -DPERL_DARWIN -mmacosx-version-min=10.15 -fno-strict-aliasing -pipe -fstack-protector-strong -I/usr/local/include'
	ccversion=''
	gccversion='4.2.1 Compatible Apple LLVM 11.0.0 (clang-1100.0.33.17)'
	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 =' -mmacosx-version-min=10.15 -fstack-protector-strong -L/usr/local/lib'
	libpth=/usr/local/lib /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/lib/clang/11.0.0/lib /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/lib /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk/usr/lib /usr/lib
	libs=-lpthread -ldbm -ldl -lm -lutil -lc
	perllibs=-lpthread -ldl -lm -lutil -lc
	libc=
	so=dylib
	useshrplib=false
	libperl=libperl.a
	gnulibc_version=''
  Dynamic Linking:
	dlsrc=dl_dlopen.xs
	dlext=bundle
	d_dlsymun=undef
	ccdlflags=' '
	cccdlflags=' '
	lddlflags=' -mmacosx-version-min=10.15 -bundle -undefined dynamic_lookup -L/usr/local/lib -fstack-protector-strong'


Characteristics of this binary (from libperl): 
  Compile-time options:
	HAS_TIMES
	PERLIO_LAYERS
	PERL_COPY_ON_WRITE
	PERL_DONT_CREATE_GVSV
	PERL_MALLOC_WRAP
	PERL_OP_PARENT
	PERL_PRESERVE_IVUV
	PERL_USE_DEVEL
	PERL_USE_SAFE_PUTENV
	USE_64_BIT_ALL
	USE_64_BIT_INT
	USE_LARGE_FILES
	USE_LOCALE
	USE_LOCALE_COLLATE
	USE_LOCALE_CTYPE
	USE_LOCALE_NUMERIC
	USE_LOCALE_TIME
	USE_PERLIO
	USE_PERL_ATOF
  Built under darwin
  Compiled at Mar 27 2020 01:45:56
  %ENV:
	PERL="/Users/brian/bin/perls/perl-latest"
	PERL5_PATH="/Users/brian/bin/perls"
	PERL6_PATH="/Users/brian/bin/perl6s:/Applications/Rakudo/bin:/Applications/Rakudo/share/perl6/site/bin"
	PERLDOTCOM_AUTHOR="brian d foy"
  @INC:
	/usr/local/perls/perl-5.31.10/lib/site_perl/5.31.10/darwin-2level
	/usr/local/perls/perl-5.31.10/lib/site_perl/5.31.10
	/usr/local/perls/perl-5.31.10/lib/5.31.10/darwin-2level
	/usr/local/perls/perl-5.31.10/lib/5.31.10

[xsawyerx]

I consider a single FETCH to be correct user behavior.

[karenetheridge]

should this be a 5.32 blocker?

[jkeenan]

should this be a 5.32 blocker?

This is already on the "Milestone 5.32.0" list -- which I take to be the 'blocker' list for 5.32.

[toddr]

Relevant mailing list thread: https://www.nntp.perl.org/group/perl.perl5.porters/2020/04/msg257322.html

[xsawyerx]

I'm going to revise my position. The semantics do dictate the FETCH be done twice. It's only correct. However, user expectation may fall short of that.

Instead, I am in favor of a documentation update as @davidnicol suggested.

[briandfoy]

David Nicol suggested for the tied case that you capture the variable:

my $x = ...
if( 1 < $x < 10 ) { ... }

But, that assumes you know that something is tied, and the point of tie is that you don't have to know that. It's a feature where it takes more time to explain the edge case than the common case, and historically that sort of wart has been a pain point for Perl adoption. To effectively guard against the edge case, you are now checking the input for every case, which means it's easier to not use this feature.

I'm not sure which semantics dictate what. The mailing list thread was all implementation details, and specifically not language details. The language can decide what it wants to do with a feature, although I take it the implementation is harder because this isn't tokenized differently and is actually just shoehorning it into old syntax.

[khwilliamson]

@xenu submitted a comment that didn't make it into this ticket. The operation of this new feature is not as clear-cut as people previously thought. Shouldn't we make it experimental in 5.32? Since I think all new features should be experimental, based on my previous school of hard knocks, I agree with him.

[jkeenan]

On 4/16/20 11:34 AM, Karl Williamson wrote: @xenu <https://github.com/xenu> submitted a comment that didn't make it into this ticket. The operation of this new feature is not as clear-cut as people previously thought. Shouldn't we make it experimental in 5.32? Since I think all new features should be experimental, based on my previous school of hard knocks, I agree with him.

I concur with Karl here; make it experimental.

[demerphq]

It is documented that it does this. Yves

…

On Thu, 9 Apr 2020 at 13:55, Sawyer X ***@***.***> wrote: I'm going to revise my position. The semantics do dictate the FETCH be done twice. It's only correct. However, user expectation may fall short of that. Instead, I am in favor of a documentation update as @davidnicol <https://github.com/davidnicol> suggested. — You are receiving this because you are subscribed to this thread. Reply to this email directly, view it on GitHub <#17692 (comment)>, or unsubscribe <https://github.com/notifications/unsubscribe-auth/AAAZ5R7UYUHG7CXOBVFA3LDRLWZSLANCNFSM4L5U4YIQ> .

-- perl -Mre=debug -e "/just|another|perl|hacker/"

[demerphq]

This how it is documented to behave, why should this be a bug? Yves

…

On Sat, 4 Apr 2020 at 10:31, brian d foy ***@***.***> wrote: *Description* A chained comparison in 5.31.10 appears to evaluate the middle condition twice for a tied scalar. *Steps to Reproduce* use v5.31.10; package UnstableScalar { use parent qw(Tie::Scalar); sub TIESCALAR { bless {}, $_[0] } sub FETCH { my $value = int rand 10; say "Fetching scalar with $value"; return $value; } } tie my $unstable, 'UnstableScalar'; say $unstable foreach ( 0 .. 2 ); say "Comparing----"; if( 5 < $unstable < 9 ) { say "Found a value between 5 and 9"; } The comparison short circuits just fine, but if the first comparison is true, the tied scalar is evaluated again (so things such as Tie::Cycle will unexpectedly progress or skip values): $ perl5.31.10 ~/Desktop/test.pl Fetching scalar with 2 $ perl5.31.10 ~/Desktop/test.pl Fetching scalar with 0 $ perl5.31.10 ~/Desktop/test.pl Fetching scalar with 9 Fetching scalar with 2 Found a value between 5 and 9 $ perl5.31.10 ~/Desktop/test.pl Fetching scalar with 6 Fetching scalar with 9 *Expected behavior* I expect that the scalar would only call FETCH once and reuse the result in the second comparison. *Perl configuration* Summary of my perl5 (revision 5 version 31 subversion 10) configuration: Platform: osname=darwin osvers=19.3.0 archname=darwin-2level uname='darwin otter.local 19.3.0 darwin kernel version 19.3.0: thu jan 9 20:58:23 pst 2020; root:xnu-6153.81.5~1release_x86_64 x86_64 ' config_args='-des -Dprefix=/usr/local/perls/perl-5.31.10 -Dusedevel' hint=recommended useposix=true d_sigaction=define useithreads=undef usemultiplicity=undef use64bitint=define use64bitall=define uselongdouble=undef usemymalloc=n default_inc_excludes_dot=define bincompat5005=undef Compiler: cc='cc' ccflags ='-fno-common -DPERL_DARWIN -mmacosx-version-min=10.15 -fno-strict-aliasing -pipe -fstack-protector-strong -I/usr/local/include -DPERL_USE_SAFE_PUTENV' optimize='-O3' cppflags='-fno-common -DPERL_DARWIN -mmacosx-version-min=10.15 -fno-strict-aliasing -pipe -fstack-protector-strong -I/usr/local/include' ccversion='' gccversion='4.2.1 Compatible Apple LLVM 11.0.0 (clang-1100.0.33.17)' 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 =' -mmacosx-version-min=10.15 -fstack-protector-strong -L/usr/local/lib' libpth=/usr/local/lib /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/lib/clang/11.0.0/lib /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/lib /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk/usr/lib /usr/lib libs=-lpthread -ldbm -ldl -lm -lutil -lc perllibs=-lpthread -ldl -lm -lutil -lc libc= so=dylib useshrplib=false libperl=libperl.a gnulibc_version='' Dynamic Linking: dlsrc=dl_dlopen.xs dlext=bundle d_dlsymun=undef ccdlflags=' ' cccdlflags=' ' lddlflags=' -mmacosx-version-min=10.15 -bundle -undefined dynamic_lookup -L/usr/local/lib -fstack-protector-strong' Characteristics of this binary (from libperl): Compile-time options: HAS_TIMES PERLIO_LAYERS PERL_COPY_ON_WRITE PERL_DONT_CREATE_GVSV PERL_MALLOC_WRAP PERL_OP_PARENT PERL_PRESERVE_IVUV PERL_USE_DEVEL PERL_USE_SAFE_PUTENV USE_64_BIT_ALL USE_64_BIT_INT USE_LARGE_FILES USE_LOCALE USE_LOCALE_COLLATE USE_LOCALE_CTYPE USE_LOCALE_NUMERIC USE_LOCALE_TIME USE_PERLIO USE_PERL_ATOF Built under darwin Compiled at Mar 27 2020 01:45:56 %ENV: PERL="/Users/brian/bin/perls/perl-latest" PERL5_PATH="/Users/brian/bin/perls" PERL6_PATH="/Users/brian/bin/perl6s:/Applications/Rakudo/bin:/Applications/Rakudo/share/perl6/site/bin" PERLDOTCOM_AUTHOR="brian d foy" @inc: /usr/local/perls/perl-5.31.10/lib/site_perl/5.31.10/darwin-2level /usr/local/perls/perl-5.31.10/lib/site_perl/5.31.10 /usr/local/perls/perl-5.31.10/lib/5.31.10/darwin-2level /usr/local/perls/perl-5.31.10/lib/5.31.10 — You are receiving this because you are subscribed to this thread. Reply to this email directly, view it on GitHub <#17692>, or unsubscribe <https://github.com/notifications/unsubscribe-auth/AAAZ5R3YT56CGT33UJ4HXUDRK3V57ANCNFSM4L5U4YIQ> .

-- perl -Mre=debug -e "/just|another|perl|hacker/"