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Feature: get "name" of lvalue expression #16328

p5pRT opened this issue Dec 20, 2017 · 4 comments

Feature: get "name" of lvalue expression #16328

p5pRT opened this issue Dec 20, 2017 · 4 comments


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@p5pRT p5pRT commented Dec 20, 2017

Migrated from (status was 'rejected')

Searchable as RT132628$

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@p5pRT p5pRT commented Dec 20, 2017

From @epa

Created by @epa

Often an error message ends up repeating the name of the variable it's dealing with.

  die "bad height $height" unless (50 <= $height && $height <= 150);
  die "bad width $width" unless (20 <= $width && $width <= 70);
  die "bad length $length" unless (50 <= $length && $length <= 200);

One would want to refactor the above code into something like

  my @​checks = ([ $height, 50, 150 ], [ $width, 20, 70 ], [ $length, 50, 200 ]);
  foreach (@​checks) {
  my ($v, $min, $max) = @​$_;
  die "bad value $v" unless ($min <= $v && $v <= $max);

But then you lose the useful text in the error message. Of course you
can then add it as a fourth item in the tuple, but for a quick bit of
scripting, wouldn't it be nice if the name of the variable itself
could get into the error message?

If you forgo lexically scoped variables then you can do this. You
could have the string 'height' and then do ${$varname}. But this
feature request is for going the other way - given an lvalue scalar,
find the variable it corresponds to.

Clearly, this could not be expected to work in all circumstances.
Many scalars do not correspond to any variable, lexical or otherwise.
In other cases two different variable names might end up holding the
same scalar. But remember that we are just talking about error
messages here. Just as Perl includes the line number in the error
message (even though that line number is an internal implementation
detail of the program) it could try to print a variable name too.

  die "bad value $v from variable " . nameof($v);

If the scalar passed to nameof() can be found in the current scope
then it returns that name (so '$height' in this case). Otherwise it
could return '(anonymous scalar at 0xabcd)' or perhaps undef.

Since the normal use would be only for debugging messages and error
checks, it doesn't need to be fast. It could linearly scan the symbol

(An alternative way to cut down on typing would be some kind of compile
time foreach​:

  my ($height, $width, $length);
  static_foreach $varname ('height', 'width', 'length') {
  my $v = $$varname;

but I think that is going too far for a dynamic language like Perl.
Better to allow the code to be written in the normal style, but allow
a bit of 'peeking' at the time an error has to be reported.)

Perl Info


Site configuration information for perl 5.22.2:

Configured by Red Hat, Inc. at Fri Nov  4 14:35:02 UTC 2016.

Summary of my perl5 (revision 5 version 22 subversion 2) configuration:
    osname=linux, osvers=4.7.9-200.fc24.x86_64, archname=x86_64-linux-thread-multi
    uname='linux 4.7.9-200.fc24.x86_64 #1 smp thu oct 20 14:26:16 utc 2016 x86_64 x86_64 x86_64 gnulinux '
    config_args='-des -Doptimize=none -Dccflags=-O2 -g -pipe -Wall -Werror=format-security -Wp,-D_FORTIFY_SOURCE=2 -fexceptions -fstack-protector-strong --param=ssp-buffer-size=4 -grecord-gcc-switches  -m64 -mtune=generic -Dldflags=-Wl,-z,relro  -Dccdlflags=-Wl,--enable-new-dtags -Wl,-z,relro  -Dlddlflags=-shared -Wl,-z,relro  -Dshrpdir=/usr/lib64 -DDEBUGGING=-g -Dversion=5.22.2 -Dmyhostname=localhost -Dperladmin=root@localhost -Dcc=gcc -Dcf_by=Red Hat, Inc. -Dprefix=/usr -Dvendorprefix=/usr -Dsiteprefix=/usr/local -Dsitelib=/usr/local/share/perl5 -Dsitearch=/usr/local/lib64/perl5 -Dprivlib=/usr/share/perl5 -Dvendorlib=/usr/share/perl5/vendor_perl -Darchlib=/usr/lib64/perl5 -Dvendorarch=/usr/lib64/perl5/vendor_perl -Darchname=x86_64-linux-thread-multi -Dlibpth=/usr/local/lib64 /lib64 /usr/lib64 -Duseshrplib -Dusethreads -Duseithreads -Dusedtrace=/usr/bin/dtrace -Duselargefiles -Dd_semctl_semun -Di_db -Ui_ndbm -Di_gdbm -Di_shadow -Di_syslog -Dman3ext=3pm -Duseperlio -Dinstallusrbinperl=n -Ubincompat5005 -Uversiononly -Dpager=/usr/bin/less -isr -Dd_gethostent_r_proto -Ud_endhostent_r_proto -Ud_sethostent_r_proto -Ud_endprotoent_r_proto -Ud_setprotoent_r_proto -Ud_endservent_r_proto -Ud_setservent_r_proto -Dscriptdir=/usr/bin -Dusesitecustomize'
    hint=recommended, useposix=true, d_sigaction=define
    useithreads=define, usemultiplicity=define
    use64bitint=define, use64bitall=define, uselongdouble=undef
    usemymalloc=n, bincompat5005=undef
    cc='gcc', ccflags ='-D_REENTRANT -D_GNU_SOURCE -O2 -g -pipe -Wall -Werror=format-security -Wp,-D_FORTIFY_SOURCE=2 -fexceptions -fstack-protector-strong --param=ssp-buffer-size=4 -grecord-gcc-switches -m64 -mtune=generic -fwrapv -fno-strict-aliasing -I/usr/local/include -D_LARGEFILE_SOURCE -D_FILE_OFFSET_BITS=64',
    optimize='  -g',
    cppflags='-D_REENTRANT -D_GNU_SOURCE -O2 -g -pipe -Wall -Werror=format-security -Wp,-D_FORTIFY_SOURCE=2 -fexceptions -fstack-protector-strong --param=ssp-buffer-size=4 -grecord-gcc-switches -m64 -mtune=generic -fwrapv -fno-strict-aliasing -I/usr/local/include'
    ccversion='', gccversion='5.3.1 20160406 (Red Hat 5.3.1-6)', 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='gcc', ldflags ='-Wl,-z,relro  -fstack-protector-strong -L/usr/local/lib'
    libpth=/usr/local/lib64 /lib64 /usr/lib64 /usr/local/lib /usr/lib /lib/../lib64 /usr/lib/../lib64 /lib
    libs=-lpthread -lresolv -lnsl -lgdbm -ldb -ldl -lm -lcrypt -lutil -lc -lgdbm_compat
    perllibs=-lpthread -lresolv -lnsl -ldl -lm -lcrypt -lutil -lc, so=so, useshrplib=true,
  Dynamic Linking:
    dlsrc=dl_dlopen.xs, dlext=so, d_dlsymun=undef, ccdlflags='-Wl,--enable-new-dtags -Wl,-z,relro '
    cccdlflags='-fPIC', lddlflags='-shared -Wl,-z,relro  -L/usr/local/lib -fstack-protector-strong'

Locally applied patches:
    Fedora Patch1: Removes date check, Fedora/RHEL specific
    Fedora Patch3: support for libdir64
    Fedora Patch4: use libresolv instead of libbind
    Fedora Patch5: USE_MM_LD_RUN_PATH
    Fedora Patch6: Skip hostname tests, due to builders not being network capable
    Fedora Patch7: Dont run one io test due to random builder failures
    Fedora Patch15: Define SONAME for
    Fedora Patch16: Install to -Dshrpdir value
    Fedora Patch22: Document Math::BigInt::CalcEmu requires Math::BigInt (CPAN RT#85015)
    Fedora Patch26: Make *DBM_File desctructors thread-safe (RT#61912)
    Fedora Patch27: Make PadlistNAMES() lvalue again (CPAN RT#101063)
    Fedora Patch28: Make magic vtable writable as a work-around for Coro (CPAN RT#101063)
    Fedora Patch29: Fix duplicating PerlIO::encoding when spawning threads (RT#31923)
    Fedora Patch30: Do not let XSLoader load relative paths (CVE-2016-6185)
    Fedora Patch31: Avoid loading optional modules from default . (CVE-2016-1238)
    Fedora Patch32: Fix a crash in lexical scope warnings (RT#128597)
    Fedora Patch33: Do not mangle errno from failed socket calls (RT#128316)
    Fedora Patch34: Fix crash in "evalbytes S" (RT#129196)
    Fedora Patch35: Fix crash in "evalbytes S" (RT#129196)
    Fedora Patch36: Fix crash in "evalbytes S" (RT#129196)
    Fedora Patch37: Fix crash in splice (RT#129164, RT#129166, RT#129167)
    Fedora Patch38: Fix string overrun in Perl_gv_fetchmethod_pvn_flags (RT#129267)
    Fedora Patch39: Fix string overrun in Perl_gv_fetchmethod_pvn_flags (RT#129267)
    Fedora Patch40: Fix string overrun in Perl_gv_fetchmethod_pvn_flags (RT#129267)
    Fedora Patch41: Fix string overrun in Perl_gv_fetchmethod_pvn_flags (RT#129267)
    Fedora Patch42: Fix string overrun in Perl_gv_fetchmethod_pvn_flags (RT#129267)
    Fedora Patch43: Fix crash when matching UTF-8 string with non-UTF-8 substrings (RT#129350)
    Fedora Patch44: Fix parsing perl options in shell bang line (RT#129336)
    Fedora Patch45: Fix firstchar bitmap under UTF-8 with prefix optimization (RT#129950)
    Fedora Patch46: Avoid infinite loop in h2xs tool if enum and type have the same name (RT130001)
    Fedora Patch47: Fix stack handling when calling chdir without an argument (RT#129130)
    Fedora Patch200: Link XS modules to with EU::CBuilder on Linux
    Fedora Patch201: Link XS modules to with EU::MM on Linux

@INC for perl 5.22.2:

Environment for perl 5.22.2:
    LANGUAGE (unset)
    LD_LIBRARY_PATH (unset)
    LOGDIR (unset)
    PERL_BADLANG (unset)

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@p5pRT p5pRT commented Dec 20, 2017

From @epa

An obvious flaw in my proposal is that nameof($v) returns '$v'.
You would need to pass a reference​:

  sub check {
  my ($ref, $min, $max) = @​$_;
  unless ($min <= $$ref && $$ref <= $max) {
  die "bad value $$ref for variable​: " . nameof($$ref);

At this point you no longer have a 'ponies' solution which just prints out whatever the name is of the variable you were dealing with. So the bug might be best closed; as nice as it would be, the magic probably doesn't exist.

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@p5pRT p5pRT commented Dec 20, 2017

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

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@p5pRT p5pRT commented Dec 21, 2017

@iabyn - Status changed from 'open' to 'rejected'

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