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[perl #126749] FH: get more benefit from ||, &&

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From:
KES
Date:
November 27, 2015 11:35
Subject:
[perl #126749] FH: get more benefit from ||, &&
Message ID:
rt-4.0.18-24248-1448624091-1138.126749-75-0@perl.org
# New Ticket Created by  KES 
# Please include the string:  [perl #126749]
# in the subject line of all future correspondence about this issue. 
# <URL: https://rt.perl.org/Ticket/Display.html?id=126749 >


To: perlbug@perl.org
Cc: kes-kes@yandex.ru
Subject: FH: get more benefit from ||, &&
From: kes-kes@yandex.ru
Reply-To: kes-kes@yandex.ru
Message-Id: <5.22.0_6261_1448609478@keswork>


This is a bug report for perl from kes-kes@yandex.ru,
generated with the help of perlbug 1.40 running under perl 5.22.0.


-----------------------------------------------------------------
[Please describe your issue here]

A few days ago I ran into the problem the expression does not work as
expected:

    eval: $z=7; ( $fh ) =  $z =~/^(\d+)$/  ||  $fh; print $fh;
    result: 1
    expected: 7

I have tried the DOC: http://perldoc.perl.org/perlop.html#C-style-Logical-Or
I have found only one interesting thing:

    Scalar or list context propagates down to the right operand if it is evaluated.

I would never have thought, but thank to 'mst'. He point me to:

    http://perldoc.perl.org/perlop.html#Logical-Defined-Or

And I have found I am interesting in:

    @a = @b || @c;          # this is wrong
    @a = scalar(@b) || @c;  # really meant this
    @a = @b ? @b : @c;      # this works fine, though

So behaviour for my case is documented and therefore should be expected and
my assumption is wrong. (sighs)

But what about DWIM (Do What I Mean) principle?
When I write '@a = @b || @c' I mean: assign first list, if it is empty assign other.
And I am surprised to have the list of (1) in @a

I ask people on the #perl channel and they argue that we need scalar to calcuate
the result of boolean expression so @b IS CALCULATED IN SCALAR CONTEXT

Diggig deeper and deeper...

When looking to assignment operators http://perldoc.perl.org/perlop.html#Assignment-Operators
Everybody will agree that:

    $x += 2            is equivalent to      $x = $x + 2
    @x ||= (1,2,3)     is equivalent to      @x = @x || (1,2,3)

    # @x += 2 This seems meaningless but maybe   @x = map $_+2, @x    # ??? =)

Use cases:

    @x ||= (1,2,3); # Can't modify array dereference in logical or assignment (||=) at
    print "@x\n";

    @x = (0,1);
    @x ||= (1,2,3); # Can't modify array dereference in logical or assignment (||=) at
    print "@x\n";


    @y = @y || (1,2,3);
    print "@y\n";            # 1 2 3

    @y = (0,1);
    @y = @y || (1,2,3);
    print "@y\n";            # 2

First surprise: why the short form does not work whereas long does?
Second suprise:

    people on irc said
    truth is a scalar concept. it can't work at all otherwise.

    @y = (0,1);
    print "@y"   if @y = @y; # (0,1) <--- Why this works?? it is list, not scalar

I have not found info (http://perldoc.perl.org/perlsyn.html#Compound-Statements)
in which context the if EXPR is executed so I assume it requires the scalar
also I relay on the 'truth is a scalar concept'. Now I suppose this:

    if scalar( EXPR )

Because of @y || @y also checks left @y for truth I am should be forced to suppose:

    scalar( @y ) || @y

This is same as described here: http://perldoc.perl.org/perlop.html#Logical-Defined-Or
But stop, || is more than just check for truth, here we require the result
of expression. Actually two results required:

    @y = @y || @y;
    1. for ||
    2. for =

Please pay attention and do not forget about 'if @y = @y' and about its two steps:

    1. We calculate the expression in its own context
    2. We force THE EXPRESSION RESULT into scalar to make truth concept to work

If we apply same logic @y = EXPR || EXPR:

    1. We require the EXPR result in list context,
       so calculate EXPR in list context
    2. force THE EXPRESSION RESULT into scalar to make check by ||
       if true return THE EXPRESSION RESULT else calculate EXPR in required context

in pseudo code that may look like (http://paste.scsys.co.uk/501911):

    1 sub OR {
    2    my( $left, $right ) = @_;
    3
    4    if( wantarray ) {
    5        @result = $left->();                      # list
    6        return @result if @result;
    7
    8        return $right->();
    9    }
    10   else {
    11       $result = $left->();                      # scalar
    12       return $result if $result;
    13
    14       return $right->();
    15   }
    16 }
    17
    18
    19 @l =  foo() OR bar(); # foo is called in list context
    20 $s =  foo() OR bar(); # foo is called in scalar context

The call context of fn_this should depend on context of ||

          fn_this() || fn_that()  # void
    $x =  fn_this() || fn_that()  # scalar
    @x =  fn_this() || fn_that()  # list

    sub t {
        print defined wantarray ?
            wantarray ? 'LIST - ' : 'SCALAR - ':
            'VOID - ';

        undef;
    }

Should be:

          t || t;    # VOID - VOID -
    $x =  t || t;    # SCALAR - SCALAR -
    @x =  t || t;    # LIST - LIST -

Instead of current:

          t || t;    # SCALAR - VOID -
    $x =  t || t;    # SCALAR - SCALAR -
    @x =  t || t;    # SCALAR - LIST -


What benefits we get:

    @x ||=  qw/ s o m e   d e f a u l t s /;       # ('s','o'...'s') if @x is empty
    @x =  $str =~ /^(\d)(\d)(\d)$/  ||  ();        # (7,5,3) if $str eq '753', NOT (3)
    @x =  fn_this() || fn_that();                  # faster: only one call to fn_this
    @x =  fn_this() ?  fn_this()  :  fn_that() ;   # slower: fn_this called twice

For the if statement there is not change in behaviour:

    1 if @x || @y;
    1 if t  || t;    # SCALAR - SCALAR -

PS. I have not thiat about but this logic may be applied to && also.


[Please do not change anything below this line]
-----------------------------------------------------------------
---
Flags:
    category=core
    severity=wishlist
---
Site configuration information for perl 5.22.0:

Configured by kes at Tue Sep 29 16:12:55 EEST 2015.

Summary of my perl5 (revision 5 version 22 subversion 0) configuration:
   
  Platform:
    osname=linux, osvers=3.13.0-37-generic, archname=x86_64-linux
    uname='linux keswork 3.13.0-37-generic #64-ubuntu smp mon sep 22 21:28:38 utc 2014 x86_64 x86_64 x86_64 gnulinux '
    config_args='-des -Dprefix=/home/kes/perl'
    hint=recommended, useposix=true, d_sigaction=define
    useithreads=undef, usemultiplicity=undef
    use64bitint=define, use64bitall=define, uselongdouble=undef
    usemymalloc=n, bincompat5005=undef
  Compiler:
    cc='cc', ccflags ='-fwrapv -fno-strict-aliasing -pipe -fstack-protector -I/usr/local/include -D_LARGEFILE_SOURCE -D_FILE_OFFSET_BITS=64',
    optimize='-O2',
    cppflags='-fwrapv -fno-strict-aliasing -pipe -fstack-protector -I/usr/local/include'
    ccversion='', gccversion='4.8.4', 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 -L/usr/local/lib'
    libpth=/usr/local/lib /usr/lib/gcc/x86_64-linux-gnu/4.8/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 -ldl -lm -lcrypt -lutil -lc
    perllibs=-lpthread -lnsl -ldl -lm -lcrypt -lutil -lc
    libc=libc-2.19.so, so=so, useshrplib=false, libperl=libperl.a
    gnulibc_version='2.19'
  Dynamic Linking:
    dlsrc=dl_dlopen.xs, dlext=so, d_dlsymun=undef, ccdlflags='-Wl,-E'
    cccdlflags='-fPIC', lddlflags='-shared -O2 -L/usr/local/lib -fstack-protector'


---
@INC for perl 5.22.0:
    /home/kes/perl5/lib/perl5/x86_64-linux
    /home/kes/perl5/lib/perl5
    /home/kes/perl/lib/site_perl/5.22.0/x86_64-linux
    /home/kes/perl/lib/site_perl/5.22.0
    /home/kes/perl/lib/5.22.0/x86_64-linux
    /home/kes/perl/lib/5.22.0
    .

---
Environment for perl 5.22.0:
    HOME=/home/kes
    LANG=ru_UA.UTF-8
    LANGUAGE=en
    LC_MESSAGES=en_US.UTF-8
    LD_LIBRARY_PATH (unset)
    LOGDIR (unset)
    PATH=/home/kes/perl/bin:/home/kes/perl/bin:/home/kes/perl5/bin:/home/kes/bin:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/usr/games:/usr/local/games
    PERL5LIB=/home/kes/perl5/lib/perl5
    PERLDB_OPTS=RemotePort=keswork:9001
    PERL_BADLANG (unset)
    PERL_MB_OPT=--install_base "/home/kes/perl5"
    PERL_MM_OPT=INSTALL_BASE=/home/kes/perl5
    SHELL=/bin/bash


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