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Inchworm vs scalar: on boolean false value

by rsFalse (Chaplain)
on Dec 17, 2019 at 13:53 UTC ( [id://11110273]=perlquestion: print w/replies, xml ) Need Help??

rsFalse has asked for the wisdom of the Perl Monks concerning the following question:

Hello,

Today I've found for myself that inchworm "~~" (perlsecret operator: two consecutive unary inverse operators) gives '0' on "false", not ''. Can someone explain why behaviour of 'inchworm' and 'scalar' operators differ here? 
#!/usr/bin/perl -l

use strict;
use warnings;

map { print "[$_]" }
    scalar( 1 == 0 ),
    ~~ '',
    ~~ scalar( 1 == 0 ),
    ~~ ( 1 == 0 );

[download]
output: 
[]
[]
[0]
[0]

[download]
It is strange for me, that print ~~ scalar( 1 == 0 ) outputs '0' even when scalar( 1 == 0 ) corresponds to ''.

Replies are listed 'Best First'.
Re: Inchworm vs scalar: on boolean false value
by Eily (Monsignor) on Dec 17, 2019 at 14:20 UTC

    There are two things at work here. First, the false value of perl is actually neither strictly '' nor 0, it's kinda both (link to the doc when I find it). As demonstrated by 

    perl -E "use strict; use warnings; say( '' + 1 ); say( !1 + 1 ); say !
+1; say '!1 is defined' if defined !1"
Argument "" isn't numeric in addition (+) at -e line 1.
1
1

!1 is defined

    [download]
    The warning about the non numeric argument happens only once, because !1 is treated as a number in arithmetic context. But printing !1 doesn't show anything because false is the empty string in string context.

    ~ however only works in two ways: bitwise not on a number, or bitwise not on a binary string. And the output will be of the same type. This means that ~~ is only equivalent to scalar when the operand is a number or a string, but it will force another type (probably string) on any other value (an object, a reference...).

    Edit: one place where the special value of perl's false is mentioned: Relational Operators.
    Also, ~~ is not equivalent to scalar for some big numbers and negative numbers: 

      DB<1> say ~~ (-1)
18446744073709551615

    [download]
    So it's better not to use it outside of obfuscated code.

[reply]
[d/l]
[select]
      the false value of perl is actually neither strictly '' nor 0, it's kinda both (link to the doc when I find it).

      Truth and Falsehood :-)

[reply]
[d/l]
Re: Inchworm vs scalar: on boolean false value
by LanX (Saint) on Dec 17, 2019 at 13:59 UTC
    ~ is a binary operation not a Boolean, IIRC.

    If you want Boolean results use !! instead.

    Update

    Uhm ... I think it might not do what you think it does.

    Binary "~~" does a smartmatch between its arguments.

    Better run B::Deparse to understand what the parser sees.

    Cheers Rolf
    (addicted to the Perl Programming Language :)
    Wikisyntax for the Monastery FootballPerl is like chess, only without the dice

[reply]
[d/l]

      Link to what he's using in perlsecret. Interestingly enough Deparse doesn't get much to work with (looks like the constant values generated are what's available; 5.26.1 using -MO=Deparse,-p,-q) 

      BEGIN { $/ = "\n"; $\ = "\n"; }
use strict;
use warnings;
map({print((('[' . $_) . ']'));} scalar((!1)), '', 0, 0);

      [download]

      The cake is a lie.
      The cake is a lie.
      The cake is a lie.

[reply]
[d/l]
[select]

      Binary "~~" does a smartmatch between its arguments.
      Here it's the unary ~~ though (or rather, two unary ~ in a row), because of the commas, the same way that A, -B is a list of two values, not the difference between the list (A,) and the scalar B.

[reply]
[d/l]
        It looked unary for me too, but I'd rather prefer being sure.

        That's why I mentioned Deparse.

        Cheers Rolf
        (addicted to the Perl Programming Language :)
        Wikisyntax for the Monastery FootballPerl is like chess, only without the dice

[reply]
Re: Inchworm vs scalar: on boolean false value
by Eily (Monsignor) on Dec 17, 2019 at 14:44 UTC

    Also, if you're not using the ouput of map, it's shorter and easier to read (IMHO at least) to use a postfix for loop: 

    print "[$_]" for
    scalar( 1 == 0 ),
    ~~ '',
    ~~ scalar( 1 == 0 ),
    ~~ ( 1 == 0 );

    [download]

[reply]
[d/l]
Re: Inchworm vs scalar: on boolean false value
by LanX (Saint) on Dec 17, 2019 at 14:15 UTC
    Another thing to consider is IIRC that "false" or !!0 is a dual var which becomes 0 in numeric context and empty in string context.

    So is ~ now operating on the empty string or the 0?

    Cheers Rolf
    (addicted to the Perl Programming Language :)
    Wikisyntax for the Monastery FootballPerl is like chess, only without the dice

[reply]

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