Inchworm vs scalar: on boolean false value

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

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]
Re^2: Inchworm vs scalar: on boolean false value by haukex (Archbishop) on Dec 17, 2019 at 21:55 UTC
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]
Re^2: Inchworm vs scalar: on boolean false value by Fletch (Bishop) on Dec 17, 2019 at 14:32 UTC
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((('[' . $_) . ']'));} scalar((!1)), '', 0, 0);` [download] The cake is a lie. The cake is a lie. The cake is a lie.	[reply] [d/l] [select]
Re^2: Inchworm vs scalar: on boolean false value by Eily (Monsignor) on Dec 17, 2019 at 14:30 UTC
Binary "~~" does a smartmatch between its arguments. Here it's the unary ~~ though (or rather, two unary ~ in a row), because of the commas, the same way that `A, -B` is a list of two values, not the difference between the list (A,) and the scalar B.	[reply] [d/l]
Re^3: Inchworm vs scalar: on boolean false value by LanX (Saint) on Dec 17, 2019 at 15:59 UTC
It looked unary for me too, but I'd rather prefer being sure. That's why I mentioned Deparse. Cheers Rolf _{(addicted to the Perl Programming Language :) Wikisyntax for the Monastery FootballPerl is like chess, only without the dice}	[reply]
Re: Inchworm vs scalar: on boolean false value by Eily (Monsignor) on Dec 17, 2019 at 14:44 UTC
Also, if you're not using the ouput of map, it's shorter and easier to read (IMHO at least) to use a postfix for loop: `print "[$_]" 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]

Update