The mystery of double bitwise negation

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

Hello again Monks!

Another day - another mystery. This time is about double bitwise negation (or strange form of smartmatch)

First encountered in DBM::Deep:

my %is_legal_filter = map {
    $_ => ~~1
} qw(
    store_key store_value
    fetch_key fetch_value
);
[download]

Than, in Devel::Declare (even stranger form):

  set_in_declare(~~@{$temp_name||[]});
[download]

Deparse says that at least first one is double negation, no-op. So - what for?

P.S. is the second example just a weird replacement for scalar ?

Comment on The mystery of double bitwise negation Select or Download Code

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Re: The mystery of double bitwise negation by BrowserUk (Patriarch) on Jul 12, 2015 at 18:04 UTC
I'd say that the first one was completely pointless. Double bit-wise inversion of the constant 1 serves no purpose at all: `C:\test>perl -MDevel::Peek -le" my($m,$n) = (1,~~1); print Dump($m), ' + ', Dump( $n )" SV = IV(0x2ca374) at 0x15501c REFCNT = 1 FLAGS = (PADBUSY,PADMY,IOK,pIOK) IV = 1 SV = IV(0x2ca378) at 0x155010 REFCNT = 1 FLAGS = (PADBUSY,PADMY,IOK,pIOK) IV = 1` [download] The second is a golfed version of scalar. Why? No idea. With the rise and rise of 'Social' network sites: 'Computers are making people easier to use everyday' Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error. "Science is about questioning the status quo. Questioning authority". In the absence of evidence, opinion is indistinguishable from prejudice. I'm with torvalds on this Agile (and TDD) debunked I told'em LLVM was the way to go. But did they listen!	[reply] [d/l]
Re: The mystery of double negation by shmem (Chancellor) on Jul 12, 2015 at 17:20 UTC
The point of Double Negation is casting any true value to 1, and any false value to 0. See also: Re^2: Secret Perl Operators: The Highlander List Asserter perl -le'print map{pack c,($-++?1:13)+ord}split//,ESEL'	[reply]
Re^2: The mystery of double negation by hurricup (Pilgrim) on Jul 12, 2015 at 17:34 UTC
Nope, it's bitwise negation and doesn't work this way. It's basically - noop. Keeps value the same.	[reply]
Re^3: The mystery of double negation (bool 0) by tye (Sage) on Jul 12, 2015 at 22:51 UTC
The point of Double Negation is casting [...] any false value to 0. Nope, it's bitwise negation and doesn't work this way. It's basically - noop. Keeps value the same. Um: `% say "(!1)" % say "~~(!1)" 0` [download] So it could be an idiom that is applied to all Boolean values (so that Boolean false shows up as `'0'` instead of as the "invisible" `''`). It just does nothing when the Boolean value is already 1. - tye	[reply] [d/l] [select]
Re^4: The mystery of double negation (bool 0) by zwon (Abbot) on Jul 13, 2015 at 20:55 UTC
Re^5: The mystery of double negation (SvPOK) by tye (Sage) on Jul 13, 2015 at 21:51 UTC
Re^3: The mystery of double negation by shmem (Chancellor) on Jul 12, 2015 at 17:41 UTC
Right you are. Then... maybe chromatic can enlighten us? perl -le'print map{pack c,($-++?1:13)+ord}split//,ESEL'	[reply]
Re^3: The mystery of double negation by Laurent_R (Canon) on Jul 12, 2015 at 18:01 UTC
You're right, but, still, `~~1` will return 1, i.e a true value. Why the author of the module did that, I have no idea.	[reply] [d/l]
Re: The mystery of double bitwise negation by ikegami (Patriarch) on Jul 13, 2015 at 17:04 UTC
For integers, it's no-op that evaluates its operand in scalar context. So, `set_in_declare(~~@{$temp_name\|\|[]});` [download] is being used in place of `set_in_declare(scalar(@{$temp_name\|\|[]}));` [download]	[reply] [d/l] [select]

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