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Re^2: Why is Dumper returning "!!1" for true file test?
by nysus (Parson) on Jan 20, 2024 at 17:58 UTC
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I just started using the latest version of Perl from version 5.32. I hadn't heard about the new booleans. Hadn't heard about !!0 and !!1 either.
These are definitely a weird beast. When I did print Dumper !!1 I expected it to print 1 but in fact it printed $VAR 1= !!1. This must be the dual var nature you refer to.
Is there a basic tutorial on all this?
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Perl 5.36 does have "new" booleans. Copies of the immortal &PL_sv_yes and &PL_sv_no values are still identifiable as booleans. This means they can be serialized more accurately. The latest versions of JSON::PP, Data::Dumper, and Storable all support this.
Prior to perl 5.36 it was not possible to identify a boolean true value after it had been copied. The exact same SV type, flags, and values could be produced by normal usage of a string "1". A boolean false would be more identifiable as it would never be produced naturally, requiring dualvar to construct.
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> What was missing is a function whose sole purpose was to return them. That's what was recently introduced. And that's why we used !!0 and !!1 to produce them before that.
Are you referring to builtin::true etc. in builtin? Nobody has mentioned them yet in this thread.
Or are you talking about the internal &PL_sv_yes ?
On a side note: shouldn't it be possible to activate true and false while silencing the warnings with just one pragma?
use experimental qw/builtin/;
use builtin qw/true false/;
use Data::Dump;
ddx $];
dd [true,false]
__END__
# bool.pl:6: "5.036000"
[1, ""]
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> Hadn't heard about !!0 and !!1 either.
I hope it's clear that ! is just the logical negation operator.
Double negations are a very common idiom, you will find it in many languages, also in JS.
!!$a is in the same idiomatic niche like 0+$a or "".$a of explicit casting to a certain type without altering the value. Just Boolean now instead of number or string.
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I'm not aware of a single perldoc discussing all details, like it's done in MDN for JS.
(Especially as it's mostly internal stuff which could change and the effects are supposed to be DWIM. Like I said most problems arise when converting to other languages, esp. JS)
But you can gather them from various places like perlop or perlglossary .
FWIW and I don't think !!1 used to be dualvar (see Scalar::Util ) prior to 5.36.
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FWIW and I don't think !!1 used to be dualvar (see Scalar::Util ) prior to 5.36.
It's actually a triplevar, and it has been so at least as far back as 5.6, but probably a lot further back.
I don't have 5.6, but here's 5.8.9:
$ 5.8.9/bin/perl -MDevel::Peek -e'Dump(!!1)'
SV = PVNV(0x55e2de0d1348) at 0x55e2dd477310
REFCNT = 2147483647
FLAGS = (IOK,NOK,POK,READONLY,pIOK,pNOK,pPOK)
IV = 1
NV = 1
PV = 0x55e2de0d2320 "1"\0
CUR = 1
LEN = 8
$ 5.8.9/bin/perl -MDevel::Peek -e'Dump(!!0)'
SV = PVNV(0x55aeb71e8320) at 0x55aeb5e05da0
REFCNT = 2147483647
FLAGS = (IOK,NOK,POK,READONLY,pIOK,pNOK,pPOK)
IV = 0
NV = 0
PV = 0x55aeb71e8300 ""\0
CUR = 0
LEN = 8
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Re^2: Why is Dumper returning "!!1" for true file test?
by NERDVANA (Priest) on Jan 20, 2024 at 23:38 UTC
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The real question is why it returns "!!0" and "!!1" instead of "!1" and "!0". Saves a character and a runtime op... | [reply] |
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> Saves a character and a runtime op...
And is confusing because you need to mentally negate it again, while !! is a common idiom.
But the performance question occurred to me already I kind of expected constant folding to happen.
update
ha, indeed! :)
DB<5> p B::Deparse->new()->coderef2text( sub{!!1} )
{
use feature ... ; # shortened
!0;
}
DB<6> p B::Deparse->new()->coderef2text( sub{!!0} )
{
use feature ... ; # yadda
!1;
}
DB<7> p $]
5.038002
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