Re: [PATCH] gitweb: merge boolean feature subroutines

From: demerphq <demerphq@gmail.com>
Date: Jan 3, 2009, 17:41 UTC
Message-ID: <9b18b3110901030941n3aa04e5ejb34c6f82c11bce4c@mail.gmail.com>
In-Reply-To: <20090103171333.GB4205@ftbfs.org>

2009/1/3 Matt Kraai <kraai@ftbfs.org>:

Show 41 quoted lines

> On Sat, Jan 03, 2009 at 05:51:50PM +0100, demerphq wrote:
>> 2009/1/3 Matt Kraai <kraai@ftbfs.org>:
>> > I don't think Perl has *a* false value, but rather has multiple values
>> > that are treated as false, such as undef, zero, and the empty string.
>> > Personally, I find 0 clearer than the empty string, but that's
>> > probably just my C bias sneaking in.
>>
>> Yes it definitely does have a false value, PL_sv_no, and a true value,
>> PL_sv_yes (although it is much less important).  It is these values
>> which are copied to signify true and false in the cases where the
>> internals need to, such as for boolean operators that must return
>> false, and things like negation and (in)equality checks.
>>
>> It is a so called "dual var" SvPVNV, with 0 in the NV (numeric) slot
>> and the empty string in the PV (string) slot.
>>
>> You can see one example of its behaviour in my previous mail, and can
>> see it further here:
>>
>> $ perl -MDevel::Peek -e'print Dump(shift @ARGV eq "true")'
>> SV = PVNV(0x952eb10) at 0x952b6f0
>>   REFCNT = 2147483647
>>   FLAGS = (IOK,NOK,POK,READONLY,pIOK,pNOK,pPOK)
>>   IV = 0
>>   NV = 0
>>   PV = 0x952eae8 ""\0
>>   CUR = 0
>>   LEN = 4
>>
>> Compare that to:
>>
>> perl -MDevel::Peek -e'print Dump(shift @ARGV eq "true" ? 1 : 0)'
>> SV = IV(0x94d8398) at 0x94bd678
>>   REFCNT = 1
>>   FLAGS = (PADBUSY,PADTMP,IOK,READONLY,pIOK)
>>   IV = 0
>
> Wow, I had no idea about this.  Thanks for the information.
>
> It seems like using these values would require obfuscating the code,
> though.  :(  They only seem to be exposed directly via XS.

Depend how you look at it. You have access to them via the negation and inequality operators. In a way you can think of the 'not' (or !) operator as being the "inverted boolean constructor". So for instance the kind of surprising

  my $bool= !!$val;

can be used to get a copy of the appropriate PL_sv_yes/no. But the need to do is rare. Luckily. :-)

But yeah, they are not exposed directly at the perl level. There is no keyword to use to generate them like there is for undef/PL_sv_undef, although I guess it would be fairly easy to expose them in a similar fashion via Scalar::Util.

cheers, Yves

-- 
perl -Mre=debug -e "/just|another|perl|hacker/"