Re: [PATCH] gitweb: merge boolean feature subroutines

[Matt Kraai <kraai@ftbfs.org>]

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.

-- 
Matt                                                 http://ftbfs.org/