2009/1/3 Matt Kraai <kraai@xxxxxxxxx>: > On Sat, Jan 03, 2009 at 05:51:50PM +0100, demerphq wrote: >> 2009/1/3 Matt Kraai <kraai@xxxxxxxxx>: >> > 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/" -- To unsubscribe from this list: send the line "unsubscribe git" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
