Dr.Ruud wrote:
>
> Rob Dixon schreef:
>>
>> Dr.Ruud wrote:
>>>
>>> Rob Dixon wrote:
>>>>
>>>> A scalar cannot have multiple 'personalities' - it can have a
>>>> maximum of two (its inherent type and a string equivalent) at any
>>>> one time, but usually has only one.
>>>
>>> It can at least have 3: an integer numeric one, a floating numeric
>>> one, and a string one. It can have more.
>>
>> No. It can be just a string; an integer and a string; a double and a
>> string; or a reference (pointer) and a string. After that there's
>> some weird stuff that goes on with tied scalars and so on. Basically
>> every scalar data type keeps space for a pointer to its string
>> equivalent as well, presumably because it's a useful thing to
>> remember once it has been evaluated once.
>
>
> 1: all three of IOK and NOK and POK are set.
>
> perl -Mstrict -Mwarnings -MData::Dumper -MDevel::Peek=Dump -wle'
>
> my $s = "x";
> defined($s) and do {
> no warnings "numeric";
> $s eq "" and 1;
> $s == $_ and 1 for 0, 0.1;
> };
> print "\n", Dumper($s);
> Dump $s;
> '
>
> $VAR1 = 'x';
>
> SV = PVNV(0x8a19cb8) at 0x89bad2c
> REFCNT = 1
> FLAGS = (PADBUSY,PADMY,IOK,NOK,POK,pIOK,pNOK,pPOK)
> IV = 0
> NV = 0
> PV = 0x89b6548 "x"\0
> CUR = 1
> LEN = 2
>
>
> 2: lossy, so no IOK.
>
> perl -Mstrict -Mwarnings -MData::Dumper -MDevel::Peek=Dump -wle'
>
> my $s = "122.99999999999999";
> defined($s) and do {
> no warnings "numeric";
> $s eq "" and 1;
> $s == $_ and 1 for 0, 0.1;
> };
> print "\n", Dumper($s);
> Dump $s;
> '
>
> $VAR1 = '122.99999999999999';
>
> SV = PVNV(0x88aba40) at 0x884cc3c
> REFCNT = 1
> FLAGS = (PADBUSY,PADMY,NOK,POK,pIOK,pNOK,pPOK)
> IV = 122
> NV = 123
> PV = 0x8848700 "122.99999999999999"\0
> CUR = 18
> LEN = 19
>
>
> 3: a numeric string, touched by numeric comparisons, so returned as a
> number by Dumper; again all of IOK, NOK and POK are set.
>
> perl -Mstrict -Mwarnings -MData::Dumper -MDevel::Peek=Dump -wle'
>
> my $s = "123";
> $ARGV[0] and defined($s) and do {
> no warnings "numeric";
> $s eq "" and 1;
> $s == $_ and 1 for 0, 0.1;
> };
> print "\n", Dumper($s);
> Dump $s;
> ' 1
>
> $VAR1 = 123;
>
> SV = PVNV(0x8160dd0) at 0x8101be8
> REFCNT = 1
> FLAGS = (PADBUSY,PADMY,IOK,NOK,POK,pIOK,pNOK,pPOK)
> IV = 123
> NV = 123
> PV = 0x80fd450 "123"\0
> CUR = 3
> LEN = 4
>
>
> I would say that 1 and 3 have three personalities.
> And 2 has nearly three. :)
First of all I would say that all of these are NV values (double floats), and as
I have said scalar values always allow for PV (string) equivalents. What I was
unaware of was that the NV structure also includes space for an IV (integer)
equivalent. Other than that your results are as I described, and what you are
really defending is your comment here
> Dr.Ruud wrote:
>
> There is no real way to test if a value inside a variable has a numeric
> "personality". Variables can have multiple "personalities", each with there
> own binary value.
> Rob Dixon wrote:
>
> A scalar cannot have multiple 'personalities' - it can have a maximum of two
> (its inherent type and a string equivalent) at any one time, but usually has
> only one. It will be given an additional string value only if it needs one for
> some reason. This is one reason why quoting variables - "$val" - is a bad idea
> in general, because it both forces conversion and occupies memory
> unnecessarily.
>
> There is indeed a simple way to test in Perl whether a scalar value is being
> treated as a string (I'm sure this is one or Randal's, but I can't find the
> reference)
>
> print "Non-string" if ($val & ~$val) eq '0';
>
> And also, since Data::Dumper does it there clearly is a way for a module to do
> it in general.
And to be honest I think that if you're allowing 'multiple' to include 'three'
it may as well include 'two' as well :)
> 4: make $s blessed too.
>
> perl -Mstrict -Mwarnings -MData::Dumper -MDevel::Peek=Dump -wle'
>
> my $s = "123";
> bless \$s, "test";
> defined($s) and do {
> no warnings "numeric";
> $s eq "" and 1;
> $s == $_ and 1 for 0.1, 0;
> };
> print "\n", Dumper($s);
> Dump $s;
> '
>
> $VAR1 = 123;
>
> SV = PVMG(0x90f5618) at 0x9068c44
> REFCNT = 1
> FLAGS = (PADBUSY,PADMY,OBJECT,IOK,NOK,POK,pIOK,pNOK,pPOK)
> IV = 123
> NV = 123
> PV = 0x9064460 "123"\0
> CUR = 3
> LEN = 4
> STASH = 0x9063cf4 "test"
>
> That last one is an example of what I named "more".
A PVMG is a 'magical', and that is a whole separate consideration that I don't
think belongs here.
I hope it summarises this thread to your satisfaction if I make these points
- A simple scalar variable may have up to three valid values simultaneously: a
string value, and integer value, and a double float value
- It is straightforward for a Perl module with a C component to examine which
values are valid, and behave accordingly
- Forcing a conversion to a string or a numeric is easy in Perl, using
$val.''
or
$val+0
respectively.
Just to make things worse, I see that Perl 5.10 has moved the pointer to the
string value from the body of the structure up to the header, so that it is now
svu_pv instead of xp_pv. Also, string and integer structures (xpv and xpviv) now
have space for a double value as well. Oddly though, there is no allowance in
xpv for an integer value. This stuff has stayed pretty much constant since Perl
5.4 and it seems I have a lot of reading to do!
Rob
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