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Postings from July 2010
On Wed, Jul 28, 2010 at 11:21:31AM -0400, Chas. Owens wrote:
> I took a [quick look][0] to see how well documented "0 but true" is.
> It looks like it is only mentioned tangentially. Similarly, "0E0" gets
> mentioned once in perlop:
>
> The final sequence number in a range has the string "E0" appended to it,
> which doesn't affect its numeric value, but gives you something to search
> for if you want to exclude the endpoint.
>
> They should probably be documented in the [Truth and Falsehood][1]
> section of perlsyn. A patch will be forthcoming. Does anybody know
> the full list of strings that are not warned about? I know that
> strings that match the following regex are all true but do not warn in
> numeric context:
>
> my $zero_but_true = qr{
> \A
> \s*
> (?:
> 0 but true | # "0 but true"
> 00+ | # "00", "000", etc.
> 0+[.]0* | # "0.", "00.00", etc.
> [.]0+ | # ".0", ".0000", etc.
> 0+[eE][0-9]+ | # "0e0", "00e19", etc.
> 0+[.]0*[eE][0-9]+ | # "0.e0", "000.000E19", etc.
> [.]0+[eE][0-9]+ # ".0e0", ".000e500", etc.
> )
> \s*
> \z
> }x;
>
> [0]: http://www.google.com/search?q=+site:perldoc.perl.org+%220+but+true%22
> [1]: http://perldoc.perl.org/perlsyn.html#Truth-and-Falsehood
"0 but true" is the only special case in the number conversion code. The
others are all valid floating point representations.
If you're up for it, I don't think that it's documented anywhere that the
false value returned by a logical operator is a special case:
$ perl -wle '$a = 0 == 1; print ">$a<"; print $a + 0'
><
0
it is both the empty string, and a numeric zero.
(That's not a special case in any conversion code. It's a dual value)
Also, $! is a dual value, with the string value being the error string, and
numeric value being the error code. On this system, the error string for 0
*is* an empty string, hence this is false:
$ perl -wle '$! = 0; print ">$!<"; print $! + 0'
><
0
And if you're really up for it, there's a bug with no obvious fix if your
system supports negative zero:
$ perl -wle '$a = 0/-1; print $a ? "T" : "F"; print $a; print $a ? "T" : "F";'
F
-0
T
the problem being that the implementation of true/false is
is there a string value?
Y: is it "0" or ""?
is there a numeric value?
Y: is it == 0
...
IEEE floating point has -0 numerically equal to 0, so that makes it false.
But if you then cause that value to be used as a string (such as that print)
then the string representation is cached, and it doesn't meet the value of
falsehood.
This *is* inconsistent with the documentation, and with the general intended
behaviour, but I can't see a way to fix it. Whereas the behaviour of strings
such as "0e0" and "00" and "0.0" is consistent with both the intent and the
documentation.
Nicholas Clark
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