RE: [F&O] 17.8 Two definitions for decimal -> double

> Sarah:
> Thank you for your comment.  This has been fixed.  The 
> (single) rule now
> reads:
> 
> - If ST is xs:float or xs:decimal, or types derived from 
> them, then TV is xs:double(SV cast as xs:string) and the 
> conversion is complete.
> 

Sarah also asked:

Also, why would float need to be first cast to a string 
when double represents a superset of its values (both for the actual 
value as well as mantissa and exponent)?


I have to say I am confused by this myself. I have also heard it suggested
that the reason xs:float is not defined in XML Schema as a subtype of
xs:double is that the value space of xs:float is not a subset of the value
space of xs:double.


The Java language spec says:

The finite nonzero values of type float are of the form s.m.2^e, where s is
+1 or -1, m is a positive integer less than 2^24, and e is an integer
between -149
and 104, inclusive. Values of that form such that m is positive but less
than 2^23
and e is equal to -149 are said to be denormalized.

The finite nonzero values of type double are of the form s.m.2^e, where s is
+1 or -1, m is a positive integer less than 2^53, and e is an integer
between -1075
and 970, inclusive. Values of that form such that m is positive but less
than 2^52
and e is equal to -1075 are said to be denormalized.

These definitions certainly suggest that the value space of float is a
subspace of double.

So why isn't float a subtype of double, and why do we convert from one to
the other via a string?

Michael Kay