Le 21/11/14 02:51, Eliot Miranda a écrit :
Hi All,
64-bit Spur can usefully provide an immediate float, a 61-bit subset of the ieee double precision float. The scheme steals bits from the mantissa to use for the immediate's 3-bit tag pattern. So values have the same precision as ieee doubles, but can only represent the subset with exponents between 10^-38 and 10^38, the single-precision range. The issue here is how to organize the class hierarchy.
The approach that looks best to me is to modify class Float to be an abstract class, and add two subclasses, BoxedFloat and SmallFloat, such that existing boxed instances of Float outside the SmallFloat range will become instances of BoxedFloat and instances within that range will be replaced by references to the relevant SmallFloat.
With this approach ...
- Float pi etc can still be used, even though they will answer instances of SmallFloat. But tests such as "self assert: result class == Float." will need to be rewritten to e.g. "self assert: result isFloat".
- BoxedFloat and SmallFloat will not be mentioned much at all since floats print themselves literally, and so the fact that the classes have changed won't be obvious.
- the boxed Float primitives (receiver is a boxed float) live in BoxedFloat and the immediate ones live in SmallFloat. Making SmallFloat a subclass of Float poses problems for all the primitives that do a super send to retry, since the boxed Float prims will be above the unboxed ones and so the boxed ones would have to test for an immediate receiver.
An alternative, that VW took (because it has both Float and Double) is to add a superclass, e.g. LimitedPrecisionReal, move most of the methods into it, and keep Float as Float, and add SmallFloat as a subclass of LimitedPrecisionReal. Then while class-side methods such as pi would likely be implemented in LimitedPrecisionReal class, sends to Float to access them find them via inheritance. An automatic reorganization which moves only primitives out of LimitedPrecisionReal is easy to write.
Eliot the solution proposed by VM is nice in the sense that we can continue to use Float pi... On the other hand, I like the design used on objective-c when the superclass act as a factory for its subclass hidding concrete implementation classes. Now I do not know if his would make sense for Float. Another point, I do not really like Boxed because this is the first time we use it. May be SmallFloat would be in the same spirit than SmallInteger.
Thoughts? -- best, Eliot