[swift-dev] Why are BinaryFloatingPoint's RawSignificand and RawExponent different type?

[Stephen Canon]

Assuming RawSignificand really is the type you want, I think this does what you’re looking for?

protocol BinaryFloatingPointWithBitPattern: BinaryFloatingPoint {
    init(bitPattern: RawSignificand)
    var bitPattern: RawSignificand { get }
}

extension Float: BinaryFloatingPointWithBitPattern { }
extension Double: BinaryFloatingPointWithBitPattern { }

extension BinaryFloatingPointWithBitPattern {
    init(unitRangeFromRawSignificand s: RawSignificand) {
        self = Self(bitPattern: Self(1).bitPattern | s) - 1
    }
}

> On Aug 26, 2016, at 7:38 PM, Stephen Canon via swift-dev <swift-dev at swift.org> wrote:
> 
> Where does your RawSignificand input come from?  Is that really the type that you want?
> 
> I don’t think you really need very much boilerplate at all here.
> 
>> On Aug 26, 2016, at 7:30 PM, Jens Persson <jens at bitcycle.com> wrote:
>> 
>> I understand.
>> It's just very tempting to try and use the new static computed properties for eg 23 and 52 etc.
>> I guess I'll just have to write a lot of boilerplate, or perhaps a protocol that is just implemented by Double and Float (that will be very similar to BinaryFloatingPoint in a lot of ways).
>> /Jens
>> 
>> On Sat, Aug 27, 2016 at 1:25 AM, Stephen Canon <scanon at apple.com> wrote:
>> This doesn’t really scale up very well, though.  BinaryFloatingPoint needs to also be able to model e.g. Float2048 or similar; we generally don't want to require that RawExponent to be the same type as RawSignificand (which I think is what you’re really suggesting), because in typical bignum usage significands are much larger than exponents.
>> 
>> It sounds like maybe you actually want to be operating directly on bitPatterns, rather than the abstract fields of the types.
>> 
>> – Steve
>> 
>>> On Aug 26, 2016, at 7:21 PM, Jens Persson <jens at bitcycle.com> wrote:
>>> 
>>> Oh, to more directly answer your question: I don't like having to create a UInt (UInt64) value when all my bit manipulaton code happens in UInt32 (for Float) for example.
>>> 
>>> The most probable context for using these computed properties and types of BinaryFloatingPoint is one in which specific fixed width types really matters a lot (look at the name of the protocol and the properties and assocated types we are talking about).
>>> 
>>> /Jens
>>> 
>>> 
>>> On Sat, Aug 27, 2016 at 1:15 AM, Jens Persson <jens at bitcycle.com> wrote:
>>> Reason for asking is that I have this:
>>> 
>>> extension Double {
>>>     init(unitRangeFromRawSignificand s: RawSignificand) {
>>>         let bitPattern = s | (1023 << 52)
>>>         self = unsafeBitCast(bitPattern, to: Double.self) - 1.0
>>>     }
>>> }
>>> extension Float {
>>>     init(unitRangeFromRawSignificand s: RawSignificand) {
>>>         let bitPattern = s | (127 << 23)
>>>         self = unsafeBitCast(bitPattern, to: Float.self) - 1.0
>>>     }
>>> }
>>> 
>>> But they would be better as:
>>> extension BinaryFloatingPoint {
>>>     init(unitRangeFromRawSignificand s: RawSignificand) {
>>>         ... problems here, have to try casting things into RawSignificand's type ...
>>>     }
>>> }
>>> 
>>> Please have a go at that and perhaps you see what I mean or you will come up with a nice solution that I have missed. (Speed is very important btw.)
>>> 
>>> /Jens
>>> 
>>> 
>>> On Sat, Aug 27, 2016 at 1:02 AM, Stephen Canon <scanon at apple.com> wrote:
>>> > On Aug 26, 2016, at 6:06 PM, Jens Persson via swift-dev <swift-dev at swift.org> wrote:
>>> >
>>> > I can understand why
>>> > Double.RawSignificand is UInt64
>>> > and
>>> > Float.RawSignificand is UInt32
>>> >
>>> > But I can't understand why both
>>> > Double.RawExponent
>>> > and
>>> > Float.RawExponent
>>> > should be UInt.
>>> >
>>> > Why aren't they also just UInt64 and UInt32, resp.?
>>> 
>>> Let me flip the question: why would they be UInt64 and UInt32?  Absent a reason to prefer a specific fixed-with type, Swift integers should generally default to being [U]Int (and ideally Int, but RawExponent is Unsigned).
>>> 
>>> – Steve
>>> 
>>> 
>> 
>> 
> 
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