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

[Stephen Canon]

If BinaryFloatingPoint had init(_: RawSignificand), you could also just write:

extension BinaryFloatingPoint {
    init(unitRangeFromRawSignificand s: RawSignificand) {
        self = Self(s) * .ulpOfOne
    }
}

(this is why I ask if RawSignificand is really the type you want; if you use some concrete integer type this will work).  But once we have all the new integer protocol conformances, we’ll have a generic init from any integer type (this was already reviewed for FloatingPoint, but isn’t implementable without the Integer support), which will also make this possible.

> On Aug 26, 2016, at 7:47 PM, Stephen Canon via swift-dev <swift-dev at swift.org> wrote:
> 
> 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 <mailto: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 <mailto: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
>>>> 
>>>> 
>>> 
>>> 
>> 
>> _______________________________________________
>> swift-dev mailing list
>> swift-dev at swift.org <mailto:swift-dev at swift.org>
>> https://lists.swift.org/mailman/listinfo/swift-dev
> 
> _______________________________________________
> swift-dev mailing list
> swift-dev at swift.org
> https://lists.swift.org/mailman/listinfo/swift-dev

-------------- next part --------------
An HTML attachment was scrubbed...
URL: <https://lists.swift.org/pipermail/swift-dev/attachments/20160826/f666b894/attachment.html>