> On Feb 21, 2017, at 3:27 PM, Xiaodi Wu <[email protected]> wrote: > Hmm, with respect to both endianness and Jordan's comments re popcount (and > by extension, trailing zeros and leading zeros), I think these go to the dual > purpose of `BinaryInteger` (and its refinement, `FixedWidthInteger`). Both > concede by their very name that they are modeling not merely a numeric type > but one that has a certain machine representation. Endian conversions aren't > numeric operations, but they are appropriately operations that can be generic > over fixed width integers. I'm not convinced (though I haven't thought about > it for too long yet) of what `LittleEndian<Int>` would gain us.
It isolates non-native and native integer representations so that you can't easily mix them. It's an extremely common bug in serialization code to forget to byte-swap integers, and we should not naively introduce that bug into Swift by encouraging programmers to use the same type to represent them. The easiest way to achieve that is to try to prevent programmers from ever creating an Int32 whose bytes aren't in native order in the first place. That's not always possible — notably, C doesn't distinguish these in the type system, and there are C APIs that expect to be given a big-endian number — but it's easy enough for BigEndian to have an "init(bitPattern: T)" and a "var bitPattern: T" to cover for cases like that. John. > > > On Tue, Feb 21, 2017 at 2:12 PM, John McCall via swift-evolution > <[email protected] <mailto:[email protected]>> wrote: >> On Feb 21, 2017, at 3:08 PM, John McCall via swift-evolution >> <[email protected] <mailto:[email protected]>> wrote: >>> On Feb 21, 2017, at 2:15 PM, Dave Abrahams via swift-evolution >>> <[email protected] <mailto:[email protected]>> wrote: >>> Sent from my moss-covered three-handled family gradunza >>> >>> On Feb 21, 2017, at 9:04 AM, Jordan Rose <[email protected] >>> <mailto:[email protected]>> wrote: >>> >>>> [Proposal: >>>> https://github.com/apple/swift-evolution/blob/master/proposals/0104-improved-integers.md >>>> >>>> <https://github.com/apple/swift-evolution/blob/master/proposals/0104-improved-integers.md>] >>>> >>>> Hi, Max (and Dave). I did have some questions about this revision: >>>> >>>>> Arithmetic and SignedArithmetic protocols have been renamed to Number and >>>>> SignedNumber. >>>> >>>> What happens to NSNumber here? It feels like the same problem as Character >>>> and (NS)CharacterSet. >>>> >>>> >>>>> Endian-converting initializers and properties were added to the >>>>> FixedWidthInteger protocol. >>>> >>>> This is the thing I have the biggest problem with. Endian conversions >>>> aren't numeric operations, and you can't meaningfully mix numbers of >>>> different endianness. That implies to me that numbers with different >>>> endianness should have different types. I think there's a design to >>>> explore with LittleEndian<Int> and BigEndian<Int>, and explicitly using >>>> those types whenever you need to convert. >>> >>> I disagree. Nobody actually wants to compute with numbers in the wrong >>> endianness for the machine. This is just used for corrections at the ends >>> of wire protocols, where static type has no meaning. >> >> I think Jordan's suggestion is not that LittleEndian<Int> or BigEndian<Int> >> would be artihmetic types, but that they would be different types, primarily >> opaque, that can be explicitly converted to/from Int. When you read >> something off the wire, you ask for the bytes as one of those two types (as >> appropriate) and then convert to the underlying type. Ideally, Int doesn't >> even conform to the "this type can be read off the wire" protocol, >> eliminating the common mistake of serializing something using native >> endianness. > > Of course, you would not want LittleEndian<Int> to be directly serializable > either, because it is not a fixed-size type; but I think the underlying point > stands. > > John. > >> >> John. >> >>> >>>> Here's a sketch of such a thing: >>>> >>>> struct LittleEndian<Value: FixedWidthInteger> { >>>> private var storage: Value >>>> >>>> public var value: Value { >>>> #if little_endian >>>> return storage >>>> #else >>>> return swapBytes(storage) >>>> #endif >>>> } >>>> >>>> public var bitPattern: Value { >>>> return storage >>>> } >>>> >>>> public var asBigEndian: BigEndian<Value> { >>>> return BigEndian(value: self.value) >>>> } >>>> >>>> public init(value: Value) { >>>> #if little_endian >>>> storage = value >>>> #else >>>> storage = swapBytes(value) >>>> #endif >>>> } >>>> >>>> public init(bitPattern: Value) { >>>> storage = bitPattern >>>> } >>>> } >>>> >>>> I'm not saying this is the right solution, just that I suspect adding >>>> Self-producing properties that change endianness is the wrong one. >>>> >>>>> /// The number of bits equal to 1 in this value's binary representation. >>>>> /// >>>>> /// For example, in a fixed-width integer type with a `bitWidth` value >>>>> of 8, >>>>> /// the number 31 has five bits equal to 1. >>>>> /// >>>>> /// let x: Int8 = 0b0001_1111 >>>>> /// // x == 31 >>>>> /// // x.popcount == 5 >>>>> var popcount: Int { get >>>>> } >>>> >>>> Is this property actually useful enough to put into a protocol? I know >>>> it's defaulted, but it's already an esoteric operation; it seems unlikely >>>> that one would need it in a generic context. (It's also definable for >>>> arbitrary UnsignedIntegers as well as arbitrary FixedWidthIntegers.) >>> >>> The whole point is that you want to dispatch down to an LLVM instruction >>> for this and not rely on the optimizer to collapse your loop into one. >>> >>>> >>>> (I'm also still not happy with the non-Swifty name, but I see >>>> "populationCount" or "numberOfOneBits" would probably be worse.) >>>> >>>> >>>> Thanks in advance, >>>> Jordan >>> _______________________________________________ >>> swift-evolution mailing list >>> [email protected] <mailto:[email protected]> >>> https://lists.swift.org/mailman/listinfo/swift-evolution >>> <https://lists.swift.org/mailman/listinfo/swift-evolution> >> >> _______________________________________________ >> swift-evolution mailing list >> [email protected] <mailto:[email protected]> >> https://lists.swift.org/mailman/listinfo/swift-evolution >> <https://lists.swift.org/mailman/listinfo/swift-evolution> > > _______________________________________________ > swift-evolution mailing list > [email protected] <mailto:[email protected]> > https://lists.swift.org/mailman/listinfo/swift-evolution > <https://lists.swift.org/mailman/listinfo/swift-evolution> > >
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