[swift-evolution] [swift-evolution-announce] [Re-Review] SE-0104: Protocol-oriented integers

[Jordan Rose]

[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. 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.)

(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
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