> On 18 Oct 2016, at 20:46, Guoye Zhang via swift-evolution
> <swift-evolution@swift.org> wrote:
>
>>
>> 在 2016年10月18日，15:30，David Waite <da...@alkaline-solutions.com
>> <mailto:da...@alkaline-solutions.com>> 写道：
>>
>>
>>> On Oct 18, 2016, at 12:17 PM, Guoye Zhang via swift-evolution
>>> <swift-evolution@swift.org <mailto:swift-evolution@swift.org>> wrote:
>>> I propose to ban the top value in Int/UInt which is 0xFFFF... in hex. Int
>>> family would lose its smallest value, and UInt family would lose its
>>> largest value. Top value is reserved for nil in optionals. An additional
>>> benefit is that negating an Int would never crash.
>>
>> There are two ways to do this (using Int8 for example)
>> 1. 0xFF reserved to mean nil. As this normally means -1, all negative
>> numbers now use complements rather than two’s complement form. This breaks a
>> lot of binary math.
>>
>> 2. 0x80 reserved to mean nil. This is normally -128. Overflow would have to
>> be modified in order to support this (otherwise, 127 + 1 == nil). bit
>> padding no longer works (0x80 would expand to 0xFF80 for a Int16 with bit
>> padding, not 0x8000)
>
> Yes, 0x80 is better for arithmetic, checking for nil might be slower.
>
>>
>>>
>>> Interacting with C/Obj-C is a major concern, but since we are already
>>> importing some of the unsigned integers as Int which loses half the values,
>>> one value is not such big a drawback. Alternatively, we could leave current
>>> behavior as CInt/CUInt. Converting them to the new Int?/UInt? doesn't
>>> generate any instructions since the invalid value already represents nil.
>>>
>>
>> As the appropriate integer minimum value may already be in use in C or
>> Objective C code, I believe you would need to define a new integer types to
>> support this sort of constrained type.
>>
>> Where I would see something like this be most appropriate would be for
>> supporting a “BigNumber” type in the language, preferably as the default
>> integer type. Ruby does this for example with Fixnum/Bignum - all values in
>> Ruby are actually tagged pointers (where the lower bits are set to cause
>> invalid alignment of a pointer in order to indicate it is a special case
>> immediate value). So if the lowest bit is set, the value is a FixNum integer
>> with a lower max/higher min than a traditional integer. On overflow, the
>> value is promoted to be a BigNum, which is a reference to an arbitrary sized
>> integer on the heap.
>>
>> -DW
>
> I would also like to see big number some day.
I've just finished the implementation of Decimal in Foundation on Linux, which
provides for a greater (though not unlimited) space of numbers.
https://github.com/apple/swift-corelibs-foundation/pull/687
Alex
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