> 在 2016年10月18日，15:30，David Waite <da...@alkaline-solutions.com> 写道：
>> On Oct 18, 2016, at 12:17 PM, Guoye Zhang via swift-evolution
>> <firstname.lastname@example.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.
I would also like to see big number some day.
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