[swift-evolution] [Pitch] Ban the top value in Int/UInt

[Guoye Zhang]

> 在 2016年10月18日，15:30，David Waite <david at alkaline-solutions.com> 写道：
> 
> 
>> On Oct 18, 2016, at 12:17 PM, Guoye Zhang via swift-evolution <swift-evolution at 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.

- Guoye