> 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 >> <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. - Guoye _______________________________________________ swift-evolution mailing list swift-evolution@swift.org https://lists.swift.org/mailman/listinfo/swift-evolution
