Treating the abusive unsigned syndrome

[Andrei Alexandrescu]

Steven Schveighoffer wrote:
> "Andrei Alexandrescu" wrote
>> I remembered a couple more details. The names bits8, bits16, bits32, and 
>> bits64 were a possible choice for undecided-sign integrals. Walter and I 
>> liked that quite some. Walter also suggested that we make those actually 
>> full types accessible to programmers. We both were concerned that they'd 
>> add to the already large panoply of integral types in D. Dropping bits8 and 
>> bits16 would reduce bloating at the cost of consistency.
>>
>> So we're contemplating:
>>
>> (a) Add bits8, bits16, bit32, bits64 public types.
>> (b) Add bit32, bits64 public types.
>> (c) Add bits8, bits16, bit32, bits64 compiler-internal types.
>> (d) Add bit32, bits64 compiler-internal types.
>>
>> Make your pick or add more choices!
> 
> One other thing to contemplate:
> 
> What happens if you add a bits32 to a bits64, long, or ulong value?  This 
> needs to be illegal since you don't know whether to sign-extend or not.  Or 
> you could reinterpret the expression to promote the original types to 64-bit 
> first?

Good point. There's no (or not much) arithmetic mixing bits32 and some 
64-bit integral because it's unclear whether extending the bits32 
operand should extend the sign bit or not.

> This makes the version with 8 and 16 bit types less attractive.
> 
> Another alternative is to select the bits type based on the entire 
> expression.  Of course, you'd have to disallow them as public types.  And 
> you'd want to do some special optimizations.  You could represent it 
> conceptually as calculating for all the bits types until the one that is 
> decided is used, and then the compiler can optimize out the unused ones, 
> which would at least keep it context-free.
> 
> -Steve 

That's the intent of defining arithmetic on sign-ambiguous values. The 
type information propagates in a complex expression. I haven't heard of 
typechecking on entire expression patterns and I think it would be a 
rather unclean technique (it means either that there are values that you 
can't tell the type of, or that a given value has a context-dependent type).


Andrei