Treating the abusive unsigned syndrome

[Andrei Alexandrescu]

(I lost track of quotes, so I yanked them all beyond Don's message.)

Don wrote:
> The problem with that, is that you're then forcing the 'unsigned is a 
> natural' interpretation when it may be erroneous.
> 
> uint.max - 10 is a uint.
> 
> It's an interesting case, because int = u1 - u2 is definitely incorrect 
> when u1 > int.max.
> 
> uint = u1 - u2 may be incorrect when u1 < u2, _if you think of unsigned 
> as a positive number_.
> But, if you think of it as a natural modulo 2^32, uint = u1-u2 is always 
> correct, since that's what's happening mathematically.

Sounds good. One important consideration is that modulo arithmetic is 
considerably easier to understand when two's complement and signs are 
not involved.

> I'm strongly of the opinion that you shouldn't be able to generate an 
> unsigned accidentally -- you should need to either declare a type as 
> uint, or use the 'u' suffix on a literal.
> Right now, properties like 'length' being uint means you get too many 
> surprising uints, especially when using 'auto'.

I am not surprised by length being unsigned. I'm also not surprised by 
hexadecimal constants being unsigned. (They are unsigned in C. Walter 
made them signed or not, depending on their value.)

> I take your point about not wanting to give up the full 32 bits of 
> address space. The problem is, that if you have an object x which is 
>  >2GB, and a small object y, then  x.length - y.length will erroneously 
> be negative. If we want code (especially in libraries) to cope with such 
> large objects, we need to ensure that any time there's a subtraction 
> involving a length, the first is larger than the second. I think that 
> would preclude the combination:
> 
> length is uint
> byte[].length can exceed 2GB, and code is correct when it does
> uint - uint is an int (or even, can implicitly convert to int)
> 
> As far as I can tell, at least one of these has to go.

Well none has to go in the latest design:

(a) One unsigned makes everything unsigned

(b) unsigned -> signed is allowed

(c) signed -> unsigned is disallowed

Of course the latest design has imperfections, but precludes neither of 
the three things you mention.


Andrei