Treating the abusive unsigned syndrome

[Andrei Alexandrescu]

Sean Kelly wrote:
> Andrei Alexandrescu wrote:
>>
>> Notice that the fact that one operand is a literal does not solve all 
>> of the problems I mentioned. There is for example no progress in 
>> typing u1 - u2 appropriately.
> 
> What /is/ the appropriate type here?  For example:
> 
>     uint a = uint.max;
>     uint b = 0;
>     uint c = uint.max - 1;
> 
>     int  x = a - b; // wrong, should be uint
>     uint y = c - a; // wrong, should be int
> 
> I don't see any way to reliably produce a "safe" result at the language 
> level.

There are several schools of thought (for the lack of a better phrase):

1. The Purist Mathematician: We want unsigned to approximate natural 
numbers, natural numbers aren't closed for subtraction, therefore u1 - 
u2 should be disallowed.

2. The Practical Mathematician: we want unsigned to approximate natural 
numbers and natural numbers aren't closed for subtraction but closed for 
a subset satisfying u1 >= u2. We can rely on the programmer to check the 
condition before, and fall back on modulo difference when the condition 
isn't satisfied. They'll understand.

3. The C Veteran: Everything should be allowed. And when unsigned is 
within a mile, the type is unsigned. I'll take care of the rest.

4. The Assembly Programmer: Use whatever type you want. The assembly 
language operation for subtraction is the same.

5. The Dynamic Language Fan: Allow whatever and check it dynamically.

6. The Static Typing Nut: Use some scheme to magically weed out 73.56% 
mistakes and disallow only 14.95% valid uses.

Your example is in fact perfect. It shows how the result of a 
subtraction has ultimately its fate decided by case-by-case use, not 
picked properly by a rule. The example perfectly underlines the 
advantage of my scheme: the decision of how to type u1 - u2 is left to 
the only entity able to account: the user of the operation. Of course 
there remains the question, should all that be implicit or should the 
user employ more syntax to specify what they want? I don't know.


Andrei