Positive

[KennyTM~]

Andrei Alexandrescu wrote:
> KennyTM~ wrote:
>> Bruce Adams wrote:
>>> On Sun, 05 Oct 2008 11:18:24 +0100, KennyTM~ <kennytm at gmail.com> wrote:
>>>>
>>>> Probably they'd used (0, +∞) and [0, +∞) instead.
>>>>
>>>> ---
>>>>
>>>> BTW, negative real numbers can also be indicated as ℝ⁻ (R^- if 
>>>> Unicode is not supported). Is it now special enough to deserve a 
>>>> Negative!() template? :p
>>>
>>> Nice use of unicode.
>>>
>>>>
>>>> I think these sign checking should be done through contracts or 
>>>> "conditional template" (? whatever it's called; I haven't used one 
>>>> of these yet) instead. Unless you can runtime check that
>>>>
>>>>    Positive!(double) x = 6;
>>>>    Positive!(double) y = 12;
>>>
>>> Compile time check
>>>
>>>>    Positive!(double) z = void;
>>>
>>> This must be an syntax error surely?
>>
>> type x = void; means don't initialize x. Or have I missed something?
>>
>>>
>>>>    z = x - y;  // raises error.
>>>
>>> The return type of subtraction goes outside the postive domain:
>>> i.e. if it needed to exist (which it probably doesn't) the function 
>>> spec would be:
>>>
>>> double opMinus(Positive!(double) y, Positive!(double) y);
>>>
>>>
>>> So this is actually:
>>>
>>> z = cast(Positive!(double))(x - y)
>>>
>>> Hence there is the necessary check which could be either run-time or 
>>> compile time.
>>>
>>
>> Most of the time it can't be compile time because if I've initialized 
>> x and y to be some runtime-dependent function (e.g. sin(current time)) 
>> then you can't be sure of the sign of x-y. So the check must be made 
>> into runtime unless the expression can be CTFE-ed.
>>
>>>
>>>>    Positive!(double) w;
>>>>    din.readf("%g", &w);  // raises error if user enters negative 
>>>> number.
>>>>
>>>> But I don't think uint, etc now even do these checks.
>>>
>>> This would ideally work as above but I see your problem.
>>> The contract of readf is not tight enough. You should rewrite as:
>>>
>>> double wtemp;
>>> din.read("%g", &wtemp);
>>> Positive!(double) w = wtemp;    // type check applied here
>>>
>>
>> But this is getting ugly :)
> 
> Of course. It's because it uses the "!". :o)
> 
> Andrei

I mean using temporary variable explicitly is ugly -_-