[std-interval] More on interval computations as proofs

[Sylvain Pion]

Prof.Dr.J.Wolff von Gudenberg wrote:
> Dear Intervalers,
>  during the SCAN 2006 meeting in Duisburg the following points have been 
> discussed.
> 
> The GAMM activity group for Computer Arithmetic and Scientific Computing 
> strongly supports the efforts to standardize
> interval arithmetic in C++
> 
> The discussion showed that
> Standard functions MUST set a flag when the argument interval is not 
> completely contained in the domain of the function
> Otherwise (i.e without the flag) verification of the result with fixed 
> points or other assertions that need continuity is not possible
> A whole branch of interval arithmetic will not be supported by the 
> standard in that case.

I'm not opposed to this.

> We propose (together with G. Melquiond) That this flag can be managed by 
> the user himself.
> The elementary functions (at least those with bounded domain) have to 
> set this flag in the case
> when the argument interval is not completely contained in the domain of 
> the function
> interval<T> sqrt ( interval<T> x, bool& flag)
> will be a proper interface
> Such a function is also necessary for the division operator
> interval<T> divide ( interval<T> x, interval<T> y, bool& flag)
> interval<T> divide ( T x, interval<T>y,  bool& flag)
> multiplication and addition are ok.

An alternative implementation is to have a global flag like errno,
which allows to check the flag without modifying all function calls.
What is the motivation behind your choice ?

-- 
Sylvain