Re: about emp (was: Motion 42: NO)
Guillaume
On 8 Feb 2013, at 06:05, Guillaume Melquiond wrote:
> Let me quote you the relevant parts of 8.8.6:
>
> "w_dw = phi(v_dv)" where v_dv is a box "v_dv_1, ..., v_dv_k"
> "w \subseteq Rge(phi | v)" (23)
> "p_dv_0(phi,v)" (24)
> "dw = min(dv_0,dv_1,...,dv_k)" (25)
> "where the minimum is taken with respect to the propagation order dac >
> def > trv > emp > ill" (26)
>
> As you can see from (25) above, the decoration dw of the output can
> never be improved so that it becomes higher than the decorations
> dv_0, ..., dv_k of the inputs.
Yes, that is true according to the current text. I have held to that principle for some time, partly for conceptual simplicity, partly because Nate strongly supported it.
But I think we should allow it to be varied. The simplest example is multiplying by a known zero, somewhere in an expression, so
f(x) = c*g(x) where c=0
Interval evaluation will do
ff = [0,0]*gg
and if gg has a better decoration than trv, surely ff should be given the best decoration, so com if supported? So def or dac can -> com.
Another IMPORTANT situation is the case(c,g,h) function where Dmitry has pointed out the 8.8.6 rules are incompatible with the intention that one can "streamline" case(cc,gg,hh) by not evaluating hh [resp. gg] if cc<0 [resp. cc>=0].
John Pryce