Re: Meaning of the ill decoration (was: Motion 42: NO)
On 2013-02-07 14:54:36 +0100, Guillaume Melquiond wrote:
> Le jeudi 07 février 2013 à 14:07 +0100, Vincent Lefevre a écrit :
> > On 2013-02-06 22:07:15 +0100, Guillaume Melquiond wrote:
> > > - I miss the point of the ill decoration as defined in Section 8.8.2,
> > > since it is undecidable whether Dom(f) is empty for an arbitrary
> > > real-valued function f. (And it does not even have to be that arbitrary:
> > > you just need addition, multiplication, floor, conditional, and a
> > > function that is not defined on the whole real line, say square root.)
> >
> > The fact that it is undecidable whether Dom(f) is empty is not a
> > problem, since an implementation can return emp instead (the best
> > decoration is not required).
>
> That is right: as an implementer, I would always return emp, because I
> would have no way to decide whether the function has an empty domain.
> This defeats the point of such a definition of ill.
The "Dom(f) is empty." was introduced by John on 13 Nov 2012. Now I
wonder why. I thought it was copied from ndf ("nowhere defined") of
Motion 27A, but ndf was more like emp.
An implementation that can analyse the expressions (e.g. if P1788
is implemented by a compiler) could return ill in some cases, such
as sqrt(-x^2-1). But...
> Do people really care about functions that are defined nowhere?
I don't think so.
--
Vincent Lefèvre <vincent@xxxxxxxxxx> - Web: <http://www.vinc17.net/>
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Work: CR INRIA - computer arithmetic / AriC project (LIP, ENS-Lyon)