Motion 42: NO
I vote NO on Motion 42 "Decoration system".
Below are the points I cannot make sense of or am in disagreement with.
- The motion states that "the final result is decorated com if and only
if the evaluation of the whole expression was common as defined in 5.4"
in Section 6.3. I understand the "only if" direction, but not the "if"
direction. Indeed, Section 5.4 deals with level 1 while 6.3 deals with
level 2, where overflow, precision, and so on, matter. I guess that the
fact the sentence uses "f" instead of "phi" is not unrelated to my
confusion.
- 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 note at the end of that section does not say otherwise. Section
8.8.3 later gives a different meaning to ill, which makes much more
sense to me, but its relation to the definition of 8.8.2 eludes me.
- I do not understand why intersectionDec and convexHullDec do not have
symmetric behaviors with respect to propagating decorations. If
convexHullDec returns the tightest decoration containing both input
decorations, then intersectionDec should return the tightest decoration
contained in both input decorations. (I know it does not always exist,
but maybe it is the symptom of a bigger issue.) Or, in the other
direction, if intersectionDec returns the min of the input decorations,
then convexHullDec should return the max. In other words, the lattice
structure is lost once decorations are added to intervals according to
Section 8.8.7, which troubles me.
- Sections 8.8.2 and 8.8.10 define decorations as both a property of the
input intervals and the behavior of the function on these inputs. I am
fine with decorations characterizing the behavior of the function, but
not with taking inputs into account. More precisely, I feel that def,
dac, and com, should not require the inputs to be nonempty, and com
should not require the inputs to be bounded. They should only require
that input intervals are part of the domain. Note that I am fine with
both definitions of newDec and I am also fine with the way decorations
propagate from inputs to outputs.
- I do not agree with com requiring the computed interval to be bounded
at level 2. I feel that the boundedness should only be required at level
1. In particular, I do not see what is gained from stripping com in case
of a harmless overflow. What is the point of com if an unbounded
interval from the point of view of the interval type is necessarily
unbounded from the point of view of the decorations? Any information
about what the mathematical function actually computes is lost.
- Finally, the emp decoration seems superfluous to me. There is no point
in decorating a nonempty interval with an emp decoration (except maybe
for wreaking havoc in an interval library), so the emp decoration could
just as well be removed. An empty interval would then be decorated with
trv when it does not designate a NaI.
Best regards,
Guillaume