About Bug 1742275: featurep(false, 'complex)

On 7/14/07, Harald Geyer <Harald.Geyer at gmx.at> wrote:

> I have thought about bug 1742275 "featurep(false, 'complex) -> true"
> a bit and i basically see two possible solutions:

Well, featurep has evolved toward evaluating predicates of the
form is(x in S) where S is a set; I think featurep recognizes
integers, rationals, reals, complex, and pure imaginary, maybe
some others. That is a useful idea, but at present featurep is not
a very good implementation of it.

In that spirit, feature(false, 'complex) should return false
because false is a Boolean literal and therefore demonstrably
not an element of set of complex numbers.
Likewise featurep(true, 'complex) and featurep("foo", 'complex)
should return false.

featurep(A, 'complex) where A is a matrix, list, or set should
return false (or maybe featurep should distribute over aggregate
objects; dunno if I'm for or against that).

> a)
> Just remove the condition ((eq ind '$complex) t) from $featurep.
> With this we would get
>   declare(z, 'complex); featurep(z, 'complex); -> true
> because of the generic condition for symbol properties. But
>   featurep(1+%i, 'complex); -> false

Hmm. I guess I don't like this one.

> b)
> Change the condition to
> ((eq ind '$complex)
>  (let ((ris (trisplit e)))
>        (not (or (zerop1 (car ris)) (zerop1 (cdr ris))))))
> which is equivalent to realpart(expr) != 0 and imagpart(expr) != 0

At present featurep seems to embody the notion that
featurep(X, S) => featurep(X, T) when S is a subset of T.
That makes sense to me and I don't want to change it at
the moment. Requiring that realpart and imagpart be nonzero
makes featurep true isn't consistent with that.
So I guess I don't like this one either.

As mentioned by others, featurep has accumulated some baggage
over the years and probably we're best off separating the
stuff which has only to do with declared properties of symbols,
and potentially-inferred properties of symbols or expressions.

For the moment I think changing featurep(foo, complex) to
return false when foo is a Boolean or string literal is a step forward.

In the long run, I would like to see declarations expanded to
expressions as well as symbols, which would allow stuff like:

  declare (x in Z);
  declare (matrixp (y));
  declare (z in R and z > 0);

I'd like to merge the declare and assume stuff --- these could
equally well be assume(x in Z), assume(matrixp(y)), etc.

For the moment this is just a daydream.

FWIW
Robert