Ralf Hemmecke wrote:
>
> > [[x, y, z] for t in allTriples
> > | (x := t(1))^3 + (y := t(2))^3 = (z := t(3))^3]
> >
> > If allTriples goes without repetitions trough all triples of
> > integers this stream is finite, but I do not want to
> > exclude such streams.
>
> Well, the dangerous thing with the above stream is that (if there are
> just N entries in this stream) you shouldn't ask for an element that
> comes after the last one (i.e. wanting to access the (N+1)-th element,
> will never succeed).
>
> Honestly, I wouldn't suddently want to run into a never returning
> function call. In other words, without otherwise ensuring that only
> computable stream elements are accessed, I wouldn't want to see such
> stream, no matter how nice they look.
The point is that it is programmer job to ensure that there is
enough values. I do not expect compiler to try to prove that
stream is infinite and reject the ones for which it can not
decide.
Also, note that there is kind of dual to this: reasonable
streams which sometimes are finite and you can easily detect
finiteness: consider nonzero Taylor coefficients of a
rational function with on pole at 0. If this function
happens to be a polynomial (which is easy to check), then
we get finite stream, otherwise infinite. Do you want
to throw errors when such function is given a polynomial?
--
Waldek Hebisch
hebi...@math.uni.wroc.pl
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