On 10/17/2012 5:18 AM, Peter Alexander wrote:
So, std.range.isInfinite checks if r.empty is a compile time boolean equal to
false. This works most of the time, but not always. Some ranges are infinite,
but cannot be determined to be so at compile time (or even run time!)
cycle([1, 2, 3]).until(4); // infinite, but not isInfinite
cycle(new int[0]); // not infinite, but isInfinite
collatzSequence(n).until(1); // infiniteness an open problem!
In short, isInfinite does not -- and cannot -- work as a way of telling if a
range is finite or infinite in general.
isInfinite tells you if it is statically determinable to be infinite, not if it
might be at run time.
On resolution is to take isInfinite at face value: it only tells you if a range
is statically determined to be infinite. If isInfinite is false, it could still
be infinite.
Right. isInfinite doesn't pretend to solve the halting problem.
This leaves us with the tricky cases like cycle(new int[0]). There's three
resolutions to this (as far as I can tell):
1. Change cycle to not be an infinite range.
2. Make cycle assert when the source range is empty.
3. Ignore this issue.
Option 1 is sound, but kind of sucks, because generally cycle is infinite.
Option 2 is sound, but I don't like the idea of asserting on logically valid
input just because the problem is too hard.
It isn't just hard, it is impossible. Re the halting problem.
